Download Patterns of sensory intermodality relationships in the cerebral cortex

THE JOURNAL OF COMPARATIVE NEUROLOGY 308:432-456 (1991)
Patterns of Sensory Intermodality
Relationships in the Cerebral Cortex
of the Rat
TAMAR PAPERNA AND RAFAEL MALACH
Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel 76 100
ABSTRACT
Patterns of connections underlying cross-modality integration were studied by injecting
distinguishable, retrograde tracers (Fluoro-Gold and diamidino yellow) in pairwise manner into
different sensory representations (visual, somatosensory, and auditory) in the cerebral cortex of
the rat.
In agreement with previous single tracer studies, our results indicate that the central core
of sensory areas receives projections mainly from a set of association areas located in a ringlike
fashion along the margin of the cortical mantle. The visual cortex received projections from
areas 48/49, area 29d, posterior agranular medial cortex (AGm), area 11,area 13, and area 35.
All these areas were also connected to the auditory cortex with the exception of areas 29d and
AGm. However, lateral to area 29d and posterior AGm, a band of neurons projecting to the
auditory cortex was present. Somatosensory cortex was connected mainly with the more
anterior aspect of the hemisphere, which included primary motor area, area 11,and area 13.
The patterns of intermodality relationships revealed in the present study were of two main
categories. In the anterior and lateral areas, an intermingling of cells projecting to different
sensory modalities was observed. In contrast, in areas located along the medial aspect of the
hemisphere, cells connected to different sensory modality representations tended to be
segregated from each other. Postsubicular cortex (areas 48/49) contained both intermingled
and segregated groups of cells. The incidence of clearly identified double-labeled cells
concurrently projecting to two different sensory representations was extremely rare. These
patterns may form a substrate for different levels of cross-modal sensory integration in the rat
cortex.
Key words: auditory, somatosensory,visual, connections,association cortex
The integration of different sensory inputs in the brain is
a necessary stage before behavior can be executed. However, this aspect of sensory processing is still poorly understood. One approach to the problem in recent years was to
identify candidate areas that might subserve cross-modality integration. Several physiological and anatomical studies (e.g., Jones and Powell, '70; Benevento et al., '77; Bruce
et al., '81; Pandya and Yeterian, '85; Goldman-Rakic, '88)
have suggested such "polysensory" areas in the cerebral
cortex of different mammalian species.
In the rat, tract tracing studies have also implicated a
number of areas as possible candidates for cross-modality
integration by virtue of their connections with different
sensory modality representations. Overall, these association areas appear to be arranged in a peripheral ring, which
includes most of the limbic cortex and part of the frontal
cortex of the rat. Thus on the medial margin of the
hemisphere, posterior and anterior cingulate cortices were
reported to be connected to the visual cortex, whereas a
0 1991
WILEY-LISS, INC.
neighboring band in area 18b contained auditory projections (Vogt and Miller, '83; Miller and Vogt, '84a; Torrealba
et al., '84). More anteriorly, the posterior medial part of the
agranular medial cortex (AGm) of Donoghue and Wise ('82)
was shown to receive projections from visual, somatosensory, and auditory cortices (Miller and Vogt, '84a; Reep et
al., '84; Torrealba et al., '84). This area partially overlaps
area 8 of Vogt and Miller ('83).
At the frontal pole, area 11was reported to be connected
to the visual cortex (Vogt and Miller, '83; Miller and Vogt,
'84a) but connections to other sensory areas were not
described. On the lateral side of the hemisphere, the
perirhinal areas 13 and 35 (Miller and Vogt, '84a) were
found to be connected to visual, auditory, and somatosensory cortices (Deacon et al., '83; Guldin and Markowitsch
Accepted October 19,1990.
Address reprint requests to R. Malach, Department of Neurobiology,
Perlman building, Weizmann Institute of Science, Rehovot, Israel 76 100.
433
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
'83). Finally, at the posterior tip of the cortex, connections
to visual areas were found with parts of the postsubicular
and parasubicular cortices (areas 48 and 49, respectively,
Vogt and Miller, '83). However, we were unable to find
further information about the connection of these sensory
areas with other peripheral areas.
In all these studies, the polysensory potential of these
areas was inferred indirectly by pooling results across
different cases. Thus it is still not clear whether in areas
considered to be polysensory, there is a precise overlap of
connections to different sensory representations or whether
interdigitation of unisensory columns might exist. Alternatively, in areas considered to be unisensory, is there a sharp
transition from one sensory modality to another, or do
these areas contain broad overlap zones that might also
subserve intermodality integration? Finally, an interesting
question pertains to the nature of feedback connections
from polysensory areas: Do neurons projecting from such
areas remain segregated by modality or do they bifurcate to
innervate directly two different sensory areas?
To answer these questions directly, we employed a doublelabel tracing paradigm, which enabled us to relate, within
the same case, the organization of connections of two
different sensory areas. Our results show that within the
set of the peripheral association areas, two patterns of
neuronal labeling were present. In areas located on the
medial margin of the hemisphere, the neurons projecting to
different unimodal cortical areas occupied separate but
neighboring territories. In contrast, in lateral and anterior
areas, neurons linked to different unimodal sensory areas
were intermingled within the same territory. These different neuronal arrangements may relate to the role of the
peripheral areas in cross-modality interactions.
METHODS
The experimental paradigm employed in this study involved injections of two distinguishable tracers into different sensory representations in the cerebral cortex of the
rat. This approach enabled exact analysis of the relative
distribution of neurons projecting to different targets in the
same animal. Below is a detailed description of the methodology employed.
Abbreviations
A1
AC
AGm
Ap
BLA
DLG
Ent
L
LD
LP
M1
MG
Pir
Po
R
s1
s2
v1
VL
VPL
primary auditory cortex
anterior cingulate cortex
agranular medial cortex
anterior pretectal nucleus
anterior basolateral amygdaloid nucleus
dorsal lateral genicuiate nucleus
entorhinal cortex
lateral
lateral dorsal nucleus
lateral posterior nucleus
primary motor cortex
media1 geniculate nucleus
piriform cortex
posterior nuclei group
rostral
primary somatosensory cortex
secondary somatosensory cortex
primary visual cortex
ventral lateral nucleus
ventral posterolateral nucleus
Tracer injections
A total of 36 adult hooded rats was used in this study. The
rats were anesthetized with Nembutal (40 mgkg) supplemented with ketamine (Vetalar, doses of 5 mg i.m.1. TWO
separate injections of the fluorescent tracers Fluoro-Gold
(FG, Fluorochrome, Englewood, Co., Schmued and Fallon,
'86) and diamidino yellow dihydrochloride (DY, Keizer et
al., '83) were placed in different sensory areas in the right
cortical hemisphere by using stereotactic coordinates. FG
was injected iontophoretically through glass pipettes (4% in
0.1 M sodium acetate buffer pH.3.3, 40-70 pm tip diameter) by pulses of 5 4 , 7 sec on 7 sec off, direct current for 15
minutes. DY was pressure injected through a Hamilton
syringe attached to a glass pipette (0.1-0.15 p1 of 2% DY
aqueous solution). After a week of recovery, the rats were
reanesthetized and a large flap of bone overlying the left
hemisphere was removed, exposing major portions of the
parietal and occipital cortices and smaller portions of the
frontal and temporal cortices. Twenty-five to 35 injections
of horseradish peroxidase (HRP, each injection 0.25-0.35
pl, 10-20% solution) were evenly placed over the exposed
cortex for staining of the callosal connections (Olavarria
and Montero, '84, Malach, '88).
Histology
Two days after injections of HRP, the rats were deeply
anaesthetized with 10% chloral hydrate, perfused with a
brief wash of 0.1 M phosphate-buffered saline (PBS),
followed by 4% paraformaldehyde containing 5% sucrose in
PBS solution. Six brains were prepared for coronal sectioning, which included gelatin embedding of the brain and
postfixation overnight in a 4% paraformaldehyde solution
containing 30% sucrose in phosphate buffer.
In the other 30 cases, the right cortex was flattened
between two glass slides (Welker and Woolsey, '74; Olavarria and Montero, '84; Malach, '88) and postfixed overnight
in 4% paraformaldehyde, 20% sucrose PBS solution. Sections were cut on a freezing microtome at 60 pm for coronal
sections, 40 pm for tangential sections of the flattened
cortices. Five out of six sections from cases sectioned in the
coronal plane were collected for histological and neuroanatomical analysis. This included Nissl stain, acetylcholinesterase (AchE) histochemistry (Geneser-Jensen and Blackstad, '711, myelin stain (Hutchins and Weber, '83), HRP
reaction with tetramethyl benzidine as a chromogen (Mesulam, '78) according to a modified procedure (Malach, '881,
and scanning of the fluorescence labeling, which was done
on an otherwise unprocessed section.
All tangential sections from the flattened cortices were
collected, one out of two sections was kept unprocessed for
mapping of fluorescent label. The remaining sections were
processed alternately for HRP and Nissl stain. Photographs
of coronal and tangential unprocessed sections were taken
prior to mounting on Kodak LPD-4 film under brightfield
illumination. This procedure reveals myeloarchitectonic
patterns in the same sections scanned later for fluorescence
(Malach, '89).
