Download Organization and Significance of Neurons That Detect Change of

THE JOURNAL OF COMPARATIVE NEUROLOGY 424:356 –376 (2000)
Organization and Significance of
Neurons That Detect Change of Visual
Depth in the Hawk Moth Manduca sexta
MARTINA WICKLEIN AND NICHOLAS J. STRAUSFELD*
Arizona Research Laboratories Division of Neurobiology, University of Arizona,
Tucson, Arizona 85721
ABSTRACT
Visual stimuli representing looming or receding objects can be decomposed into four parameters: change in luminance; increase or decrease of area; increase or decrease of object perimeter
length; and motion of the object’s perimeter or edge. This paper describes intracellular recordings
from visual neurons in the optic lobes of Manduca sexta that are selectively activated by certain
of these parameters. Two classes of wide-field neurons have been identified that respond selectively to looming and receding stimuli. Class 1 cells respond to parameters of the image other
than motion stimuli. They discriminate an approaching or receding disc from an outwardly or
inwardly rotating spiral, being activated only by the disc and not by the spiral. Class 2 neurons
respond to moving edges. They respond both to movement of the spiral and to an approaching or
receding disc. These two classes are further subdivided into neurons that are excited by image
expansion (looming) and are inhibited by image contraction (antilooming). Class 2 neurons also
respond to horizontal and vertical movement of gratings over the retina. Stimulating class 1 and
2 neurons with white discs against a dark background results in the same activation as stimulation with dark discs against a white background, demonstrating that changes in luminance
play no role in the detection of looming or antilooming. The present results show that the two
types of looming-sensitive neurons in M. sexta use different mechanisms to detect the approach
or retreat of an object. It is proposed that cardinal parameters for this are change of perimeter
length detected by class 1 neurons and expansion or contraction visual flow fields detected by
class 2 neurons. These two classes also differ with respect to their polarity, the former comprising
centripetal cells from the optic lobes to the midbrain, the latter comprising centrifugal neurons
from the midbrain to the optic lobes. The significance of these arrangements with respect to
hovering flight is discussed. J. Comp. Neurol. 424:356 –376, 2000. © 2000 Wiley-Liss, Inc.
Indexing terms: looming neurons; insect vision; neuroanatomy; electrophysiology; hovering flight
Perception of depth is a key feature of vision. It is
used to detect and avoid objects; to pursue visual targets; to maintain distance from targets; and to discriminate among objects in the third dimension. Animals
employ many strategies to compute depth perception.
These include stereopsis, in which images from each
retina are combined centrally to provide representation
of depth by computing binocular retinal disparities (Julesz, 1972; Livingston and Hubel, 1988). Vergence is
another binocular mechanism that uses the degree of
rotation of each eye, measured by eye muscle spindles,
to compute absolute depth of a perceived target (Judge,
1991). Vergence could, in principle, be used by arthropods with movable eyes on stalks, although it appears
that distance perception by triangulation is the method
employed (Zeil et al., 1986).
© 2000 WILEY-LISS, INC.
Insect eyes are integrated in the head cuticle and are
virtually immobile. In almost all insect species, the distance between the two eyes is not sufficient for stereopsis
Grant sponsor: NIH National Center for Research Resources; Grant
number: RR08688; Grant sponsor: A. v. Humboldt Foundation (Feodor
Lynen stipend); Grant sponsor: University of Arizona’s NSF Integrative
Graduate Education and Research Traineeship Program; Grant sponsor:
NSF Plant-Insect Interactions Group.
Martina Wicklein’s current address is: the Salk Institute, 10010 Torrey
Pines Rd., La Jolla, CA 92037-1099.
*Correspondence to: Nicholas J. Strausfeld, Arizona Research Laboratories Division of Neurobiology, 611 Gould-Simpson, University of Arizona,
Tucson, AZ 85721. E-mail: [email protected]
Received 2 December 1999; Revised 25 April 2000; Accepted 27 April
2000
LOOMING-SENSITIVE NEURONS IN MANDUCA
Fig. 1. Differences between stimuli used for this study. A: The
retinal image of an approaching (or receding) disc expands (or contracts) over the retina: the area of the image increases; the perimeter
length of the image increases; and the edge of the expanding image
provides motion stimuli across ommatidia. B: A spiral that is fixed at
one plane subtends an unchanging area of the retina. However, when
it turns, the edges of the spiral move across ommatidia, outward or
inward depending on the direction of rotation. This stimulus provides
motion stimuli only, without changing perimeter length or image
area.
or vergence other than in odonate larvae and in the preying mantis, which use true stereopsis (Baldus, 1926; Rossel, 1986). A special case of binocular vision is used by
male calliphorid flies, which possess an area of high retinal acuity that provides information about the centered
image of a pursued target. This is usually a female fly, the
immediate proximity of which is detected when its image
falls on both retinas (Collett and Land, 1975).
Both vertebrates and arthropods also employ monocular
mechanisms for depth perception. Not yet demonstrated
in insects, but known from human psychophysics, is depth
perception by occlusion where a depth map is created by
objects that are close to the moving eye appearing to pass
in front of more distant objects, blocking them and relatively larger areas of the background from view. Distance
perception by motion parallax has been identified in insects, particularly in locusts (Collett, 1978; Erikson, 1980)
and in honey bees (Srinivasan et al., 1989). This type of
depth perception relies on a comparison of velocities and
motion directions at different planes in the visual scene. It
should be noted that depth perception by occlusion or by
motion parallax requires information about self-motion
and about background.
