Download Evidence of Basal Temporo-occipital Cortex

Cerebral Cortex January 2005;15:117--122
doi:10.1093/cercor/bhh114
Advance Access publication June 24, 2004
Evidence of Basal Temporo-occipital
Cortex Involvement in Stereoscopic Vision
in Humans: A Study with Subdural
Electrode Recordings
Francisco Gonzalez1,2, José Luis Relova1,4, Angel Prieto3 and
Manuel Peleteiro1,4
Stereoscopic vision is based on small differences in both retinal
images known as retinal disparities. We investigated the cortical
responses to retinal disparities in a patient suffering from occipital
epilepsy by recording evoked potentials to random dot stereograms
(RDS) from subdural electrodes placed in the parieto-occipitotemporal junction, medial surface of the occipital lobe (pericalcarine cortex) and basal surface of the occipital and temporal lobes
(fusiform gyrus). Clear responses to disparity present in RDS were
found in the fusiform cortex. We observed that the fusiform
responses discriminate the onset from the offset of the stimulus,
correlation from uncorrelation, and they show a longer latency than
responses found in the pericalcarine cortex. Our findings indicate
that the fusiform area is involved in the processing of the stereoscopic information and shows responses that suggest a high level
of stereoscopic processing.
tion is processed throughout the various sensory pathways and
how it reaches conscious perception. This technique combined
with evoked potential recordings has been used to study several
aspects of sensory cortical functions in humans (Ray et al., 1999;
Grunwald et al., 2003; Mundel et al., 2003).
The goal of the study reported here was to determine
whether some cortical regions from the occipital and temporal lobes are involved in disparity processing, a requirement
needed to achieve stereopsis. For this, we studied the cortical
visually evoked responses to RDS recorded from subdural
electrodes in a patient who suffered from occipital epilepsy.
To our knowledge this is the first report to address this question
that uses this recording technique combined with RDS in
humans.
Keywords: fusiform gyrus, humans, random dot stereograms, stereopsis,
subdural recording
Introduction
Stereopsis is traditionally defined as the perception of depth
based on small positional differences, known as retinal disparities. Neurophysiological studies in monkeys showed that there
is a widespread distribution of retinal disparity sensitive cells
throughout many cortical areas of nonhuman primates. Sensitivity to retinal disparity has been recently found in cells from
the inferotemporal (IT) cortex (Janssen et al., 1999, 2003; Uka
et al., 2000), which indicates that the ventral visual pathway is
involved in this modality of visual perception. It has also been
observed that, in monkeys, removal of the IT impaired global
stereopsis (Cowey and Porter, 1979).
Cortical activation associated with stereopsis in humans was
mostly studied by using functional magnetic resonance imaging
(fMRI). These studies basically show that stereoscopic stimuli
produces weak or variable activity in areas V1, V2, V3 and MT,
strong activity in area V3A, V7 and posterior IPS, and stronger
activity in V4d-topo, whereas more complex objects produce
activity in the lateral occipital cortex (LO) and the fusiform
gyrus (Kwee et al., 1999; Mendola et al., 1999; Backus et al.,
2001; Gilaie-Dotan et al., 2001; Nishida et al., 2001; Iwami et al.,
2002; Tsao et al., 2003). Scalp visual evoked potentials have also
been used to study stereopsis (Braddick and Atkinson, 1983;
Norcia and Tyler, 1984), but this technique has limited spatial
resolution and therefore it is not appropriate to identify cortical
visual areas. On the contrary, subdural electrodes have high
spatial and temporal resolution. The implant of subdural electrodes in patients undergoing evaluation for surgery therefore
offers a unique opportunity to examine how sensory informaCerebral Cortex V 15 N 1 Oxford University Press 2005; all rights reserved
1
Department of Physiology, School of Medicine, University
of Santiago de Compostela, Santiago de Compostela,
Spain, 2Service of Ophthalmology, Complejo Hospitalario
Universitario de Santiago de Compostela, Santiago de
Compostela, Spain, 3Service of Neurosurgery, Complejo
Hospitalario Universitario de Santiago de Compostela, Santiago
de Compostela, Spain and 4Service of Clinical Neurophysiology,
Complejo Hospitalario Universitario de Santiago de
Compostela, Santiago de Compostela, Spain
Materials and Methods
A 47-year-old right-handed woman suffering from medically intractable
right occipital epilepsy who referred complex partial seizures with
visual aura was scheduled for surgery in order to study her epileptic
seizures and evaluate possible further surgical treatment. For this,
subdural electrode arrays covering the mesial occipital (MO, 8 3 1
electrodes), lateral occipito-temporo-parietal (LOTP, 8 3 4 electrodes),
and basal occipito-temporal (BOT, 6 3 1 electrodes) cortices of the right
hemisphere were implanted (Fig. 1). The electrode arrays were made of
platinum--iridium (Ad-Tech, Racine, WI) with a 1 mm diameter exposure
of each electrode and the electrode centers separated by 10 mm. They
were inlaid as fixed rectangular or linear arrays in a thin transparent
silastic plate. Shielded cables transmitted the signals to a customized
patch panel that allowed the pairing of any two electrodes for recordings. Subdural electrodes were placed over the cortical surface between
the dura and the pia-aracnhoid, followed by closure of the dura.