Sections were mounted on gelatin-coated slides. Myelin,
AchE, and Nissl-stained sections were dehydrated in increasing alcohols, passed through three changes of xylene, and
mounted with Permount. HRP stained sections and sections kept for fluorescence scan were rapidly dehydrated in
increasing alcohols, passed through xylene, and mounted
with Fluoromount.
434
T. PAF'ERNA AND R. MALACH
Data processing
in Nissl-stained sections by their dense layer I1 but absence
of layer IV (Miller and Vogt, '84a). The frontal orbital area
11, at the ventral tip of the cortex, was demarcated by a
dense layer I1 (Miller and Vogt, '84a).
Tangential sections. We have constructed a cortical
areal map (Fig. 2C) of the flattened cortex, which is based
on data obtained from coronal sections, as well as previously published studies on tangential sections. Using myeloarchitecture, as visualized in the photographs of the
unmounted sections, cytoarchitecture, and the callosal
pattern of connections, we parcelled the flat cortex according to the following criteria. Area V1 and the secondary
visual cortex (area 18a and area 18b) were easily identified
by the appearance of a dense myelinated zone in area V1
and a slightly lighter zone in area 18a (see Fig. 2A) and by
the typical pattern of callosal connections (see Fig. 2B;
Olavarria and Montero, '84; Olavarria and Van Sluyters,
'85; Thomas and Espinoza, '87; Malach, '89). Area A1
appeared as a myelin dense region lateral to area 18a (Fig.
2A; Krieg, '46b). Anterior and medial to area A1 is area 22
(Krieg's area 39, '46b), which borders with secondary
somatosensory cortex ($32)and area 18a and consists of a
narrow, myelin light strip (Caviness, '75; Olavarria and
Van Sluyters, '85).
In the tangential section, primary somatosensory cortex
(Sl) and area S2 are recognized by their typical myelin
dense appearance (Fig. 1A). The border between these two
areas was determined according to Nissl stain cytoarchitecture (Welker, '71, '76; Welker and Sinha, '721, and by the
lateral callosal band of labeling through which the border
passed (Figs. 2B,C; Olavarria et al., '84). The medial border
of area S1 was also delineated by bands of dense callosal
labeling (Akers and Killackey, '78; Olavarriaet al., '84). The
borders of the motor areas in the frontal cortex, area M1
Definition of cortical areas
Coronal sections. The cortical areas in the coronal and area AGm, were estimated by transforming measured
sections were delineated according to the Nissl, AchE, distances from coronal sections, relying on previous parcelmyelin stains, and their relation to the pattern of callosal lation schemes (Donoghue and Wise, '82; Zilles, '85).
As in the coronal sections, the lateral border of the
connections. The cytoarchitectonic borders of the visual
areas, including the primary visual cortex (Vl), area Ma, cingulate cortex in tangential sections is defined by a dense
and area 18b (Krieg, '46b; Miller and Vogt, '84b) were myelination (Fig. 2A,C). We did not attempt to delineate
correlated with myelin (Zilles, '85) and AchE stains, as further the subdivisions of area 29 because of the distortion
demonstrated in Figure 1, and the pattern of callosal caused by the high curvature of the tissue at this position.
connections (not shown). The borders of the somatosensory The location of the adjacent postsubicular cortex (area
cortex (Krieg, '46b; Welker, '71, '76; Welker and Sinha, '72) 48/49) was estimated according to its position as seen in
and the auditory cortex, including the secondary auditory cortical reconstruction diagrams (Zilles et al., '80; Zilles,
areas (area 22, area 36, area 20), which surround the '85).
primary auditory cortex, area Al, (Krieg, '46b; Cipolloni
Perirhinal and entorhinal cortices, situated lateral to
and Peters, '79; Vaughan, '83; Sally and Kelly, '88; Roger postsubicular cortex, were positioned according to reconand Arnault, '89) were distinguished by their cyto- and struction schemes from coronal sections and by using the
myeloarchitectonic characteristics. We could not distin- rhinal fissure as a marker (Miller and Vogt, '84a; Zilles,
guish clearly in our material the borders between area 36 '85). The anterolateral tip of the tangential section consists
and area 20 (Miller and Vogt, '84a).
of the ventral orbital area (Zilles, '851, designated also area
Primary motor area (Ml) was defined cytoarchitectonically by the appearance of the broad layer V, containing 11 (Caviness, '75; Miller and Vogt, '84a). A myelin dense
large, dark, Nissl-stained cells. This area corresponds to region that appeared consistently in the location of this
area AG1 of Donoghue and Wise ('82). More medially, area area served to mark its border (see Fig. 2A,C).
Determination of the laminar position of a tangential
AGm was easily differentiated from the neighboring areas
anterior cingulate (AC) and M1 by its pale band of layer I11 section was based on its relative distance from layer IV,
(Donoghue and Wise, '82). Posterior cingulate cortex (area which was conspicuously marked by the "barrels" of the
29) was subdivided after Vogt and Peters ('Sl), the lateral somatosensory cortex (Welker, '76) and high myelination
border of area 29d was correlated with a sharp drop of within area 17 (Olavarria and Van Sluyters, '85). These
myelin and AchE staining (Fig. lC,D). Anterior and poste- typical patterns could be easily visualized in the sections
rior perirhinal areas (areas 13 and area 35) were identified prior to mounting (Fig. 2).
Fluorescent labeling of retrogradely filled cells was viewed
with an epifluorescence microscope (Zeiss Universal), with
filter setting of 0 2 - W for FG, 18-blue violet for DY. DY
was seen also at 02-UV, but its greenish color and the oval,
smooth shape of the labeled nuclei (Keizer et al., '83) were
clearly distinguishable from the orange-golden color and
starlike granular appearance of the FG labeled cytoplasm
and processes (Schmued and Fallon, '86) as can be seen in
Figure 7. Double-labeled cells were identified by their
golden FG haze, which disappeared under blue violet
excitation, surrounding the DY labeled nucleus, which was
visible at both wavelengths. Drawings were prepared from
coronal sections stained for AchE, Nissl substance, HRPlabeled callosal connections, and myeloarchitecture including vascular landmarks, which served to align the drawings
of the patterns of fluorescent-labeled neurons.
To map the distribution of fluorescent-labeled cells,
sections were scanned with a computer controlled system.
The system consisted of optical rulers attached to the
microscope stage, which recorded its x-y coordinates at 1
km resolution. In addition, a pointer was viewed in the
microscope field with the aid of a drawing tube facing a
computer screen. By moving the pointer and the microscope
stage, the operator could scan the entire section at high
power and record the location of fluorescent-labeled cells, as
well as vascular and other landmarks, into the computer
memory rapidly and accurately. The stored coordinates
were translated into a color coded map of the location of the
cells in the section. The distribution of fluorescent labeling
was subsequently aligned with drawings of cytoarchitectonic and callosal patterns by means of the vascular landmarks.
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
Fig. 1. Histology and pattern of labelingin adjacent coronal sections
taken from case SV-6. This case is fully presented in Figure 12. A.
Schematic diagram showing the distribution of fluorescent labeled cells.
Small dots indicate labeling following visual cortex injection. Two large
dots indicate labeling following somatosensory cortex injection. Visual
cortex injection site is shown by diagonal lines. Parcellation of the
cortical areas was done as described in the text, according to sections
shown in B, C, and D, which were aligned using major blood vessels
(e.g., arrows within sections) and other prominent features. Small
RESULTS
Classification of injection sites
435
arrows outlining the section indicate borders of the cortical areas.
Medial is to the left. B. Brightfield photograph of a Nissl-stained
section. Note the dense band of cells of layer IVin areaV1. Arrows point
to the same areal boundaries as in A. C. Brightfield photograph of a
myelin-stained section. The myelin dense regions in area 29, area V1,
area 18a, and area A1 demarcate the borders of these areas. D. AchE
stain of a section in a brightfield photograph. Note the conspicuous
AchE stained dark regions in areas 29 and V1. Scale bar = 1mm.
see Table 1).Figure 3 depicts the injection sites for each
group, as well as the injection sites of cases with only a
single injection (Fig. 3C).
The rats used in this study were divided into three
We have based the conclusions of this study on the
groups, according to the sensory modalities represented in analysis of cases in which each tracer deposit was Eestricted
the areas injected: auditory-visual(Aud-Vis),somatosensory- to a single unimodal region (both primary and secondary
visual (Som-Vis), and somatosensory-auditory (Som-Aud; sensory areas), verified by analysis of the pattern of tha-
T. PAPERNA AND R. MALACH
436
Figure 2
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
437
We analyzed both coronally and tangentially sectioned
brains of the Aud-Vis experimental cases. A representative
coronally sectioned case is depicted in Figure 4. However,
the overall pattern of labeling is more clearly seen in the
tangentially sectioned cases, as shown in figures 5 and 6.
Both auditory and visual cortices appear to be extensively
connected with a series of areas arranged in a ringlike
manner around the sensory cortex. The auditory cortex is
connected to area 11, posteromedial area M1, anterior and
medial area 18b, posterior area 13, area 35, and areas 48/49.
Injections in visual cortex resulted in labeling within area
11,posterior medial area AGm, posterior areas S1 and S2,
area 22, posterior area 13, area 35, and areas 48/49. In area
29 a band of labeled cells was present, following visual
cortex injections, near the lateral border. In coronal sections this band was localized to the lateral subdivision, area
29d. A few labeled cells were also found in the anterior
cingulate cortex; however, this labeling was difficult to
detect in tangential sections because of the flattening
procedure.