These strategies contrast with yet another mechanism
for detecting change in depth: that of looming. This is the
perception of an approaching object, the image of which
expands (or contracts) across the retinal surface. Looming
is also an interesting case of depth perception because it
involves neither background features nor self-motion of
357
Fig. 2. Histogram shows four examples of depth-sensitive neurons
(mean responses of five repeated stimulations). Both class 1 and 2
looming-sensitive neurons respond to an expanding disc with an increase of spike frequency (preferred direction) and a contracting (receding) disc with a decrease of spontaneous spike activity (null direction). The class 1 and 2 antilooming-sensitive neurons respond in the
opposite manner: to a contracting disc with an increase of spike
frequency (preferred direction) and to an expanding (approaching)
disc with a decrease of spontaneous spike activity (null direction). The
symbols used here and in other figures are as follows: a wedge pointing to the left indicates expansion; a wedge pointing to the right
indicates contraction.
the observer. The only requirement is that the object has
to contrast against a background. On its own, looming
cannot be used to define a three-dimensional (3D) map of
a scene. However, the perception of a looming stimulus
initiates escape behavior by locusts and flies and is used
by nectar feeding moths and hummingbirds for maintaining station in front of a target. It is thus not surprising
that looming-sensitive neurons can be found in insects
and vertebrates. Examples are in sphingid moths (Wicklein, unpublished data), grasshoppers (Rind, 1999; Gabianni et al., 1999), flies (Holmqvist and Srinivasan, 1991),
pigeons (Sun and Frost, 1999), and primates (Tanaka and
Saito, 1989; Graziano et al., 1994)).
This account addresses the question of what features of
the looming stimulus are detected by uniquely identifiable
neurons and how such neurons might collaborate to provide information that would regulate the flight motor to
maintain distance from a target. The paper describes experiments performed on the tobacco hawk moth Manduca
sexta—a crepuscular and nocturnal sphingid. Sphingids
are rapid and highly maneuverable flyers with large compound eyes, which hover in front of flowers where they
perform stationary flight while ingesting nectar. Typically, a moth will approach a flower at high velocity and
then abruptly decelerate to arrive at a distance from the
flower that just permits insertion of the proboscis. To feed
successfully, the moth maintains its position even if the
flower moves, irrespective of the direction of movement.
Flowers selected as targets are those that offer stark contrast in front of uniform foliage, which under these light
conditions comprises a virtually unstructured background.
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M. WICKLEIN AND N.J. STRAUSFELD
MATERIALS AND METHODS
Animals
Manduca sexta (Lepidoptera: Sphingidae) were reared
on an artificial diet (modified after Bell and Joachim 1976)
and maintained on a long-day photoperiod (17-hour light,
7-hour dark) at 25°C at 50 – 60% relative humidity. Stage
18 pupae were taken out of the colony and put into an
environmental chamber to hatch. Their light cycle was
reversed in the chamber, and animals were kept under
reversed conditions for at least a full cycle to ensure that
they switched to a diurnal rhythm.
Electrophysiology: preparation
Adult 1- to 3-day-old M. sexta were cold anesthetized
and waxed to a holder to stabilize the thorax ventrally.
The head was tilted forwards at a 30° angle from the
horizontal axis and waxed in place, leaving the eyes uncovered. The proboscis was cut to prevent muscle action,
and the cut was sealed with low melting point wax. A
narrow window was cut into either the left or right posterior surface of the head capsule, directly above the optic
peduncle. Tracheal air sacs and the neural sheath were
carefully removed to expose the optic peduncle and the
most medial part of the optic lobes. The preparation was
perfused with a Ringer’s solution to prevent desiccation
(Christensen and Hildebrand, 1987). The head of the moth
was aligned in the rig in a standard fashion using the
antennal bases and the base of the proboscis as fiducial
points that were oriented to markers on the table. The
eyes’ pseudopupils were used to orient the head so that it
assumed a standard position with respect to the stimuli,
or the monitor, in front of it. To achieve this alignment the
pseudopupils were revealed by illumination through an
operation microscope, and the head was moved until the
pseudopupils were in the most medial and dorsal corner of
each eye. These were then aligned with vertical and horizontal markers on the recording table.
Intracellular recording and staining
Intracellular recordings were made from neurons in the
optic lobes. Intracellular recording and dye injection were
performed using silicon glass microelectrodes filled with a
1% neurobiotin-KCl solution backed up with 1 M KCl. The
electrodes had tip resistances of between 60 and 90 M⍀
(when in contact with the tissue). A silver wire inserted
into the thorax served as the indifferent electrode. Recordings were amplified (Axoprobe 1A, Axon Instruments,
Burlingame, CA) and displayed using standard equipment
and stored (Video Recorder, Vetter, Rebersburg, PA). After recording, the cells were filled iontophoretically for
5–10 minutes using a 1-nA negative current. Signals were
analyzed off-line on a desktop computer using a data analysis system (Spike2, CED, Cambridge, UK). The response
frequency was calculated by subtracting the spontaneous
activity of the cell from the spike frequency during stimulation. An estimate of spontaneous activity was obtained
by calculating the means from the spike rate measured in
a 2-second period before and after each set of motion
stimuli.
Stimuli
Both physical objects (light or dark discs, gratings, rotating spirals) and their simulations were used as stimuli.
All stimuli subtended a minimum of 30° on the retina.
Stimuli used to test looming responses were a white disc
(5 cm in diameter) in front of a black background or a
black disc in front of a white background. The disc was
moved with a constant velocity of 1 m/sec toward the
animal, stopped 10 cm in front of the moth where it subtended an angle of 30° on the retina, and then moved back
to its starting position (30 cm away from the moth).
Spirals (10 cm in diameter) of different degrees of curvature, and presented at a standard distance from the eye,
were rotated using a small motor that could be reversed
(to provide apparent expansion or contraction). The spiral
could be rotated at a range of velocities. In each experiment, the spiral was rotated with constant velocity,
stopped, and then rotated in the opposite direction with
the same velocity. Spiral stimuli were presented to the
front part of the visual field. To compare the effects of
different stimuli, all the descriptions below refer to stimulation of this frontal field of view.
The stimulus used to test horizontal and vertical motion
sensitivity consisted of a frame holding a movable grating.
Gratings were moved vertically and horizontally in different parts of the visual field of the animal to test the
response of a cell to translatory motion.
Stimuli were also simulated: displayed on a fast computer monitor (NEC M500, 160-Hz refresh rate) and presented to the animal via a surface-glazed mirror positioned to stimulate the frontal visual field (extending 70°
in the horizontal plane and 50° in the vertical plane).
These stimuli consisted of moving horizontal and vertical
gratings; expanding and contracting light and dark discs;
inwardly and outwardly rotating spirals. The starting and
end size of the discs and the velocity with which they
expanded and retreated could be preprogrammed, as could
the size and curvature of the spiral and its velocity of
rotation.