Visual Stimulation
The visual stimuli were projected by a computer-controlled standard
multimedia projector onto a frontoparallel flat surface facing the subject
at a distance of 115 cm. RDS were used as stimulus to study disparity
sensitivity. They were generated by using a conventional personal
computer running software developed in our own laboratory (Gonzalez
and Krause, 1994). The patient wore red/green eyeglasses and viewed
the frontal surface where the RDS were projected. To obtain a separate
viewing for each eye red and green dot patterns were used. The picture
viewed by each eye was a rectangular area made up of a matrix of 320 3
200 pixels subtending 38.8 3 24 of visual field (‘Background’ in Fig. 2).
To generate the RDS, 10% of pixels were bright and 90% were dark. The
patient had to fixate a small target with both eyes (0.36 3 0.36, ‘Fixation
target’ in Fig. 2) located in the center of the screen. For evoking cortical
potentials, all dots within an area of 1.2 3 1.2 centered within the left
hemifield (x = –1.7, y = 0; ‘Figure’ in Fig. 2) were shifted in opposite
directions producing various negative (crossed) and positive (uncrossed) horizontal disparities. The disparity was maintained for a period
of 275 ms. Repetitive disparity presentations were made every 2 s. When
the disparity was present a small square was perceived either in front
(negative disparity) or behind (positive disparity) the background,
Figure 1. Location of the electrode arrays. (A) lateral plain radiograph of the patient. The electrode arrays are outlined and the first and last electrodes are indicated with a number.
LOPT, lateral occipito-temporo-parietal grid with 32 electrodes; MO, mesial occipital array with eight electrodes; BOT, basal occipito-temporal array with six electrodes. (B) The
same view indicating the pairs of electrodes from which recordings were made. (C) coronal MRI showing electrodes of the LOPT array and the electrode 2 of the BOT array. (D)
sagital MRI showing the location of the MO array; electrodes 4 and 8 are outlined.
Figure 2. Visual stimulus. Schematic representation of the stimuli used in this study.
To study sensitivity to disparity we used random dot stereograms (RDS). Whenever
horizontal disparity was presented in a central region a stereofigure was perceived
(‘Figure’) either in front or behind the surrounding field of dots (‘Background’).
Uncorrelated stereograms were similar but instead of horizontally shifting a small
region of the random dot field (‘Figure’), all dots of this region were scrambled in such
a way that each eye viewed different patterns. The stimulus presentation (stereofigure) lasted for a period of 275 ms. In those cases we tested hemifield dominance
the same stimulus was delivered in a symmetrical position on the right hemifield.
whereas with zero disparity no figure was perceived. Disparities ranging
from +0.75 to –0.75 were used. Both dynamic (frame change every 1/
60th s) and static RDS were used to assess disparity sensitivity. The
luminance was 18.0 cd/m2 for the random dot fields (10% bright pixel
density). After filtered by the eyeglasses, the luminance was 2.2 cd/m2
for the red pattern and 1.43 cd/m2 for the green pattern.
Recording Procedure
Recordings started 2 days after the electrodes were implanted and were
made for three consecutive days. Direct inspection of the patient was
118 Stereopsis in the Fusiform Gyrus
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Gonzalez et al.
continuously made to ensure the reliability of the procedure. The
patient reported to perceive the central ‘Figure’ (Fig. 2) in front or
behind the ‘Background’ whenever negative or positive disparity was
present in both static and dynamic RDS.
Evoked potentials were recorded from pairs of subdural electrodes by
using standard equipment (Viking IV IOM; Nicolet Biomedical Ltd,
Wisconsin) from electrodes 2--3, 3--4 and 5--6 of the BOT strip, 3--4 and
4--8 of the MO strip, and 1--2, 3--4, 5--6 and 7--8 of the LOTP grid (Fig. 1).