Additional labeling could be discerned in more central
portions of the cortex, in areas located roughly between the
different sensory modalities. Cells labeled by the auditory
injection were present at an area located between the
somatosensory and visual representations (e.g., Fig. 6B,C),
and cells connected to the visual cortex could be seen in area
22 bordering on area S2 (which may be considered as an
auditory-somatosensory transition zone).
In almost every injection involving the visual cortex, a
few labeled cells were found in the auditory cortex and vice
versa (see Figs. 5,6). The belt of secondary sensory areas
surrounding the primary areas usually contained a larger
number of cells sending projections to the other sensory
modality. Area 36, which has been ascribed both visual and
auditory functions (Cipolloniand Peters, '79; Vaughan, '83;
Miller and Vogt, '84a), was accordingly labeled by both
injections (see Fig. 6).
We did not conduct a thorough laminar analysis of the
labeling pattern; however, our impression was that in most
of the outer ring areas, labeled cells were located in layers
11-111 and V. No clear laminar, modality-specific segregation was observed in areas that appeared to project to both
sensory modalities (Fig. 4). Since the pattern of labeling
appeared to vary mostly along the tangential plane, we
emphasized in this study the analysis of the flattened cortex
preparation. Comparison of the different sections from a
tangentially sectioned brain (Fig. 5) showed that the pattern of labeling throughout the sections was fairly consistent, although its intensity might have varied. Thus both
posterior medial area AGm and area 29 labeling appeared to
be more extensive in deeper layers than superficial ones (cf.
Fig. 5 section #5 and #13).
A closer examination of the arrangement of labeled cells
in the peripheral ring areas revealed two types of connectivity patterns. In the first type neurons projecting to the
auditory areas appeared to be intermingled with neurons
projecting to visual areas. Areas manifesting such a neuronal arrangement included area 11 at the frontal pole, area
13, and area 35. The second type was a modality-specific
pattern in which neurons projecting to two sensory modalities occupied separate but neighboring territories. In this
type of pattern, a minor level of intermingling of neurons
projecting to different targets was often evident. Areas of
this segregated type included posterior medial area AGm,
posterior medial area M1, medial area 18b, and area 29.
Figures 7-11 show detailed mapping of selected areas to
illustrate the patterns observed. Below is a more detailed
description of labeling patterns in individual areas.
Area 11. This area was often difficult to preserve in the
flattening process, but in both coronally sectioned material
and in successfully flattened hemispheres a labeled cluster
of cells was present (Figs. 4-6). Within this cluster, neurons
projecting to the visual areas were clearly intermingled
with neurons projecting to the auditory cortex. This pattern
is shown in detail in Figure 7.
Areas 13 and 35. In the lateral zone encompassing areas
13 and 35, neurons projecting to the auditory and visual
cortex were intermingled (Figs. 5,6). However, within area
35, as can be seen in Figure 6 and in the more detailed
Figure 8, the intermingling was more pronounced in the
posterior part, whereas the anterior part was mostly dominated by neurons projecting to the auditory cortex.
Areas 48 and 49. The postsubicular and parasubicular
cortices are situated at the most posterior aspect of flattened cortex. The neurons labeled by the different tracer
injections were found to be partly intermingled and partly
Fig. 2. Parceliation of the cortical areas in the flattened cortex
preparation. A. Reverse contrast photograph of an unstained tangential
section, which shows the myelin-dense regions corresponding to the
primary sensory cortices, as well as area 52, and the lateral border of
area 29 (arrows). Major blood vessels are underlined with small bars. B.
The pattern of callosal connections, as viewed in a darkfield photograph
taken from an HRP-stained section adjacent to the section shown in A.
Small bars mark the same blood vessels as shown in A. C. The areal
subdivisions in the tangential section, based on the patterns shown in A
and B. The thick lines delineate borders of areas that were defined
mainly by myeloarchitectonic criteria. Thin lines demarcate borders of
areas determined using the callosal pattern (marked by shaded areas)
and other criteria (see methods). Dashed lines indicate location of
borders whose position was less certain. Scale bar = 1mm.
TABLE 1. Location of Injection Sites in Experimental Cases
Number of rats injected
Plane of Section
Aud-Vis
Tangential
Coronal
Total
11
3
14
Som-Vis
5
3
8
Sorn-Aud
Aud
Son1
Vis
Add'
4
2
2
2
4
4
2
2
2
4
'Additional cases with injections in areas estimated to be connected with the sensory
cortices.
lamic labeling. Thus labeling restricted to the lateral geniculate nucleus and the lateral posterior nucleus indicated an
uptake zone in the visual cortex (Nauta and Bucher, '54;
Montero and Guillery, '68; Takahashi, '85), medial geniculate nucleus labeled cells were indicative of an injection in
the auditory cortex (Krieg, '46a; Herkenham, '80; Vaughan,
'83; Winer and Lame, '87), and labeling in the ventral
posterior lateral nucleus and the posterior nuclei group
indicated an uptake zone within the somatosensory cortex
(Wise, '75; Wise and Jones, '77; Akers and Killackey, '78).
In order to verify the reciprocity of connections to
sensory cortices, four additional cases received tracer injections in areas identified as projection targets of sensory
cortices. In the following sections we present our results
while considering each group of cases separately.
Auditory-visual cases
438
T.PAPERNA AND R. MALACH
R
L
Fig. 3. A schematic representation of all the injection sites in the
experimental cases. A. Auditory-visual cases (prefix AV). The location
of FG injections is marked by circles, injections of DY are marked by
triangles. Full symbols represent the cases shown in this manuscript.
B. Somatosensory-visual cases (prefix SV). Symbols are the same as in
A. C. Somatosensory-auditory cases (prefix SA), represented by circles
and triangles as in A. This figure also includes cases with single
injections in sensory areas (squares), as well as cases with injections in
the medial aspect of the cortex (stars, prefix Add).
segregated into separate territories. This is shown in detail
in Figure 9. As can be seen, in the anterior part of this
region the neurons projecting to the two different sensory
areas were confined within separate territories. However,
more posteriorly there was a clear overlap zone containing
intermingled populations of cells.
Area 29. Figure 10 shows the labeling of auditory and
visually projecting cells in area 29 and the adjacent area
18b. As can be seen, the two populations of neurons were
segregated in neighboring territories, with the auditory
projecting neurons localized at the medial border of area
18b. Note, however, that even within this segregated
pattern of labeling, the separation was not absolute.
Posterior medial AGm. Strong labeling was present in
this area followingtracer deposits in the visual cortex. This
group of labeled neurons seemed to be basically unimodal in
nature as only scant connections were maintained here
with auditory cortex. This can be seen clearly in Figure 11,
which shows the typical elongated cluster of cells filled by
retrograde transport from the visual areas, together with
the corresponding computer mapping, which points out the
few auditory connected cells found in this territory. Inspection of all Aud-Vis cases (see Figs. 5,6) suggested some
tendency for clustering of the auditory projecting cells in a
bandlike arrangement, but their density was too low to
reveal a clear-cut segregation.
An obvious question is whether the “segregated” pattern
was spuriously produced by the particular positioning of
the tracer injections within the respective sensory maps. To
investigate such a possibility, the location of the injection
sites within the two sensory representations was varied in
the different cases. In all cases, the segregation or intermingling of the labeled neurons in the peripheral areas remained consistent (Fig. 6).
Finally, a result that was constant across all cases was
the striking paucity of double-labeled cells, even in areas
where neurons labeled by the two kinds of tracers were
thoroughly intermingled.
In summary, the results from the Aud-Vis experimental
group revealed that the auditory and visual cortices were
connected with several common areas, most of them located
within a peripheral cortical ring. Two patterns of labeling
could be observed within these areas. In the medially
located areas, neurons projecting to different sensory modalities were segregated into separate territories, and in the
anterior and lateral part of the cortex, these neurons were
found to be intermingled.
Somatosensory-visual cases
The pattern of connections revealed by injections into the
somatosensory cortex closely resembled the pattern previously described by Akers and Killackey (’78). The pattern of
connections as revealed in a coronally sectioned brain is
illustrated in Figure 12. Tangential cortical sections from
representative cases are shown in Figures 13 and 14. In
case SV-4 (Fig. 13)a minor spread of tracer deposit from the
visual cortex into area S2 occurred, as was evident by the
thalamic labeling; however, the resulting cortical pattern of
label was in agreement with that seen in all the other cases.
As can be seen, somatosensory cortex received projections
principally from two directions: from areas in the frontal
cortex, namely, area M1 (Donoghue and Parham, ’83),
anterolateral area AGm, and area 11, and from areas
directly lateral to area S2, namely, area 13 and a zone
located at the border of area 13 with area S2. In addition, a
few projections originated posteriorly t o the injection site,
within area 22, from an intermediate area between auditory
and somatosensory cortices.