Stationary stimuli were presented first, then moved in
one direction, stopped, and moved in the opposite direction. This cycle was repeated five times for each stimulus.
The order in which stimulus types were presented was
randomized.
Histology
The procedure for revealing neurobiotin was based on
the original method of Horikawa and Armstrong (1988),
using a protocol for marine Crustacea (Schmidt and Ache
1990) or modifications for Diptera (Gronenberg and
Strausfeld, 1992). After iontophoresis (5–10 minutes 1nA)
and 1 hour for tracer diffusion (at 4°C), the brain was
opened and immersed in 4% formaldehyde in 0.2 M Sorensen’s phosphate buffer. Half an hour later the brain
Fig. 3. Reponses of class 1 looming cells to expanding discs and
spirals. Histograms (A,B) show relative spike frequencies elicited by
a disc moving toward and away from the animal (wedges), a spiral
turning outward and inward (indicated by arrowed spirals), horizontal progressive and regressive movements of gratings (horizontal arrows), and up-to-down or down-to-up motion of gratings (vertical
arrows). The histograms show that both recorded neurons had the
same properties: strict tuning to disc expansion, inhibition to disc
contraction, and no or negligible responses to directional motion.
C–J: Spike trains supporting these conclusions. C,D: Excitation to
disc expansion, inhibition to disc contraction. E,F: No responses to
apparent looming (spiral). G–I: Movement of gratings shows no effect
on background discharge.
Figure 3
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M. WICKLEIN AND N.J. STRAUSFELD
was dissected and fixed overnight. After washing in buffer
for at least 8 hours, the tissue was dehydrated through an
ethanol series, rinsed in propylene oxide (10 minutes), and
then rehydrated. The purpose of this step is to permeabilize the tissue for a whole-mount avidin-biocytin procedure
(Gronenberg and Strausfeld, 1992). The brain was then
incubated overnight in buffer with avidin-horseradishperoxidase (Vectastain ABC-kit, Vector, Burlingame, CA)
and 1% Triton X-100. After washing in buffer for 8 hours,
the brain was presoaked for 4 hours in the dark at 4°C in
diaminobenzidine (DAB) solution (Vectastain DAB-kit,
Vector). Another buffer wash followed, and the tissue was
then transferred into the same DAB solution containing
0.001% hydrogen peroxide (Vectastain, DAB-kit, Vector)
and processed in the dark at 4°C for 2 hours. After a
1-hour rinse in buffer in the dark at room temperature,
the brain was processed for an additional hour in the dark
at room temperature. The tissue was rinsed in buffer
overnight and then incubated in osmium tetroxide for
2–12 hours in the dark at 4°C. The tissue was then dehydrated and embedded in Durcupan (Fluka, Heidelberg,
Germany).
Serial horizontal or frontal sections were cut at 35 ␮m.
Preparations were observed and photographed with a
Leitz Orthoplan microscope. Drawings and reconstructions were made using a camera lucida attachment. Selected areas were serially digitized at 2-␮m steps using a
Sony DC 5000 CCD and overlaid using Adobe Photoshop.
Single images were collapsed into one image using the
Photoshop darkening function. Reconstructions have been
normalized to show one-half of the brain, as seen from the
back or top, with the optic lobes to the left of the illustration. Schematics showing the organization of the insect
visual system are provided in Strausfeld and Lee (1991).
RESULTS
More than 200 neurons were recorded from 148 moths.
Most neurons were motion sensitive but not looming sensitive. Sixty-seven neurons that responded to looming
stimuli were recorded from the optic lobes. Of these, 14
were filled with neurobiotin, with 14 others showing partial fills or multiple fills. The latter were discarded. Looming neurons can be generally grouped into two major
classes according to their tuning properties and structure.
Class 1 neurons respond to approaching or receding discs
whereas class 2 neurons respond both to these stimuli and
to rotational motion of a spiral pattern (Fig. 1). Class 1
and 2 neurons can be subdivided into those that are excited by an object moving toward the eye (type 1a, 2a
neurons) and those that are excited by an object receding
away from the eye (type 1b, 2b neurons; Fig. 2A,B). Of
those neurons that were filled with neurobiotin, class 1
neurons were revealed to be centripetal cells leading from
the optic lobes to the brain whereas class 2 neurons are
centrifugal cells, leading from centers in the posterior
midbrain (called the posterior slope) out to the medulla.
The recorded neurons responded with action potentials
having amplitudes that ranged from 10 to 60 mV. Responses were mostly sustained tonic excitations or were
inhibition of background activity (Figs. 3A,B, 4A,B). Most
class 2 neurons showed phasic-tonic responses and higher
response frequencies at stimulus onset when stimulated
with a rotating spiral (Figs. 4A,B, 9 –11, 13).
Significantly, none of the neurons showed habituation
to repetitive stimulation. Most neurons maintain a background (resting) activity. Thus, suppression of background in response to visual stimulation indicates inhibition in the null direction. Conversely, excitation is an
increase of the spike frequency compared with the background.
All the neurons tested had large (up to 120° horizontal,
90° vertical) receptive fields in the frontal or frontoventral
part of the monocular visual field. Responses to a moving
disc or spiral were the same as those to movement simulated on a computer monitor.
Encoding of approaching and retreating
objects
Neurons responded best to objects that move along the
longitudinal body axis of the moth toward or away from
the eye (i.e., the z-axis). We classified neurons as looming
or antilooming sensitive and direction selective. Four examples are shown in Figure 2. Two cells respond with an
increase in spike frequency when stimulated with a disc
moving toward the eye and a decrease in spike frequency
when a disc recedes from the eye (cells 1,2a). These neurons are classified as looming-sensitive neurons. Two
other neurons show an increase of spike frequency when
stimulated by a disc moving away from the eye and a
decrease in spike frequency when the disc moves toward
the eye (cells 1, 2b). These cells exemplify antilooming
neurons.
Different types of stimuli that are associated with
change in distance reveal two classes of neurons that
respond selectively to specific visual cues (Fig. 1): 1) expanding or contracting light and dark discs in front of a
contrasting background provide change in object area,
change of object perimeter (edge) length, and edge motion;
2) inward and outward rotation of a spiral isolates edge
motion since this stimulus neither changes its area nor its
perimeter length; and 3) movement of horizontal and vertical gratings test for directionally selective motion sensitivity along the x and y axes.