The remaining electrodes did not allow reliable recordings. The recording of the electrode potential started 250 ms before the onset of
the stimulus and lasted for 1 s. The trigger for starting each recording
was produced by the same computer that generated the visual stimulus.
The responses evoked by 100 consecutive stimulus presentations were
averaged to obtain a final evoked potential for each type of stimulation.
We used a 1 kHz high pass filter and 100 Hz low pass filter (American
Electroencephalographic Society, 1986). Electrode impedance was <3 kX.
The resulting averaged evoked potentials were then analyzed for the
presence of deflections related to stimulus responses.
The experiments were conducted in compliance with the relevant
laws and guidelines of the Bioethical Committee of our institution. The
patient was fully informed about the objectives, details, and risks of the
experiment and a written consent was obtained before the recordings
documented in this report were made.
Results
The LOTP grid was placed under direct visual inspection
covering the postero-lateral surface of the temporal cortex
and the junction of the occipito-parieto-temporal cortex.
Because of the limitations in the surgical technique, it was not
possible to know the exact location of the MO and BOT
electrode strips by direct visual inspection. However, the image
studies (X-ray, NMR and CT) indicated that the MO strip was
covering the pericalcarine cortex with the electrode 1 anterosuperior and 8 infero-posterior, whereas the BOT strip was lying
along the fusiform gyrus with electrode 1 anterior and 6
posterior. Figure 1 shows the electrode locations, indicating
those from which recordings were made.
Responses to RDS were found in those recordings made from
electrodes 2--3 of the BOT strip and 4--8 of the MO strip.
Talairach (Talairach and Tournoux, 1988) coordinates (x, y, z)
were (34, –50, –14) for electrode BOT-2, (49, –55, –16) for
electrode BOT-3, (3, –65, 14) for electrode MO-4 and (3, –87, –2)
for electrode MO-8. We shall refer to these areas as fusiform and
pericalcarine areas, respectively. The recordings made from
the remaining electrodes including those from the LOTP array
(Fig. 1) did not show detectable responses to disparity.
As Figure 3 shows, recordings made in the fusiform area
clearly show a prominent response for a disparity of +0.25,
whereas the remaining disparities evoked either a weaker
response or no response at all. The evoked potential peaked
at ~210 ms after the stimulus onset, but there was no response
to the stimulus offset. In the pericalcarine area every disparity
produced both onset and offset potentials (Fig. 4). In this case
the peak of the response occurred at ~150 ms after the stimulus
onset and offset respectively.
To make sure the recorded responses from the fusiform area
were due to disparity and not to a change in the dot pattern, we
used dynamic RDS that changed the pattern every 1/60th of
a second. This stimulus produced similar responses as static
RDS (Fig. 3). Additionally, we recorded the responses to two
disparities (+0.25 and –0.25) with and without red/green
eyeglasses. In our RDS, when no eyeglasses are used and
disparity is present instead of a stereofigure an area of mixed
yellow, red and green dots is perceived. When eyeglasses are
used and disparity is present, a figure in front or behind the
background is perceived. Figure 5A clearly shows how, when
eyeglasses are used, a prominent response to +0.25 of disparity
is present, whereas there is no response to a disparity of –0.25.
On the contrary, when the eyeglasses are removed there is
a similar response to both disparities as shown in Figure 5B.
In order to know whether correlation and uncorrelation had
the same significance for both areas, we used uncorrelated RDS
as stimulus. Figure 6 shows the responses to uncorrelated RDS
in the fusiform and pericalcarine areas. It can be observed that
whereas in the pericalcarine area there is a clear response to the
onset and offset of the uncorrelated RDS, the response in the
fusiform area is absent.
To test hemifield dominance we used a RDS with a stereofigure that was 1.2 3 1.2 in size centred at –1.7,0 for contralateral stimulation and +1.7,0 for ipsilateral stimulation. Figure
7 shows a clear response when the stimulus is presented on the
contralateral hemifield whereas a very mild response with
longer latency was recorded when the same stimulus was
delivered on the ipsilateral hemifield.
Figure 3. Responses to static and dynamic RDS in the fusiform area. Visual evoked
potentials recorded from the fusiform area (electrodes 2--3 of the BOT array). The thick
line on the abscissa indicates the time the stereofigure was on. Each recording
represents the average response to 100 stimulus presentations. Note that the
strongest response is obtained for a disparity of þ0.25, which peaks at 210 ms after
the stimulus onset for both types of RDS.