The connections of the visual cortex, as already seen in
the Aud-Vis results, were mainly to areas located within the
peripheral ring, namely (going clockwise),posterior medial
area AGm, area 11, area 13, area 35, areas 48/49, and area
439
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
AGm
AG m
s2
13
Fig. 4. Distribution of labeled cells in a case injected in auditory and
visual cortices (AV-31, sectioned in the coronal plane. In the center, a
schematic, dorsolateral view of the cortex shows the location of auditory
and visual injection sites and the corresponding levels for each of the
coronal sections. Injection sites (section #1641 are filled for the
auditory site and contain diagonal lines for the visual site. The diffusion
zone is delineated by the empty contour around the injection sites.
Large dots represent cells labeled by the auditory injection and small
dots represent labeling from visual injections. Here, and in the following figures unless otherwise stated, each dot corresponds to 2-4 labeled
cells. Small arrows near the boundaries of the section mark the borders
of the cortical areas. Thin line within the sections marks the border
between layers IV and V. Scale bar = 1 mm.
T.PAPERNA AND R. MALACH
440
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Figure 5
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
441
29. In coronal sections labeling appeared also in the anteSomatosensory-auditory cases
rior cingulate cortex.
In this group of experiments, only flat cortical preparaA few direct connections between the visual cortex and
the somatosensory cortex were found in our cases; these tions were studied. Figure 15 shows four sections taken
were mostly restricted to the posterior lateral parts of area from case SA-2 to illustrate the tangential distribution of
the connections throughout the depth of the cortex; Figure
S1 (vibrissae and eyebrow fields; Welker, ’76; Olavarria et
al., ’84), posterior area S2, and the anterior parts of 16 shows representative sections from three other SomAud cases. These cases illustrate the typical connections of
secondary visual cortex.
The laminar pattern of labeled cells shows, as before, somatosensory and auditory cortices as shown previously.
labeling mainly in layers 11-111and V (Fig. 12). However, Thus the auditory cortex was connected with the peripherthe somatosensory projections from area 13 seemed to be ally located areas (going clockwise) medial area 18b, posteromedial M1, area 11, posterior area 13, area 35, and areas
restricted to the deeper layers (see Fig. 12, section #127).
48/49.
The somatosensory cortex was connected with area
The pattern of labeling in the Som-Vis cases showed a
tendency for neurons projecting to the somatosensory M1, antero-lateral area AGm, few cells in area 11,and area
cortex to be segregated from the neurons projecting to the 13.
The overall impression is that areas projecting to the
visual cortex. This trend was evident in all our Som-Vis
cases independent of the exact site of tracer injection in somatosensory and auditory cortices are segregated from
either sensory cortices (Figs. 12, 13, 14). It is notable that each other (Figs. 15, 16). Thus the labeled neurons projectcells labeled by injections in the somatosensory cortex were ing to the somatosensory cortex in anterior area AGm and
basically absent from posterior cortical areas such as area in area 11 were clearly segregated from the patch of
29 and areas 48/49, which contained many visual projecting neurons in area 11 projecting to the auditory cortex.
neurons. There were usually no double-labeled neurons in Similarly, the neurons projecting to the auditory and
this group of cases. Below is an area by area description of somatosensory cortices in area M1 were also segregated
from each other. In fact, the entire medial part of the
the labeling patterns.
Area AGm. The visual projecting zone in the posterior cortex, namely, medial area 18b, posterior area M1, and
medial area AGm appeared distinct from its neighboring posterior medial area AGm, was labeled exclusively by
region in area M1, which projects to the somatosensory transport from the auditory cortex. Again, lateral area 13
cortex. Only at the border between these regions could a stands out in being the only area containing a clear
small overlap zone be discerned in which neurons project- intermingled pattern of labeling by the two tracers. Note
ing to the somatosensory and visual cortices were intermin- that in all the Som-Aud cases studied, no double-labeled
gled (Figs. 12-14). It is not clear whether this overlap may cells were present.
To summarize, in the Som-Aud cases, similar to the
be caused by developmental “noise” or may reflect a
genuine mechanism for cross-modal convergence. At the Som-Vis cases, the pattern of labeled neurons points to
anterolateral aspect of AGm, only neurons projecting to the separation between the regions projecting to the somatosensory and auditory areas, with the exception of lateral area
somatosensory cortex were found.
Area 11. The projections to the somatosensory and 13.
visual cortices appeared to originate from separate zones
within area 11. The neurons projecting to the somatosenAdditional cases
sory areas were located in a distinct band, posterior to the
To verify the reciprocal nature of some of the connections
visual projecting zone (see Fig. 13B-C).
Area 13. This area was found to contain two separate studied here, we conducted an additional four experiments.
groups of labeled cells: At the border between area 13 and In these cases, tracers were injected at sites along the
area S2, a large cluster of cells was labeled mainly by medial cortex estimated to be target areas of the projections
injections in the somatosensory cortex, and more laterally, from the sensory areas. Two representative cases are
along the rhinal fissure, an elongated strip of labeled depicted in Figure 17, which includes injections placed at
neurons contained both visual and somatosensory project- area 29 and medial area 18b and injections placed at the
ing cells that were intermingled (Figs. 13,and 14).This was frontal cortex, posterior medial area AGm, and area M1.
the only area in the Som-Vis cases where a complete overlap
Case Add-11 (Fig. 17A) was injected in medial area 18b
of the zones projecting to the two sensory modalities was and in area 29. The pattern of labeling demonstrates that
revealed.
the afferents to these areas originated mostly from secondIn summary, the results described above present a pic- ary sensory areas. Thus following the injection in medial
ture of clear segregation of the areas connected to the visual area 18b, most of the labeled cells in the auditory areas were
cortex and the areas connected to the somatosensory cortex found in the secondary areas 36 and 22. Similarly, the
in the cerebral cortex of the rat, with the exception of injection in area 29 produced only a few labeled neurons in
narrow borderline zones and a lateral strip in area 13.
the primary visual cortex, whereas many labeled neurons
Fig. 5 . Tangential distribution of labeled cells followinginjections in
auditory and visual cortices (caseAV-8). The drawings are from a series
of tangential sectionsof the flattened cortex. The distribution of cells is
shown relative to the map of the cortical areas (in B the area names are
specifically given). Injection sites in this figure, and in all the following
figures showing tangential sections, are indicated by a contour filled
with dots, which represents the actual injection sihe, and by a surrounding empty contour, which represents the fluorescent halo around it. Dot
symbols as in Figure 4. Stars represent double labeled cells. A. Section
#5, estimated at layer 11-111 (seeMethods for criteria of layer identification). B. Section #9, estimated at layer 11-111. C. Section #13, estimated
at layers IV-V. Rectangles outline areas enlarged in Figure 10 (rectangle 1-2), Figure 11 (rec. 3), Figure 7 (rec, 4), and Figure 8 (rec. 5). D.
Section #17, estimated at layer V. Scale bars = 1 mm. Note that the
labeled cells are arranged in a ring of peripheral areas.
442
T. PAPERNA AND R. MALACH
A
-v
AV-14
Fig. 6. Distribution of labeled cells in representative sections taken
from four auditory-visual cases. Symbols are the same as in Figures 4
and 5. The sections in all cases were taken from middle layers IV and V.
Arrows point to fields of labeling which are of particular interest (see
text). A. Case AV-10. Area outlined by rectangle is shown in detail in
Figure 9. B. Case AV-14. C. Case AV-11. D. Case AV-4. Scale bars = 1
mm each. Note that the general pattern shown in Figure 5 remains
consistent across different cases and following displacements of the
injection sites.
443
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
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Fig. 7. Labeled cells in area 11followingan injection in auditoryand
visual cortices. Details from section #13 case AV-8 (Fig. 5C). Inset
shows the cortical location of this enlargement (filled rectangle) in
relation to the primary sensory areas. A. Detailed distribution of cells in
area 11, corresponding to rectangle #4 in Figure 5C. The outlined
rectangle in this figure delineates the area shown in B, C , and D.
Symbols are the same as in Figure 4, except that here each dot
represents 1-2 cells. Scale bar = 250 pm. B. Drawing of the cells in
rectangle of A, according to the microphotographs shown in C and D.
FG cells are depicted by the dot textured cells, the contour of DY cells is
drawn with a thick line. The blood vessels that appear in C and D are
delineated by the dashed line. Scale bar = 20 pm for B-D. C . High
power microphotograph of cells in the outlined rectangle of A, taken
under W illumination. Note the two types of labeled cells: FG cells (full
arrows), corresponding to the dotted cells in B, were more grainy in
appearance and labeled processes could be seen emerging out of the
cells. Under W illumination these cells had golden-yellowfluorescence.
DY cells (unfilled arrows) corresponding to unfilled cells in B, were
more smooth and ellipsoid in shape without any processes. Under W
illumination these cells had green fluorescence. D. Same picture as in C,
under blue-violet illumination. The FG labeled cells could hardly be
seen under this light, whereas the DY labeled cells were very conspicuous. Note the intermingled arrangement of the cells projecting to the
visual and auditory cortices.
were found in secondary areas 18a and anterior 18b.
Interestingly, an intense projection to area 29 originated
from anterior area 18b, at a region corresponding to Krieg's
('46a) area 7.
In addition, many areas within the peripheral ring contained labeled cells following these injections. Thus both of
the injections produced labeling in posterior medial area
AGm, area 11,area 35, and areas 48149.