Responses to these stimuli demonstrate that class 1 and
2 neurons are sensitive to certain, but not all features of
an advancing or receding stimulus. Class 1 cells respond
only to an approaching or retreating disc but not to rotating spirals or moving gratings. Class 2 cells respond to all
these different stimuli. Figures 3 and 4 exemplify responses of two class 1 (Fig. 3) and two class 2 neurons (Fig.
4) elicited by these stimulus elements. The bar graphs in
Figure 3A and B summarize the response properties for
the two cell classes, each bar representing the mean response of five identical stimulations. The two class 1 neurons are clearly tuned to an expanding disc. Their spike
trains (Fig. 3C–H) show actual responses to different
stimuli: an approaching (expanding) disc causes excitation
(Fig. 3D); a spiral simulating expansion elicits no response
Fig. 4. Reponses of class 2 looming cells to expanding discs and
spirals. As in Figure 3, histograms A and boxes C, E, G, and I are from
one neuron; histogram B and boxes D, F, H, and J are from a second
neuron. Both are class 2 elements that are strongly excited by disc
expansion, outward spiral rotation, regressive motion, and downto-up motion. The cells are strongly inhibited by the respective opposite directions of motion or apparent motion.
Figure 4
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M. WICKLEIN AND N.J. STRAUSFELD
(Fig. 3E,F); and likewise, horizontal (Fig. 3G,H) and vertical movement of gratings (Fig. 3I,J) are ineffective. Class
1 neurons are further divided into two types: the 1a neurons just described are looming neurons, excited by an
approaching object and inhibited by a receding one; and
the type 1b neurons are antilooming neurons, inhibited by
an expanding object and excited by a receding one. Both
types of neurons respond feebly, if at all, when presented
with moving gratings.
Class 1 neurons distinguish expansion of the retinal
image from contraction. Contraction and expansion involve three variables: change of the perimeter length (edge
length) over the retina; change in the area of the image on
the retina; and change of luminance. Change in luminance
probably plays no role in looming or antilooming detection, however. A type 1a neuron that is activated by an
expansion of a white disc against a dark background is
equally activated by an expansion of a dark disc against a
pale background (Fig. 5), where the overall luminance
decreases.
Two cells, the activities of which are shown in Figure 4,
exemplify responses of class 2 neurons. The bar graph
again summarizes the response properties for each of the
two cells; each bar represents the mean response of five
repeated stimuli. The spike trains show responses of the
neurons to the same stimuli used to test class 1 cells (Fig.
3). Figure 4A and B demonstrates that class 2 cells are
sensitive to expanding and receding discs. Here, both cells
are type 2a looming neurons, being excited by expansion
and inhibited by contraction (Fig. 4B,C). The neurons
increase their firing rates to outwardly rotating spirals
(expansion) and decrease their firing rates to inwardly
rotating spirals (Fig. 4A,B,E,F). Thus, as in humans, the
outwardly rotating spiral provides the moth with the illusion of expansion. Again, class 2 neurons are divisible into
two types: 2a responding to looming stimuli, and type 2b
responding to receding (antilooming) stimuli.
Class 2 neurons also respond to horizontal and vertical
movement of gratings over the retina (Fig. 4G–J). The
preferred directions of the neuron summarized in Figure
4A are front-to-back (regressive horizontal movement;
Fig. 4G) and down-to-up vertical movement. The second
class 2 neuron (Fig. 4B) preferred downward vertical motion and regressive horizontal motion (Fig. 4I). In each
case null directions were indicated by a reduction of the
resting frequency.
Class 2 neurons were also tested with dark discs expanding or contracting against a pale background. As is
the case for class 1 neurons, the cells respond to changes
of image size regardless of whether the stimulus is darker
or lighter than the background—in other words, independently of luminance (Fig. 5).
Anatomical differences of class 1
and 2 neurons
The two classes of looming and antilooming neurons
differ in their morphology (Figs. 6 –14). Class 1 neurons
derive from cell bodies in the optic lobes. Their terminals
invade the ipsilateral optic foci of the posterior slope of the
midbrain. Their dendrites in the optic lobes are grouped
into three distinct arbors: one in the medulla resides exclusively in its innermost stratum; a second in the lobula
consists of processes that ascend through the lobula to
provide minute twigs within its outer stratum; and a third
in the lobula plate penetrates all its strata.
Fig. 5. Class 2 type 2 neuron, excited by disc contraction independent of luminance. Contraction of a white disc against a dark background (white columns) excited the cell, as does expansion of a dark
disc against a light background (filled columns).
Class 2 neurons derive from large cell bodies situated
posteriorly in the cell body rind between the calyces of the
mushroom bodies. The dendritic trees of class 2 neurons
reside in posterior slope neuropil on one or both sides of
the brain. Their axons extend out to the medulla on the
same or the opposite side of the brain as their cell bodies.
Their terminals in the optic lobes are restricted to the
outermost medulla stratum, extending across a large area
of its retinotopic mosaic.
Descriptions of class 1 neurons
Figure 6 shows a class 1 neuron filled with neurobiotin
and reconstructed from serial sections in the vertical
plane (normal to the z-axis). The cell responds with a
higher spike frequency to a receding stimulus but no activation when stimulated with an inward rotating spiral,
thereby indicating that it is an antilooming neuron. Its
dendritic branchlets (Fig. 7A) are arranged distoproximally in the innermost stratum of the medulla and in the
outermost stratum of the lobula where they arise from
stouter branches that ascend through this neuropil (Fig.
7B). A third level of dendrites branch throughout the
depth of the lobula plate (Fig. 7C).
Figure 8A and B shows a second class 1 neuron, the
dendrites of which are concentrated mainly within the lower
half of the medulla and lobula plate, suggesting that it is
activated by stimuli subtending the lower half of the visual
hemisphere. However, its dendritic field in the lobula extends across the dorsoventral extent of this neuropil. The
neuron terminates in the posterior slope of the midbrain.