Figure 4. Responses to static RDS in the pericalcarine area. Visual evoked potentials
recorded from the pericalcarine area (electrodes 4--8 of the MO array). The thick line on
the abscissa indicates the time the stereofigure was on. Each recording represents the
average response to 100 stimulus presentations. Note that a response to the onset
and the offset of the stimulus is obtained for every disparity which peak at ~150 ms
after the stimulus onset and offset.
Discussion
Human studies aimed to find cortical areas involved in stereopsis have been made mostly using fMRI (Kwee et al., 1999;
Mendola et al., 1999; Backus et al., 2001; Gilaie-Dotan et al.,
2001; Nishida et al., 2001; Iwami et al., 2002), PET (Ptito et al.,
1993; Gulyás and Roland, 1994; Fortin et al., 2002) or in patients
with brain lesions (Ptito and Zatorre, 1988; Ptito et al., 1991).
Functional MRI studies showed that areas located in the lateral
occipital and parieto-occipital cortices are related to stereoscopic processing. However, by using subdural recordings, we
failed to record responses to RDS from the LOPT array. The
Cerebral Cortex January 2005, V 15 N 1 119
Figure 7. Ipsilateral versus contralateral hemifield stimulation. Visual evoked responses recorded from the fusiform area (electrodes 2--3 of the BOT array). The
stimulus was a static RDS and the patient wore red/green eyeglasses. The upper trace
shows the response to a stereofigure delivered on the contralateral hemifield (right).
The lower trace is the response to the same stimulus delivered on a symmetrical
position on the ipsilateral hemifield.
Figure 5. Dichoptic versus non-dichoptic stimulation. Visual evoked responses recorded from the fusiform area (electrodes 2--3 of the BOT array). The stimulus was
a static RDS with horizontal disparities of þ0.25 and 0--25. The thick line on the
abscissa indicates the time the stereofigure was on. (A) Responses recorded when the
subject wore red/green eyeglasses. Under this condition a stereofigure was perceived
either behind (positive disparity) or in front (negative disparity) of the background. Note
that there is a clearly different response for each disparity. (B) The same as in A, but
the subject did not wear the red/green eyeglasses. Under this condition the patient
perceived a mixture of red/green/yellow dots instead of a stereofigure when the
disparities were present. Note that the same response is obtained for both disparities.
Figure 6. Responses to uncorrelated static RDS. Visual evoked responses recorded
from the fusiform area (electrodes 2--3 of the BOT array) and pericalcarine area
(electrodes 4--8 of the MO array). The stimulus was a static uncorrelated RDS and the
subject wore red/green glasses. The thick line on the abscissa indicates the time the
uncorrelated stimulus was on. In the pericalcarine area a clear response is obtained for
both the onset and the offset of the stimulus whereas no response was observed in
the fusiform area.
reason may be that the electrodes of this array did not intersect
any of the areas shown to be activated by stereoscopic stimuli in
fMRI studies. On the contrary, in our study we provide evidence
that the fusiform area is involved in disparity processing. As
shown in Figure 3, responses in this area to static and dynamic
RDS are highly dependent of the stimulus disparity. We obtained
the strongest response for positive disparities which are generated by objects behind the fixation point. This is surprising
because near objects are perceptually more salient. Indeed,
120 Stereopsis in the Fusiform Gyrus
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Gonzalez et al.
Gilaie-Dotan et al. (2001) by using fMRI found preferential
activation to ‘front’ compared to ‘back’ objects. This discrepancy may be due to the spatial resolution of the subdural technique. Our electrodes had 1 mm diameter exposure separated
by 10 mm and therefore they had a restricted area or recording which may have included only fusiform regions with more
prominent responses to positive disparities.
We also found responses to static RDS in the pericalcarine
area. These responses may be due to the pattern change caused
by the onset and offset of the disparity and not by the disparity
itself. We cannot determine this because we were not able to
measure responses to dynamic RDS from this array.
The responses to RDS we found in the fusiform area were
disparity dependent whereas in the pericalcarine area were not
(see Figs 3 and 4). One explanation for this finding is that the
responses in the pericalcarine area were due to pattern change
and not to disparity. Another possible explanation is that subdural electrodes averaged population activity across several disparity columns. In monkeys, at single-cell level, there is disparity
sensitivity in areas V1, V2, V3 and V3A (Poggio et al., 1985,
1988; Gonzalez et al., 1993; Durand et al., 2002; Prince et al.,
2002); however, disparity sensitivity changes within a few hundred microns. Subdural electrodes do not have enough spatial
resolution to detect the disparity selectivity of small cellular
clusters and therefore record the activity of a large population
of cells simultaneously responding to several disparities and
to pattern change. It may be possible that in the fusiform area
cells related to positive and negative disparity are arranged in
larger patches which can cause the electrodes to record
responses selective to a given disparity.