Figure 17B shows the pattern of labeled cells following
injections in area M1 and posterior medial area AGm. Here
again, projections to the medial aspect of the cortex originated mostly from secondary sensory areas. Thus the
injection in area AGm labeled cells in lateral area 18b and in
auditory area 22, with a few labeled cells in area Al. A
cluster of labeled cells was also observed in area S1. It is not
clear whether these labeled cells are due to true connections
of posterior medial area AGm with posterior S1 (e.g., Reep,
'84; Sesack et al., '891, or to spread of label from the
injection site into area M1. Labeled cells were present also
in the peripheral ring areas: medial area 18b, area 29, and a
few cells in area 13.
The injection in area M1 (Fig. 17b) labeled several
locations mostly in the anterior half of the hemisphere: area
S1 and area S2, area 11and area 13.
444
Fig. 8. Perirhinal cortex (area 35) following an injection in auditory
and visual cortices. A detailed map from section #13 of case AV-8
(corresponding to rectangle 5 in Fig. 5C). Only cells in the perirhinal
cortex were charted in this drawing. As can be seen, the cells projecting
to the auditory and visual modalities were intermingled. Symbols are
the same as in Figure 7; scale bar = 250 pm.
To summarize, this set of experiments demonstrates that
most sensory projections to the peripheral areas 29, medial
area 18b, and posterior medial area AGm originate from
secondary auditory and visual cortices and indeed appear to
reciprocate the feedback projections from these areas t o the
sensory areas. In addition, these medially located areas
appear to be connected between themselves, as well as with
other areas located within the peripheral ring.
T. PAF’ERNA AND R. MALACH
modality representations, ignoring more detailed analysis,
e.g., the connections between primary and secondary cortices or between different parts of the sensory maps.
Another point to be considered is the possibility of “false
negatives” of double-labeledneurons; in other words, when
the labeling intensity of one tracer is far stronger than the
other, the weak tracer might fail to be detected. Since FG
and DY label separate cell compartments and have distinguishable excitation and emission spectra (Keizer et al., ’83;
Schmued and Fallon, ’861, the frequency of such misses is
likely to be low.
The main emphasis of the present study was on the
tangential distribution of connections rather than detailed
laminar analysis. This was facilitated by the method of
cortical flattening. However, in this approach, angular
parts of the cortex undergo a certain distortion, which may
lead to misinterpretation of the location of the labeled cells.
Therefore results from tangentially sectioned cortices were
complemented with data from coronal sectioned cases.
Finally, it should be emphasized that since the fluorescent tracers employed were transported only retrogradely,
most of our data concerns the organization of feedback
pathways to sensory cortex rather than the forward projection pattern. However, on the basis of our own data (e.g.,
see Fig. 17) and numerous other neuroanatomical studies,
it can be safely assumed that the vast majority of cortical
pathways are reciprocal in nature. Still, the possibility
remains that on a finer scale, differences might exist
between the patterns of forward and feedback connections.
Connections of sensory cortices in the rat,
comparison to previous single tracer studies
Our results confirm and extend the data presented by
previous studies that employed the use of a single tracer.
Figure 18 brings together in a schematic form the major
results
of the present study. As can be seen, the overall
Double-labeled neurons
layout of the sensory connections is from a centrally located
The vast majority of labeled neurons contained a single sensory surface onto a peripherally located ring of areas.
tracer. However, in several cases we did identify with Most of these areas fall into the category of association
certainty the presence of a few double-labeledneurons. The areas (e.g., Pandya and Seltzer, ’82; Miller and Vogt, ’84a).
majority of these doubly labeled neurons were in cases This layout appears to fit within the general scheme of
belonging to the Aud-Vis cases (Figs. 5 , 6, IOA, 11).They sequential flow of sensory information in the cortex sugwere found primarily in the ring of peripheral areas includ- gested by studies in primates (Pandya and Seltzer, ’82;
ing area 35, areas 49/49, area 29, and posterior medial area Pandya and Yeterian, ’85): from primary sensory areas, in
AGm, with equal frequency in areas exhibiting “segregated” successive steps to association, and then limbic cortices. In
or “intermingled” patterns of labeling.
the following section we discuss the projections to the
sensory cortices from each of the association areas.
Area 29. In agreement with Vogt and Miller (’83)and
DISCUSSION
Torrealba et al. (’84), our results show strong connections
Methodological considerations
between the cingulate cortex (area 29d) and the visual
Before discussing the results, it is important to note cortex.
Area 186 (medial). Area 18b contained a longitudinal
several methodological points. A major concern in this
study was the determination of the effective uptake zone of band of neurons projecting to the auditory cortex along the
the tracers injected. In studies of the rat cortex, this issue is medial border with area 29d. Miller and Vogt (’84a) departicularly acute because of the smallness of the cortical scribed similar connections of this area with the auditory
mantle. Indeed, some of our injections fell close to the cortex. Our results extend their finding to include the entire
border between two different sensory areas and the effec- auditory representation.
Area AGm. The extensive connections of the posterior
tive zone of transport may have included both sensory
modality representations. We have tried to reduce this medial part of area AGm with the visual cortex have been
problem by varying the location of the injection sites within noted by a number of investigators (Vogt and Miller, ’83;
each sensory area and by using the thalamic labeling as an Miller and Vogt, ’84a; Reep et al., ’84; Torrealba et al., ’84).
additional indicator in the definition of the effective uptake These connections support the suggestion that this area
zone. Furthermore, our approach was to address only may be the rat homologue of the primate frontal eye fields
global issues regarding the connectivity of entire sensory (Leonard, ’69; Hall and Lindholm, ’74; Crowne and Path-
445
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
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Fig. 9. Distribution of labeled cells in areas 48/49 after injections in
visual and auditory cortices. A. Photomicrograph taken from section
#I3 of case AV-10 (correspondingto the square in Fig. 6A), taken under
W illumination, which shows the FG labeled cells, as well as the DY
labeled cells (faint labeling at the bottom half). Arrows point to major
blood vessels; scale bar = 100 pm. B. Same area as in A, and at the same
scale. Blue-violet illumination shows the DY labeled cells which appear
brighter than in A. C. Schematic diagram showing distribution of
labeled cells in the same area shown in A and B. Symbols are the same
as in Figure 7. Arrows point to the blood vessels shown in A and B. Scale
bar = 100 pm. Note that the cells projecting to auditory and visual
areas are intermingled in the posterior portion of the area shown here.
ria, '82; Kolb et al., '84; Neafsey et al., '86; Leichnetz et al.,
'83; Miller and Vogt, '84a), we have been unable to find any
previous reference to the connection of area 11 with
auditory cortex in the rat. In addition, our study indicates
that somatosensory cortex is connected with area 11. It
should be stressed, however, that the neurons projecting to
the somatosensory cortex never appeared to intermingle
with neurons projecting to the visual or auditory cortices
(Figs. 14,18).
Areas 13 and 35. Within the borders of area 13, two
groups of labeled cells were noticeable. The large cluster of
neurons mainly connected with the somatosensory cortex,
which consistently appeared on the myeloarchitectonically
defined border between area S2 and area 13 (Fig. 18),may
correspond in location to area 14 of Krieg ('46a) and
Caviness ('75). In a similar location, other researchers have
described terminations of fibers from the primary and
secondary somatosensory cortex (Akers and Killackey, '78;
Guldin and Markowitsch, '83), and from area 18a (Miller
'87).
Connections with the auditory and somatosensory cortices have been mentioned by Reep et al. ('84; see also Sesack
et al., '89). However, in our study only weak connections
were demonstrated with the auditory cortex, whereas somatosensory cortex injections failed to label this part of area
AGm. It should be noted, however, that most of our
injections were probably not within the projection field of
the posterior medial area AGm, namely, the posterior parts
of the somatosensory cortex (see Reep et al., '84). In
contrast, our results show in several cases some labeling in
the anterolateral part of AGm, following injections in
somatosensory areas (Figs. 12-16).
Area 11. This area was found to contain neurons projecting to the visual cortex as well as neurons projecting to the
auditory cortex. Although the connections with the visual
fields have been previously documented (Vogt and Miller,
446
T. PAPERNA AND R. MALACH
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Fig. 10. Detailed maps of labeled cells' distribution in area 29
following an injection in auditory and visual cortices (case AV-8, Fig.
5C). A. Anterior area 29 (rectangle 2). B. Posterior area 29 (rectangle
1).Symbols are the same as in Figure 7. Scale bars = 200 Fm. As can be
seen in these diagrams, the two types of labeled cells are essentially
segregated, though some small scale mixing is present, especially in
medial area lab.
and Vogt, '84a). We have found few connections also with
the auditory cortex (Figs. 5,6D).
The lateral aspect of area 13 featured an elongated
cluster of neurons labeled from the somatosensory and
auditory cortices, and occasionally also the visual cortex. A
similar shaped cluster was present within area 35. As can be
seen in Figure 18, a rough topography can be discerned in
these areas, paralleling the layout of the sensory cortices
(see also Deacon et al., '83). Thus somatosensory connected
cells were located more anteriorly, with auditory and visual
projecting fields occupying more posterior locations, within
area 35. Because of the gradual nature of the shift in labeled
fields as the injection site was displaced, there were large
overlap zones connected to different sensory unimodal
areas.