Original spike trains are shown in response to stimulation
with an approaching or retreating disc (Fig. 8B) and an
outward or inward rotating spiral (Fig. 8C). These are the
typical response characteristics of type 1b antilooming neurons, showing selective activation to a receding stimulus but
no response to an inwardly rotating spiral.
Figure 9A illustrates a species of neuron that can be characterized as a type 1a looming cell, with a sustained response
to an expanding image on the retina (Fig. 9B). It is strongly
inhibited by a contracting image (Fig. 9B). An expanding
spiral may slightly diminish background frequency, although this is ambiguous (Fig. 9C). Dendrites in the medulla
LOOMING-SENSITIVE NEURONS IN MANDUCA
363
Fig. 6. Reconstruction and responses of a class 1 type 2 antilooming neuron. A: To show the full organization of this cell, the lobula is
shown separated to the right from the medulla and lobula plate.
Reconstruction of dendrites showing their distoproximal arrangements at three consecutive levels: in the medulla, lobula, and lobula
plate. Main branches ascend through the second chiasma to provide
retinotopic branchlets in the inner medulla stratum and through the
lobula plate. Branches ascend outward through the depth of the
lobula to its outer stratum, where they terminate. The inset shows an
enlargement of medulla arborizations. Abbreviations for this and
subsequent figures: me, medulla; lo, lobula; lop, lobula plate. B: Recording showing the response of this cell to an expanding and contracting disc and (C) to an outward rotating spiral. Scale bar ⫽ 100
␮m.
serve the lower frontal part of the eye’s receptive field as do
dendrites in the lobula plate. Dendrites in the lobula are
concentrated in its ventral half and have diffuse arrangements dorsally. An axon projects to neuropil flanking the
esophagus. A thin collateral leads off the axon to provide a
recurrent field of terminal processes situated at the base of
the optic peduncle (Fig. 9A, inset). Its terminal in the posterior slope was incompletely filled.
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M. WICKLEIN AND N.J. STRAUSFELD
Descriptions of class 2 neurons
The next four figures illustrate class 2 neurons, all of
which have their cell bodies and dendrites in the midbrain, as exemplified by the cell shown in Figure 10. This
cell, which has been reconstructed from serial horizontal
sections, has its terminal arborization confined to the outermost layer of the medulla. Its dendrites reside in the
lateral deutocerebrum but originate from a large cell body
situated contralaterally in the dorsal cell body rind. The
fine structure of the arborizations in the brain are suggestive of dendrites, indicating that this and other class 2
neurons receive their inputs in the midbrain (see Fig. 12
for details). This neuron showed spontaneous bursts of
activity, which were possibly autonomous to this cell type
but are more plausibly a consequence of injury and current leakage. The neuron responded to an expanding retinal image and was inhibited by a contracting one. It
showed an increase of spike frequency when stimulated
with an outward rotating spiral (illusory looming). The
initial response to this stimulus was phasic
The neurons shown in Figures 11, 13, and 14 were
reconstructed from frontal sections. Figure 11 shows a
neuron whose dendritic tree and cell body are contralateral to its terminal in the medulla. The latter provides a
dense field of beaded processes (Fig. 12A) that extend
across the whole outermost stratum of the medulla. The
dendrites branch profusely in the posterior slope of the
midbrain and some extend distally into the base of the
optic peduncle. Dendritic branches are visited by axons
originating from the lobula plate (Figs. 11A, inset, 12B,C).
The neuron is a looming cell showing an increase in activity in response to an approaching disc or an outwardly
rotating spiral.
Figure 13 shows a class 2 cell that has two fields of dendrites, one on the same side as its terminal in the medulla,
but contralateral to the cell body. Its axon prolongates over
the esophageal foramen to enter the posterior slope. Neurobiotin failed to resolve details of the dendritic tree on that
side, however. As in the previous examples, a dense system
of terminal processes extends across the outer stratum of the
medulla. The cell is a looming-sensitive neuron, typical of
other type 2a cells, responding to both an expanding disc and
outward spiral motion (Fig. 13A,B). The cell is completely
silent during stimulation with the nonpreferred direction of
movement, namely, the retreating disc or the inwardly rotating spiral.
The last example (Fig. 14) shows a type 2a neuron with
dendrites in the posterior slope of the midbrain. Its cell
body lies just at the other side of the midline, however.
The terminal processes in the medulla are restricted to the
lower half of the neuropil, in an area corresponding to the
lower half of the retina. Figure 14B and C shows that the
neuron is excited by an expanding retinal image and is
inhibited by a contracting image. An outwardly rotating
spiral vigorously excites the cell.
DISCUSSION
This paper describes neurons in the visual neuropil of
Manduca sexta that are tuned to looming stimuli. Two cell
classes can be distinguished, based on their anatomy and
their visually induced response properties. Class 1 neurons receive inputs in the inner stratum of the medulla,
the outer stratum of the lobula and through all strata of
the lobula plate. Their cell bodies are in the optic lobes,
and their axons end ipsilaterally in the posterior slope of
the midbrain. Class 2 cells are centrifugal neurons. They
receive their inputs in the posterior slope and terminate in
the medulla. Their cell bodies reside posteriorly near the
midline of the brain. The two classes of neurons also differ
in their physiological responses to visual stimuli. Class 1
cells respond only to an approaching or retreating disc but
not to a rotating spiral or moving grating. Class 2 cells
respond to both types of stimuli. The different response
properties of the neurons suggest that they detect different stimulus properties associated with looming.
In nature, tobacco hawk moths feed while they hover in
front of a flower that is selected by the moth for its high
contrast (Raguso et al., unpublished data). While feeding,
moths maintain a constant distance from the flower using
visual cues (Wicklein and Willis, unpublished data). Three
features of the target could provide information that is
used by the visual system to detect change of distance: 1)
change in the area of the retinal image; 2) change in
length of the image perimeter; and 3) detection of flow
field properties, such as expanding or contracting edges.
The present results demonstrate that at least two of these
mechanisms are realized within one and the same system.
Two components of the stimulus can provide information about change in area: a change in luminance detected
by pooling the intensity signal across the whole retina; or
detection of change of image size on the retina by recruitment or divestment of individually stimulated retinotopic
channels. Cells that increase their firing rate to an expanding bright target against a darker background also
respond in the same way to an expanding dark target
against a bright background. This excludes the possibility
that the effective signal is increased luminance because
the second stimulus condition is associated with an overall
luminance decrease.