Response latencies to the stimulus onset in the fusiform area
peak at ~210 ms, whereas in the pericalcarine area they peak at
~150 ms. This longer response delay found in the fusiform area
is indicative of a higher level in the processing of the visual
information. Whereas responses to RDS in the pericalcarine area
may be partially caused by pattern changes, this is not the case
for the fusiform area. As shown in Figure 3, there is no response
to ‘zero’ disparity nor to most of the tested disparities. This
indicates that in this area, the responses we found are caused by
the disparity present in the stimulus and not by pattern changes.
Moreover, when we used dynamic RDS we recorded similar
responses as when we used static RDS (Fig. 3). Further evidence
is shown in Figure 5. The upper part of the figure shows the
responses to a static RDS with crossed and uncrossed disparities
when the patient wore red/green glasses. Note that the elicited
response is present only when the figure was perceived
behind the background. The lower part of Figure 5 shows
the response to the same stimuli when the patient did not wear
the eyeglasses and therefore no depth was perceived. In this
case the potential was similar in amplitude and shape for both
disparities. It is interesting to observe that the morphology of
the evoked potential changed, suggesting that disparity triggers
a specific pattern of activation of the cell population in the
fusiform area.
Correlated and uncorrelated RDS evoke similar responses in
the pericalcarine area, while in the fusiform area only correlated
RDS produce responses. This finding suggests that this later area
represents a high level of the processing of retinal disparities. As
Figure 6 shows, while uncorrelated RDS produce clear on- and
offset responses in the pericalcarine area, they do not produce
any response in the fusiform area. Single-cell responses to uncorrelated stereograms have been reported in early stages of
the visual pathways such as areas V1, V2 and V3--V3A of the
monkey (Poggio et al., 1988; Gonzalez et al., 1993). It is assumed
that at some stage in the hierarchy of the visual areas, the neural
mechanisms for disparity selectivity must achieve stereo correspondence by exhibiting selectivity only for correlated random
dot stereograms. Although it is likely that the stereo correspondence problem is not achieved solely in the fusiform area, in our
study we provide evidence for an area in which this problem may
have been solved. This is in agreement with the finding that in
monkeys the end-stage of the ventral visual pathway for stereopsis may be the lower bank of the rostral superotemporal sulcus,
known as area TEs (Janssen et al., 2003).
We observed that for RDS, while there was a strong response
from the contralateral hemifield only a weak and delayed
response was obtained from the ipsilateral hemifield (Fig. 7).
Single unit recording studies in monkeys indicate that cells from
the IT cortex have receptive fields that are larger than in more
posterior areas of the ventral visual pathway, that respond
stronger at the foveal position and that prefer the contralateral
hemifield above the ipsilateral hemifield (Schwartz et al., 1983;
Komatsu and Ideura, 1993; Tovee et al., 1994; Logothetis et al.,
1995; Missal et al., 1999; Op De Beeck and Vogels, 2000). The
preference of IT cells for contralateral stimulation agrees with
our observation that the stronger response was obtained from
the contralateral hemifield. Thus, at this stage, although most
of the stereoscopic processing may already be solved, both
hemifields are not yet fully combined. We believe that this
may happen at hierarchically higher cortical areas.
It has been suggested that the 3D features of objects are
processed in the parietal areas, which belong to the dorsal visual
system (Shikata et al., 1996; Sakata et al., 1997). Recently,
neurons and regions related to binocular disparity have been
found in the IT cortex, which is part of the ventral visual
pathway, of monkeys (Janssen et al., 1999, 2003; Uka et al.,
2000). These neurons are selective for disparity-defined 3D
shapes and in the vast majority of them the selectivity depends
on the global binocular disparity gradient and not on the local
disparity, indicating a high level of stereoprocessing (Janssen
et al., 1999). Our findings support the involvement of the
ventral pathway in processing stereoscopic information.
Notes
This work was partially supported by grants BFI2001-3206 from the
Spanish Ministerio de Ciencia y Tecnologia, 02PXIC-20803PN from the
Xunta de Galicia and CIEN-Galicia (FIS-Instituto de Salud Carlos III). We
are grateful to Marisol Justo for her help in preparing this manuscript.
Address correspondence to Dr Francisco Gonzalez, Department of
Physiology, School of Medicine, E-15782 Santiago de Compostela, Spain.
Email: [email protected].
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