Areas 48 and 49. The connections of the postsubicular
and parasubicular regions (areas 48/49) with the visual
system were previously reported by Vogt and Miller ('83).
This study describes for the first time a neuronal path from
this region to the auditory cortex, thus suggesting an
additional indirect link between the auditory system and
the hippocampal formation. The neurons projecting to
these two sensory areas were intermingled in some parts of
areas 48/49 but segregated in others. It was difficult to
determine in our material whether the visually connected,
anteriorly located band in this area was part of the postsubiculum or the posterior pole of area 18a (see also Olavarria
and Montero, '84; Malach, '89).
Centrally located areas. In addition to the peripheral
association areas, labeled neurons were also found in more
centrally located regions, which due to their position between the sensory cortices may be considered as transition
zones.
At the anterior part of area 18b, we found cells that
project to the auditory cortex (Figs. 6, 10A). This site
appears to correspond to area 7 according to the parcellation of the rat's cortex (Krieg, '46a). This region has been
ascribed an associative role (Miller and Vogt, '84a; Torrealba et al., '84); however, it has previously been associated
with visuosomatic responses rather than with visuoauditory or somatoauditory responses (Wagor et al., '80; PintoHamuy et al., '87). It should be noted that in some cases
this strip of auditory projecting cells appeared to extend
anteriorly into the area previously defined as part of the
primary motor and somatosensory cortices (Donoghue and
Wise, '82; Neafsey et al., '86).
Another transitional zone may be present between the
somatosensory and auditory cortices. Area 22 is situated
within this region (Caviness, '75; Olavarria and Van
Sluyters, '85) and its anterolateral aspect probably corresponds in part to areas 39 and 40 of Miller and Vogt ('84a).
We have shown that anterior area 22 is connected with
visual and somatosensory modalities (Fig. 18);yet it seems
that its primary role is as a secondary auditory area in view
of its connections with area A1 and its physiological properties (Krieg, '46b; Sally and Kelly, '88).
447
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
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Fig. 11. Cell distribution in posterior medial area AGm following an
injection in auditory and visual cortices (case AV-8). The area shown in
this figure corresponds to rectangle 3 in Figure 5C. A. Microphotograph
taken under UV illumination, of the elongated cluster of cells labeled by
a FG injection into visual cortex. Arrows point to major bloodvessels. B.
Corresponding computer mapping of the area shown in A. Arrows point
to same blood vessels as in A. Scale bars = 100 km. Symbols are as in
Figure 7. It is evident that the vast majority of cells in this area were
labeled by visual cortex injection (small dots). Few cells were labeled by
injections in the auditory cortex (large dots).
Connections between the sensory cortices
In trying to group the areas containing segregated populations of unimodal projectingneurons versus those containing intermingled neurons, it becomes evident that the
former areas are more prevalent on the medial aspect of the
hemisphere, whereas the latter are found at the anterior
and lateral aspects. The significance of this organization is
not clear at present, but it is interesting to note that a
similar subdivision of areas was proposed on evolutionary
and embryological grounds (Sanides, ’69; Pandya and Yeterian, ’85).
Even within the so-called unimodal territories, there are
often a few neurons projecting to other sensory modalities
(for example, see area AGm in Figs. 6, l l ) , which leaves
open the possibility that some intermodality analysis does
occur. Particularly interesting is the issue of the border
zones between adjacent unimodal areas connected to different sensory modalities (e.g., see Figs. 6,14, 16) where an
intermingled pattern of labeling is more prevalent. Does
such organization present a functional principle by which
cross-modality integration is implemented, or is it merely
the result of developmental errors? A more detailed analysis of the response properties of neurons at such transition
zones will be necessary to answer this question.
It should be remembered that even within the intermingled group of cells, some micro-scale segregation might still
be present. In this respect it is important to note the
striking absence of double-labeled neurons in our results,
namely, neurons projecting simultaneously to two different
sensory modalities. It could be assumed that an area that
integrates polysensory information would have a greater
likelihood to send bifurcating axons as feedback to different
Our results (Figs. 6, 14, 161, as well as those of previous
studies, indicate that in the rat, unlike cats and primates,
direct connections also exist between the sensory cortices
themselves (Miller and Vogt, ’84a; Olavarria and Montero,
’84; Torrealba et al., ’84). The region in area S1, which is
connected to the visual cortex, appears to correspond to the
somatotopic eye and head representation (Welker ’76).
Miller and Vogt (’84a) suggested that this area may have
some role in the coordination of eye movements and the
visual stimuli observed.
Patterns of interrelationship between
modality specific pathways
The novel aspect of our study was the use of a doublelabel, tract-tracing paradigm, which enabled us to approach
directly the question of the relation between different
modality specific pathways in the association areas. If one
assumes that adjacent neurons have a higher probability
for mutual interaction, then an intermingled arrangement
of neurons, projecting to different sensory modality representations, may indicate a higher level of “cross talk”
between different modalities. Our study suggests that areas
of this type may include frontal area 11, perirhinal areas 13
and 35, and postsubicular areas 48/49 (Fig. 18). Alternatively, association areas of the “segregated” type, which
include mostly one type of modality specific cells, can be
presumed to be involved more with unimodal sensory
processing. Areas with such a segregated arrangement of
projections include posterior medial area AGm and cingulate area 29d (Fig. 18).
448
T. PAPERNA AND R. MALACH
AGm
2
Fig. 12. Distribution of labeled cells in a series of coronal sections
following an injection in somatosensory and visual cortices (case SV-6).
The position of injection sites is shown in a schematic dorsolateral view
of the cortex in the central figure. Somatosensory injection site (section
#log) is shown by the filled area, the surrounding unfilled area
delineated by a contour shows the diffusion zone. Large dots correspond
to cells labeled by this injection site. Other symbols are the same as in
Figures 4 and 5. Scale bar = 1 mm. Note that cells labeled by the
injections in somatosensory and visual cortices were located in separate
territories.
sensory areas. Yet our findings suggest that at the feedback
level, modality specificity is conserved. Alternatively, the
possibility still remains that such bifurcating axons do exist
and we have consistently failed t o inject their two target
fields simultaneously.
It is interesting to note that the auditory and visual areas
display a stronger resemblance to each other, in terms of
their pattern of connections to the association areas, than
to the somatosensory cortex. As can be seen in Figure 18,
area 11, area 35, and areas 48/49 project to both auditory
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
A
449
A
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Fig. 13. Tangential distribution of labeled cells in a series of
tangential sections taken from a case injected in somatosensory and
visual cortices (SV-4). Arrows point to labeled fields of particular
interest. Symbols as in Figure 12. A. Section #9, layers 111-IV. B.
Section #13, layers IV-V. C. Section #15, layer V. D. Section #19,
layers V-VI.Scale bars = 1mm each. It is noteworthy that apart from
lateral area 13,the cell clusters labeled by the somatosensoryand visual
cortex injectionswere segregated.
T. PAPERNA AND R. MALACH
450
A
B
sv-2
sv- 1
v
Fig. 14. Distribution of labeled cells in tangential sections of four
different cases injected in somatosensory and visual cortices. Sections
were taken from layers IV-V. Symbols are as in Figure 13.A. Case SV-3.
B. Case SV-2. C. Case SV-1. D. Case SV-5. Scale bars = 1 mm. Note
that the overall segregation of cells projecting to the somatosensory
cortex from cells projecting to the visual cortex is consistent in all these
cases in spite of the variability in the location of injection sites.
451
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
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Fig. 15. Distribution of labeled cells following injections in somatosensory and auditory cortices. These schematic diagrams show four
tangential sections, taken from the flattened cortex of case SA-2. Large
dots represent label from somatosensory injection site, small dots
represent label from auditory injection site. A. Section #5, estimated at
layers 11-111. B. Section #9, layers 11-111. C. Section #13, layer IV. D.
Section #17, layer V. Symbols are as in figure 13. Scale bar = 1mm.
Note the segregation of cells labeled by somatosensory cortex injection
from cells labeled by auditory cortex injections.
452
T. PAPERNA AND R. MALACH
A
Fig. 16. Pattern of labeling in three additional cases injected in
somatosensory and auditory cortices. Sections drawn are from layers
IV-V. A. Case SA-1. B. Case SA-3. C. Case SA-4. Symbols are as in
Figure 13.Scale bars = 1mm. Note that apart from lateral area 13,the
connections to auditory and somatosensory cortices remained segregated in different cases having different combinations of injection sites.
and visual areas, and the projection zones from the medial
cortex (area 29d, medial area 18b, posterior medial area
AGm, and posterior area M1) to these two regions are
adjacent. Projections to somatosensory cortex do not overlap any of these regions.
An interesting feature suggested by documented work of
other investigators, as well as by our data, is that the
association areas described in this study are strongly
interconnected (Meibach and Siegal, '77; Deacon et al., '83;
Vogt and Miller, '83; Miller and Vogt, '84a; Reep et al., '84).
453
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
Add- 12
Fig. 17. Pattern of labeling following injections in sensory recipient
areas A. A case (Add-11) injected in area 29 (large dots) and in medial
area 18b (small dots). Tangential section from layers IV-V. Note the
connections of both area 29 and medial area 18b with the secondary
auditory and visual areas, especially those of medial area 18b with area
Al, and the connections with the peripheral "association ring" areas,
Thus these areas may constitute a functionally related
family of association areas (Berson and Graybiel, '83; Olson
and Jeffers, '87) or a distributed network (Goldman-Rakic,
'88).