Do changes in perimeter length of the image provide
information about looming? To test this, neurons were
stimulated with expanding and contracting discs. Their
responses were compared with responses elicited by inwardly and outwardly rotating spirals. A looming object
expanding over the retina’s surface not only provides outward moving edges but also increasing area and an increase of perimeter (edge) length. A spiral rotated in one
direction provides an outward motion of edges and in the
opposite direction an inward motion of edges, in both cases
without any change in area or perimeter length. Thus, if
perimeter length is a key stimulus for looming detection,
an outwardly rotating spiral should not excite a cell that is
excited by an expanding disc.
Fig. 7. Features of the neuron shown in Figure 6. A: Tufts are
restricted to the innermost stratum (is) of the medulla’s inner layer
(il). The outer layer (ol) and serpentine layer (sl) are not invaded by
this neuron. B: Detail showing branches ascending through the inner
medulla strata (is). C: Branches also arborize through the outer (ol)
and inner (il) layers of the lobula. The inset (from the boxed area)
shows delicate brushlets of processes that invade the outer lobula
layer, at the level of terminals of type 1 transmedullary cells from the
medulla (Fig. 15). D: A third system of branches invades the lobula
plate, extending from its outer surface through all three directionand orientation-specific layers (1–3). p op t; groove in the outer surface
of the lobula carrying axons of the posterior optic tract. Scale bars ⫽
50 ␮m in A; 25 ␮m in B; 10 ␮m in C,D.
Figure 7
366
M. WICKLEIN AND N.J. STRAUSFELD
Fig. 8. A class 1 type 2 antilooming neuron. A: Localized dendritic
fields in the ventral medulla and lobula plate and across the lobula.
The axons terminate in the posterior slope (p sl). Other abbreviations:
la, lamina; mb, calyx and initial part of pedunculus of the mushroom
body; p sl, posterior slope. B: Responses to disc expansion (inhibition)
and contraction (excitation) and (C) an unchanged background activity during stimulation with an expanding (or contracting: not shown)
spiral. Scale bars ⫽ 100 ␮m.
The present results show that only class 1 neurons are
activated by expanding or contracting discs but not by
the rotating spiral. We conclude that these neurons are
tuned to changes of retinal image area or changes of
perimeter (edge) length and that luminance and moving
edges play no role in their activation. Class 2 neurons
respond to both expanding and contracting discs and to
rotating spirals. This shows that outward and inward
movement of edges are their effective stimuli and that
luminance, perimeter length, and changes of retinal
image area are not necessary to excite this class of cells.
In addition to their responses to spirals, class 2 cells are
activated by moving gratings whereas class 1 cells are
not. This supports the idea that class 2 cells are tuned
to directional motion whereas class 1 cells are not. Responses of class 2 cells to changes of image size are
independent of luminance. This fits well with natural
conditions. A white flower illuminated by moonlight or
starlight will appear pale against a dark background.
The same flower may offer a dark profile against a pale
background (the sky) at and just after sunset when the
moths are already foraging.
Anatomical polarities of class 1
and 2 neurons
Both class 1 and 2 neurons are conventional nerve
cells in that they have two separate fields of arborizations connected by an axon. Arborizations that are interpreted as dendrites can be either smooth or tapering,
or can comprise many tapering branchlets equipped
with knobs, spines, or varicosities. Structures inter-
LOOMING-SENSITIVE NEURONS IN MANDUCA
367
Fig. 9. A class 1 type 1 looming neuron. A: Reconstruction showing
that its processes are arranged across the central part of the medulla’s
retinotopic mosaic. Processes are distributed across the whole lobula
and lobula plate. The terminals arborize extensively in the neuropil,
connecting the brain with the subesophageal ganglion. A small collateral from the axon provides a field of blebbed processes (boxed; see
inset) within the optic peduncle, suggesting their relationship with
axons of other visual interneurons. B: Spike train showing excitation
elicited by an expanding disc and inhibition to a contracting disc.
C: There are no responses to an expanding spiral. Scale bar ⫽ 100 ␮m
in A; 5 ␮m in inset.
preted as terminals are composed of arborizations decorated with varicosities or bead-like specializations
(Strausfeld and Meinertzhagen, 1998). Reconstructions
of class 1 and 2 neurons suggest that the two classes are
differently polarized: class 1 neurons are centripetal,
carrying information centrally from dendrites in optic
lobe neuropils to terminals in the midbrain; class 2
neurons are centrifugal, relaying information from their
dendrites in the posterior slope out to their terminals in
the medulla. The locations of their cell bodies support
these polarities: class 1 neurons have cell bodies in the
optic lobes, and class 2 neurons have their cell bodies in
the posterior rind of the midbrain. One caveat to these
anatomical inferences is that neurons in insects, as in
368
M. WICKLEIN AND N.J. STRAUSFELD
Fig. 10. Class 2 type 1 neuron. A: Reconstruction from serial
horizontal sections shows the peripheral location of its terminals at
the medulla’s outer surface and a densely branched dendritic tree
ipsilaterally in the midbrain and its contralateral cell body. ant lob,
antennal lobe. Other abbreviations as for previous figures. Although
background bursts of spikes may be injury responses, the cell nevertheless showed a clear response to disc expansion (B) and to spiral
expansion (C). Scale bar ⫽ 100 ␮m.
deuterostomes, can have pre- and postsynaptic structures on the same arbor. We were not able to perform
physiological experiments confirming the assumed direction of information flow.
Centrifugal cells have been previously described from
the butterfly Papilio aegeus (Ibbotson et al., 1991) and
from Manduca sexta (Milde, 1993). Ibbotson et al. (1991)
found motion-sensitive neurons that connect the midbrain
to the medulla. These neurons (termed MV1 and MH1)
have cell bodies close to the midline of the brain. Their
smooth tapering arborizations are located in the posterior
slope, and their wide-field terminals reside in the outer
layers of the medulla. These cells correspond structurally
to the class 2 neurons described here. In Papilio, MH1
cells are selectively activated by horizontal motion with a
preferred and null direction. MV1 neurons respond to
vertical motion, also with a preferred and null direction.
Interestingly, the MH1 and MV1 cells of Papilio are tuned
only to horizontal or to vertical gratings, respectively,
whereas the present class 2 cells in Manduca are sensitive
to movement in both orientations, with a null and preferred direction for each. Cells termed cMt by Milde (1993)
have their cell bodies located dorsally, close to the brain’s
midline. Like the class 2 neurons described here, cMt
neurons have smooth tapering processes in the posterior
slope and beaded arborizations in the outer third of the
medulla. Whereas processes of class 2 cells extend
through large areas of the medulla’s retinotopic mosaic,
the terminals of cMt cells are restricted to small patches of
the medulla. The cMt neurons are motion sensitive and
some show directional selectivity (Milde, 1993). Neither
cMt nor MH1 and MV1 neurons were tested with looming
stimuli and thus cannot be further compared with class 2
neurons described here. We assume that all these
neurons— class 2 cells, cMt, MH1, and MV1 neurons—
belong to a larger category of motion-sensitive elements
that are supplied by afferents in the posterior slope and
feed information back to the medulla. The nature of their
inputs is discussed next.
Polarities of looming neurons
Observations of dipteran visual systems reveal that the
posterior slope of the midbrain is the main termination
area for wide-field motion-sensitive collator neurons originating in the lobula plate (Strausfeld and Bassemir,
1985a,b; Strausfeld and Gronenberg, 1990). These direc-
LOOMING-SENSITIVE NEURONS IN MANDUCA
369
Fig. 11. A class 2 type 1 neuron reconstructed from vertical sections. A: Its large dendritic tree in the right side of the brain provides
an axon that terminates in the lower frontal two-thirds of the contralateral medulla, there providing a dense plexus of beaded processes. Its dendritic tree is subdivided into branchlets (inset, upper
right) each providing a claw-like group of processes that are invested
with knobs and spicules. These appear to clasp the passage of un-
stained axons that project from the right lobula complex (not shown)
to the posterior slope. B: This neuron showed a high level of autonomous activity. The firing rate increased in response to an expanding
disc but returned to its initial rate in response to a contracting disc.
C: Its firing rate increased in response to an outwardly rotating spiral.
Scale bar ⫽ 100 ␮m.
tionally selective neurons, which are variously tuned to
specific orientations of motion, provide inputs to the dendritic trees of premotor descending neurons and are
thought to be also presynaptic to a variety of centrifugal
and heterolateral cells that extend back out into the optic
lobes (Hausen and Egelhaaf, 1989; Strausfeld et al., 1995).
The present results suggest that class 2 neurons are comparable to centrifugal neurons in Diptera, like the CH
neuron (Hausen and Egelhaaf, 1989). Moreover, observations of biocytin-filled class 2 neurons reveal that their
dendrites in the posterior slope are visited by many largediameter profiles from the lobula plate.
Given this anatomical relationship, it is not surprising
that class 2 neurons are orientation and directionally motion sensitive. They respond to a preferred vertical as well
as to a preferred horizontal direction, suggesting that
their dendrites receive inputs from two converging popu-
lations of lobula plate outputs: horizontal motion-sensitive
neurons and vertical motion-sensitive neurons. Recent
studies of the diurnal hummingbird hawk moth Macroglossum stellatarum (Wicklein and Varju, 1999) have confirmed that the sphingid lobula plate indeed provides
separate populations of vertical and horizontal motionsensitive tangential neurons that are probably homologous to the horizontal and vertical motion-sensitive neurons of many brachyceran Diptera (Buschbeck and
Strausfeld, 1997).
Organization among the dendrites of class 1 neurons is
more difficult to interpret. The entire dendritic tree provides three orderly and spatially segregated systems of
branches that supply distal processes to three discrete
levels of optic lobe: the inner stratum of the medulla; the
outermost stratum of the lobula; and all strata through
the lobula plate. Each branching system originates from a
370
Fig. 12. Details of the class 2 neuron shown in Figure 11.
A: Terminal processes are equipped with bead-like specializations
(arrowed circle) that invade retinotopic columns. B: This panel shows
an enlargement of the area boxed to the right in Figure 11, identifying
M. WICKLEIN AND N.J. STRAUSFELD
dendritic processes in the posterior slope. C: Enlargements showing
the clawed appearance of dendritic branchlets (arrows and boxes).
D: Enlargement of dendritic tuft boxed to the right in B. Scale bars ⫽
10 ␮m.
LOOMING-SENSITIVE NEURONS IN MANDUCA
371
Fig. 13. Reconstruction of a class 2 type 1 neuron showing its terminal in the outer stratum of the
medulla. A: The bilateral dendritic tree is shown only in part with the right component indicated by the
dotted line. B: The neuron showed almost no background activity and was excited by disc expansion and
outward spiral motion (C). Scale bar ⫽ 100 ␮m.
large diameter axon. Branches are ordered: those supplying
the medulla are distal to branches supplying the lobula;
these are distal to branches supplying the lobula plate.
In sphingids, as in brachyceran Diptera, levels receiving
class 1 neuron dendrites are specifically associated with
small-field retinotopic neurons that contribute to a discrete motion processing pathway (Douglass and Strausfeld, 1995, 1997) that is composed of elementary
movement-detecting circuits (EMDs; Buchner, 1976).
These circuits are repeated across the optic lobes, each
circuit detecting the sequential change of luminance between adjacent visual sampling points (Franceschini et
al., 1989). An integral part of each EMD is the type 1
transmedullary neuron (Tm1), which occurs in each retinotopic column. Tm1 neurons, which are common to both
flies and sphingid moths (Fig. 15), are modulated by directional motion (Douglass and Strausfeld, 1995). Their
collaterals invade the innermost medullary stratum, and
their endings terminate in the outermost stratum of the
lobula (Fig. 15). Tm1 neurons supply information about
movement to the next processing stage, the bushy T-cells
(T5), whose dendrites reside in the outer stratum of the
lobula, as do dendritic twigs of class 1 neurons. Bushy
T-cells terminate in orientation- and direction-specific
strata in the lobula plate. Again, class 1 neuron dendrites
invade these layers.