Finally, the possibility that the cross-modality integration may occur at the subcortical level should also be
considered in view of the wealth of cortical connections,
afferent and efferent, to subcortical structures. The cortical
association areas have been shown to be connected to the
limbic system (Meibach and Siegal, '77; Kosel et al., '82),
the thalamus (Domesick, '69; Leonard, '69, Guldin and
Markowitsch, '831, and to subcortical motor structures
(Domesick, '69). However, it was beyond the scope of this
study to investigate thoroughly this possibility.
Comparison to association areas in
cats and primates
It is obviously difficult to draw homologies between such
diverse species as the rat, cat, and monkey, but specifically
for that reason, any parallels that can be drawn carry added
significance in suggesting common mammalian principles
of organization.
Regarding the segregation of sensory pathways related to
area AGm, a suggestive parallel could be found in studies of
connections between the frontal cortex and the posterior
parietal cortex in monkeys (Cavada and Goldman-Rakic,
'89). Thus somatosensory linked area 7b and visually linked
area 7a were found to be connected with different subdivi-
r-
B. Injections in posterior medial area AGm
(small dots) and in area M1 (large dots) as seen in case Add-12. Section
is from layer IV. Scale bars = 1mm. Note the extensive connections of
posterior medial area AGm with area 29, and with the secondary
sensory areas of the visual and auditory cortices.
e.g. area 11 and area 35.
sions of the frontal cortex. A comparable phenomenon
might also exist in cat area 6 where a clear segregation of
somatosensory and visual connections was observed (Olson
and Jeffers, '87; Olson and Lawler, '87). Finally, a study of
the connections of prefrontal cortex of the cat suggests that
its various sectors maintain specific connectivity patterns
with different sensory cortices (Cavada and ReinosoSuarez, '85).
Our results suggest that orbitofrontal and perirhinal
areas should contain overlapping connections of different
modalities. Such overlapping connections were indeed described (Jones and Powell, '70; Pandya and Seltzer, '82;
Pandya and Yeterian, '85) among the peripheral areas in
the frontal and parahippocampal cortices in primates.
Interestingly, physiological studies in primates reveal auditory-visual and visual-gustatory interactions in the orbitofrontal cortex, which is a likely homologue of the rat area
11 (Benevento et al., '77; Thorpe et al., '83). A rough
rostrocaudal topography of parahippocampal connections
with the sensory areas, similar to the ones described here
for the perirhinal areas 13 and 35, was described by Room
and Groenewegen ('86) and Witter and Groenewegen ('86),
but because these studies were based on injections of single
tracers, it is difficult to assess the level of connectional
intermingling in these cases.
It is unclear whether the more centrally located transition zones in the rat cortex, e.g., area 7 of Miller and Vogt
(see also Krieg, '46a), could be considered analogues to the
454
T. PAPERNA AND R. MALACH
S0
A
VI
Fig. 18. Summary diagram of the pattern of connections of the
sensory cortices with the association areas as revealed by this study.
Three different rasters represent clusters of labeled cells, which project
to the three sensory cortices as shown left to the scheme. In regions
where only few cells were labeled, empty triangles, squares, or circles
are drawn, correspondingto visual (Vis), auditory (Aud),or somatosensory (Som) injection sites, respectively. Note the ringlike arrangement
of the areas containing the clusters of labeled cells. This diagram
illustrates the areas in which neurons projecting to different sensory
modalities are intermingled, and the areas where these neurons are
segregated according to their target sensory area. Note that the
tendency for intermingling is more prevalent on the anterior and lateral
sides of the hemisphere.
highly developed association areas described in the monkey
cortex, such as the subdivisions of area 7 (Pandya and
Seltzer, '82; Mesulam, '83; Goldman-Rakic, '88; Andersen,
'89; Cavada and Goldman-Rakic '89), or areas in the
superior temporal sulcus (Jones and Powell, '70; Benevento
et al., '77; Bruce et al., '81; Pandya and Seltzer, '82). It is
interesting to note that physiological studies suggest that
some of these transition areas might serve a polysensory
function (Benevento et al., '77; Bruce et al., '81; Duhamel et
al., '89).
cuitry revealed in the present study remains to be determined.
Conclusions
The main goal of this study was to explore directly the
connectivity patterns that might subserve cross-modality
integration in the cerebral cortex of the rat. We have found
such patterns in several cortical areas, mainly on the outer
realm of the hemisphere. The level of connectional crossmodality overlap varies markedly between these areas,
whereas on the single cell level, feedback projections remain
modality-specific. The physiological significance of the cir-
ACKNOWLEDGMENTS
We thank Drs. C.R. Olson, J. Olavarria, M. Segal, and R.
Eilam for helpful comments on the manuscript; M. Hare1
for technical assistance; G. Sirton, Division of Laboratory
Computers, for computer programing, and the Graphics
Department for assisting with the figures. This work was
supported by grants from the US-Israel B.S.F grant
88-00275 and Israeli Academy of Sciences, B.S.R grant
527189 to R. M.
LITERATURE CITED
Akers, R.A., and H.P. Killackey (1978) Organization of corticocortical
connections in the parietal cortex of the rat. J. Comp. Neurol. 181:513538.
SENSORY INTERMODALITY RELATIONSHIPS IN RAT CORTEX
Andersen, R.A. (1989) Visual and eye movement functions of the posterior
parietal cortex. Ann. Rev. Neurosci. 12:377-403.
Benevento, L.A., J. Fallon, B.J. Davis, and M. Rezak (1977) Auditory-visual
interaction in single cells on the cortex of the superior temporal sulcus
and the orbital frontal cortex of the macaque monkey. Exp. Neurol.
57,849-872.
Berson, D.M., and A.M. Graybiel(l983) Organization of the striate-recipient
zone of the cat’s lateralis posterior-pulvinar complex and its relations
with the geniculostriate system. Neuroscience 9:337-372.
Bruce, C., R. Desimone, and C.G. Gross (1981) Visual properties of neurons
in a polysensory area in superior temporal sulcus of the macaque. J.
Neurophysiol. 46r369-384.
Cavada, C., and P.S. Goldman-Rakic (1989) Posterior parietal cortex in
rhesus monkey: 11. Evidence for segregated corticocortical networks
linking sensory and limbic areas with the frontal lobe. J. Comp. Neurol.
287t422-445.
Cavada, C., and F. Reinoso-Suarez (1985) Topographical organization of the
cortical afferent connections of the prefrontal cortex in the cat. J. Comp.
Neurol. 242:293-324.
Caviness, V.S., Jr. (1975) Architectonic map of neocortex of the normal
mouse. J. Comp. Neurol. 164:247-264.
Cipolloni, P.B., and A. Peters (1979) The bilaminar and banded distribution
of the callosal terminals in the posterior neocortex of the rat. Brain Res.
176:3347.
Crowne, D.P., and M.N. Pathria (1982) Some attentional effects of unilateral
frontal lesions in the rat. Behav. Brain Res. 6:250-39.
Deacon, T.W., H. Eichenbaum, P. Rosenberg, and K.W. Eckmann (1983)
Afferent connections of the perirhinal cortex in the rat. J. Comp. Neurol.
220:168-190.
Domesick,V.B. (1969) Projections from the cingulate cortex in the rat. Brain
Res. 12296-320.
Donoghue, J.P., and C. Parham (1983) M e r e n t connections of the lateral
agranular field of the rat motor cortex. J. Comp. Neurol. 21 7:390404.
Donoghue, J.P., and S.P. Wise (1982)The motor cortex of the rat: Cytoarchitecture and microstimulation mapping. J. Comp. Neurol. 212r76-88.
Duhamel, J.R., C.L. Colby, and M.E. Goldberg (1989) Congruent visual and
somatosensory response properties of neurons in the ventral intraparietal area (VIP) in the alert monkey. SOC.
Neurosci. Abst. 15:162.
Geneser-Jensen, F.A., and J.W. Blackstad (1971) Distribution of acetylcholinesterase in the hippocampal region of the guinea pig. I. Entorhinal
area, para subiculum, and presubiculum. 2. Zellforch. Mikrosk. Anat.
114:460-481.
Goldman-Rakic, P.S. (1988) Topography of cognition: Parallel distributed
networks in primate association cortex. Ann. Rev. Neurosci. 11:137-156.
Guldin, W.O., and H.J. Markowitsch (1983) Cortical and thalamic afferent
connections of the insular and adjacent cortex of the rat. J. Comp.
Neurol. 21 5: 135-153.
Hall, R.D., and E.P. Lindholm (1974)Organization of motor and somatosensory neocortex in the albino rat. Brain Res. 66r23-38.
Herkenham, M. (1980) Laminar organization of thalamic projections to the
rat neocortex. Science 207:532-535.
Hutchins, B., and J.T. Weber (1983)A rapid myelin stain for frozen sections:
Modification of the Heidenhain procedure. J. Neurosci. Methods 7:289294.
Jones, E.G., and T.P.S. Powell (1970) An anatomical study of converging
sensory pathways within the cerebral cortex of the monkey. Brain
93~793-820.
Keizer, K., H.G.J.M. Kuypers, A.M. Huisman, and 0. Dann (1983) Diamidin0 yellow dihydrochloride (DY 2HCL); A new fluorescent retrograde
neuronal tracer, which migrates only very slowly out of the cell. Exp.
Brain Res. 51:179-191.
Kolb, B. (1984) Functions of the frontal cortex of the rat: A comparative
review. Brain Res. Rev. 8r65-98.
Kosel, K.C., G.W. Van Hoesen, and D.L. Rosene (1982) Non-hippocampal
cortical projections from the entorhinal cortex in the rat and rhesus
monkey. Brain Res. 244r201-213.
Krieg, W.J.S. (1946a) Connections of the cerebral cortex. I. The albino rat. A.
Topography of the cortical areas. J. Comp. Neurol. 84:221-275.
Krieg, W.J.S. (1946b) Connections of the cerebral cortex. I. The albino rat. B.
Structure of the cortical areas. J. Comp. Neurol. 84:277-324.
Leichnetz, G.R., S.G.P. Hardy, and M.K. Carruth (1987) Frontal projections
to the region of the oculomotor complex in the rat: A retrograde and
anterograde HRP study. J. Comp. Neurol. 263r387-399.
Leonard, C.M. (1969) The prefrontal cortex of the rat. I. Cortical projection
of the mediodorsal nucleus. 11. Efferent connections. Brain Res. 12:321343.
455
Malach, R. (1988) In vivo visualization of callosal pathways: A novel
approach to the study of cortical organization. J. Neurosci. Methods
25:225-238.
Malach, R. (1989) Patterns of connections in rat visual cortex. J. Neurosci.
9:3741-3752.
Meibach, R.C., and A. Siegal (1977) Subicular projections to the posterior
cingulate cortex in rats. Exp. Neurol. 57:264-274.
Mesulam, M.M. (1978) Tetramethyl benzidine for horseradish peroxidase
neurohistochemistry: A non-carcinogenic blue reaction product with
superior sensitivity for visualizing neural afferents and efferents. J.
Histochem. Cytochem. 25:106-117.
Mesulam, M.M. (1983) The functional anatomy and hemispheric specialization for directed attention: The role of the parietal lobe and its
connectivity. Trends Neurosci. 6:384-387.
Miller, M.W., and B.A. Vogt (1984a) Direct connections of rat visual cortex
with sensory, motor, and association cortices. J. Comp. Neurol. 226:184202.
Miller, M.W., and B.A. Vogt (1984b) Heterotopic and homotopic callosal
connections in rat visual cortex. Brain Res. 297:75-89.
Montero, V.M., and R.W. Guillery (1968) Degeneration in the dorsal lateral
geniculate nucleus of the rat following interruption of the retinal or
cortical connections. J. Comp. Neurol. 134:211-242.
Nauta, W.J.H., and V.M. Bucher (1954) Efferent connections of the striate
cortex in the albino rat. J. Comp. Neurol. 100:257-285.
Neafsey, E.J., E.L. Bold, G. Haas, K.M. Hurley-Gius, G. Quirk, C.F. Sievert,
and R.R. Terreberry (1986) The organization of the rat motor cortex: A
microstimulation mapping study. Brain Res. Rev. 11:77-96.
Olavarria, J., and V.M. Montero (1984) Relation of callosal and striateextrastriate cortical connections in the rat: Morphological definition of
extrastriate visual areas. Exp. Brain Res. 54:240-252.
Olavarria, J., and R.C. Van Sluyters (1985) Organization and postnatal
development of callosal connections in the visual cortex of the rat. J.
Comp. Neurol. 239: 1-26.
Olavarria, J., R.C. Van Sluyters, and H.P. Killackey (1984) Evidence for the
complementary organization of callosal and thalamic connections within
rat somatosensory cortex. Brain Res. 291:364-368.
Olson, C.R., and I. Jeffers (1987) Organization of cortical and subcortical
projections to area 6m of the cat. J. Comp. Neurol. 266r73-94.
Olson, C.R., and K. Lawler (1987) Cortical and subcortical afferent connections of a posterior division of feline area 7 (area 7p). J. Comp. Neurol.
259:13-30.
Pandya, D.N., and B. Seltzer (1982)Association areas of the cerebral cortex.
Trends Neurosci. 5:38&389.
Pandya, D.N., and E.H. Yeterian (1985) Architecture and connections of
cortical association areas. In A. Peters and E.G. Jones (eds): Cerebral
Cortex: Vol 4, Association and auditory cortices. New York Plenum
Press, pp. 3-61.
Pinto-Hamuy, T., J. Olavarria, E. Guic-Robles, M. Morgues, 0. Nassal, and
D. Petit (1987) Rats with lesions in anteromedial extrastriate cortex fail
to learn a visuosomatic conditional response. Behav. Brain Res. 25~221231.
Reep, R.L., J.V. Corwin, A. Hashimoto, and R.T. Watson (1984) Afferent
connections of medial precentral cortex in the rat. Neurosci. Lett.
44:247-252.
Roger, M., and P. Arnault (1989)Anatomical study of the connections of the
primary auditory area in the rat. J. Comp. Neurol. 287:339-356.
Room, P., and H.J. Groenewegen (1986) Connections of the parahippocampal cortex. I. Cortical afferents. J. Comp. Neurol. 251:415-450.
Sally, L.S., and J.B. Kelly (1988) Organization of auditory cortex in the
albino rat: Sound frequency. J. Neurophysiol. 59:1627-1638.
Sanides, F. (1969) Comparative architectonics of the neocortex of mammals
and their evolutionaryinterpretation. Ann. N.Y. Acad. Sci. 167:404-423.
Schmued, L.C., and J.H. Fallon (1986) Fluoro-Gold: A new fluorescent
retrograde axonal tracer with numerous unique properties. Brain Res.
377:147-154.
Sesack, S.R., A.Y. Deutch, R.H. Roth, and B.S. Bunney (1989) Topographical
organization of the efferent projections of the medial prefrontal cortex in
the rat: An anterograde tract-tracing study with Phaseolus vulgaris
Leucoagglutinin. J. Comp. Neurol. 290:213-242.
Takahashi, T. (1985)The organization of the lateral thalamus of the hooded
rat. J. Comp. Neurol. 231:281-309.
Thomas, H.C., and S.G. Espinoza (1987) Relationships between interhemi-
456
spheric cortical connections and visual areas in hooded rats. Brain Res.
41 7:214-224.
Thorpe, S.J., E.T. Rolls, and S. Maddison (1983) The orbitofrontal cortex:
Neuronal activity in the behaving monkey. Exp. Brain Res. 49~93-115.
Torrealba, F., J. Olavarria, and M.A. Carrasco (1984) Cortical connections of
the anteromedial extrastriate visual cortex in the rat. Exp. Brain Res.
56543-549.
Vaughan, D.W. (1983) Thalamic and callosal connections of the rat auditory
cortex. Brain Res. 260:181-189.
Vogt, B.A., and A. Peters (1981) Form and distribution of neurons in rat
cingulate cortex: Areas 32,24, and 29. J. Comp. Neurol. 195:603-625.
Vogt, B.A., and M.W. Miller (1983) Cortical connections between rat
cingulate cortex and visual, motor, and postsubicular cortices. J. Comp.
Neurol. 216; 192-2 10.
Wagor, E., N. Mangini, and A.L. Pearlman (1980) Retinotopic organization
of striate and extrastriate visual cortex in the mouse. J. Comp. Neurol.
196187-202.
Welker, C. (1971) Microelectrode delineation of fine grain somatotopic
organization of SmI cerebral neocortex in albino rat. Brain Res. 26:259275.
Welker, C. (1976) Receptive fields of barrels in the somatosensory neocortex
of therat. J. Comp. Neurol. 166:173-190.
T. PAPERNA AND R. MALACH
Welker, C., and M.M. Sinha (1972) Somatotopic organization of SmII
cerebral neocortex in albino rat. Brain Res. 37:132-136.
Welker, C., and T.A. Woolsey (1974) Structure of layer IVin the somatosensory cortex of the rat: Description and comparison with the mouse. J.
Comp. Neurol. 158;437-454.
Winer, J.A., and D.T. Larue (1987) Patterns of reciprocity in auditory
thalamocortical and corticothalamic connections: Study with horseradish peroxidase and autoradiographic methods in the rat medial geniculate body. 3. Comp. Neurol. 257~282315.
Wise, S.P. (1975) The laminar organization of certain afferent and efferent
fiber systems in the rat somatosensory cortex. Brain Res. 90:139-142.
Wise, S.P. and E.G. Jones (1977) Cells of origin and terminal distribution of
descendingprojections of the rat somatosensory cortex. J. Comp. Neurol.
175:129-158.
Witter, M.P., and H.J. Groenewegen (1986) Connections of the parahippocampal cortex in the cat. 111. Cortical and thalamic efferents. 3. Comp.
Neurol. 252;1-31.
Zilles, K. (1985) The Cortex of the Rat A Stereotaxic Atlas. Berlin:
Springer-Verlag, 121pp.
Zilles, K., B. Zilles, and A. Schliecher (1980) A quantitative approach to
cytoarchitectonics. VI.The areal pattern of the cortex of the albino rat.
Anat. Embryol. 159:335-360.