The coincidence of three levels of class 1 dendrites and
the respective levels of Tm1 collaterals, Tm1 terminals,
and T5 endings suggests that class 1 neuron dendrites
receive inputs from movement-sensitive neurons across
the whole eye. Why, though, should this occur thrice, at
successive levels in the system? And why should class 1
neurons be “blind” to translatory motion if they are supplied by motion-sensitive elements?
372
M. WICKLEIN AND N.J. STRAUSFELD
Fig. 14. A class 2 type 1 neuron with a dendritic tree in the ipsilateral posterior slope, but derived
from a contralateral cell body. A: The terminal in the medulla invades only the lower half of the
retinotopic mosaic. B: This neuron responded to an expanding disc and (C) to an outwardly expanding
spiral. Scale bar ⫽ 100 ␮m.
One suggestion is that the relationship between Tm1 neurons and class 1 neuron dendrites in the medulla and outer
lobula allows successive temporal sampling of changes of
contrast between adjacent ommatidia, as would occur when
the image perimeter expands or contracts across the retina.
In the medulla and lobula, each dendritic branchlet of a class
1 neuron thus serves as an edge detector. The neurons’ lack
of response to direction may be explained by directional
information being discarded due to interactions in the lobula
plate where class 1 neuron dendrites invade all its directionally sensitive layers.
The question of why dendrites of class 1 neurons are
distributed successively within the three optic lobe neuropils is more challenging. One possibility is that duplicating inputs from Tm1 at two levels adds redundancy to the
system and thus increases accuracy. Alternatively, dendrites staggered at three successive levels may enable the
cell to detect temporal and spatial displacements of the
image edge from one ommatidium to the next during im-
age expansion or contraction. Estimations of change of
length of the image edge as it contracts or expands across
the retina will be a function of the number of edge detectors triggered by edge motion. This calculation can ill
Fig. 15. Camera lucida drawings of small-field retinotopic neurons
homologous to those that are known to be involved in elementary
motion-detecting circuits in Diptera and that terminate in layers that
are invaded by the dendrites of class 1 looming neurons. Small and
wide-field lamina monopolar cells (L2s, w) terminate in the outer
stratum of the medulla, at the level of terminals of class 2 looming
neurons and the dendrites of type 1 small and wide-field transmedullary neurons (Tm1 s,w). These second-order neurons provide narrow
or wide-field collaterals within the inner stratum of the medulla and
terminals in the outer stratum of the lobula. The dendrites of bushy
T-cells (T4; see inset for details, T5) originate at these levels, each
sending an axon into motion- and orientation-specific levels in the
lobula plate (lop). These layers, like the inner stratum of the medulla
and outer stratum of the lobula are invaded by the class 1 neuron
dendrites. Scale bar ⫽ 100 ␮m.
Figure 15
374
afford noise incurred by synaptic delays, as velocities of
image edge expansion/contraction over the retina while
hovering are fast, calculated as 0.2– 0.4 m/sec. Possibly,
activated edge detectors are pooled first in the medulla,
then in the lobula and then in the lobula plate. The organization of class 1 dendrites at these levels could allow
comparison of rapid change in perimeter distribution on
the retina by integrating edge detection within the dendritic tree at time t1 for inputs to the medulla, t2 for inputs
to the lobula, and t3 for inputs to the lobula plate.
Why are there looming and antilooming
systems? Comparisons with other taxa
The present results identify two classes of neurons that
detect movement along the z-axis: cells that are insensitive (class 1) or sensitive to motion cues (class 2). Both
classes are further divided into two types (1, looming; 2,
antilooming) that are excited by expansion or contraction,
respectively, of a retinal image. If each of these cell types
can alone signal change of depth, what is the advantage of
having a system comprising four types of neurons that
signal the same event?
In locusts and flies a single system of neurons detects
looming to provide information about the expansion of an
image over the retina. This information triggers an escape
jump or a landing response, depending on the context in
which the stimulus is encountered. In locusts this behavior is mediated, in part, by the lobula giant motion detector (LGMD), which synapses onto the descending contralateral motion detector (DCMD: Rowell, 1971; Rind,
1984)—itself supplying the hind-leg jump circuit. The
LGMD responds to a looming stimulus that may (Gabbiani et al., 1999) or may not (Rind and Simmons, 1999)
signal time-to-collision.
In flies there is an isomorphic array of several hundred
columnar neurons called Col A cells that subserve the
whole eye. Each cell is triggered by a darkening stimulus
entering its receptive field (Gilbert and Strausfeld, 1991),
and the axon of each Col A cell is coupled by gap junctions
to dendrites of the giant descending neuron (GDN) that
supplies the midleg jump circuit (Strausfeld and Bassemir, 1983). This system reacts to a stimulus threshold
defined by perimeter length of the retinal image rather
than to time-to-collision (Holmqvist and Srinivasan,
1992).
Swimming, flying, or arboreal animals would be expected to possess neuronal circuits that inform motor
pathways about impending collision with objects. Such
circuits are not exclusive to insects. They have been proposed for crustaceans (Glantz, 1974), and these circuits
are likely to have evolved in arthropods before their appearance in chordates. Neurons having similar morphologies to those seen in insect looming systems have been
identified in the nucleus rotundus of the pigeon (Luksch et
al., 1998) and in layer SGS1 of the squirrel optic tectum
(Major, Luksch and Karten, unpublished data). In each
case the dendrites of these cells provide broad fan-like
arrays of dendritic branches, each providing a small
bottlebrush-like tuft onto which centripetal afferents terminate. As in the class 1 looming neurons, the shapes of
these elements suggest a precise retinotopic input onto
them from quite large areas of the visual field, which is
M. WICKLEIN AND N.J. STRAUSFELD
also the case for the locust LGMD and the ensemble of Col
A cells in flies.
There are interesting parallels between the selective
properties of wide-field neurons in the looming subdivisions of the nucleus rotundus of pigeons (Wang and
Frost, 1992; Wang et al., 1993) and type 1 class 1 and
type 1 class 2 neurons in Manduca. In the nucleus
rotundus, wide-field neurons are segregated at two discrete levels, the more distal cells receptive to velocity on
the x and y axes, the deeper neurons tuned to motion on
the z axis (Wang et al., 1993). Like class 1 cells, these
neurons are not sensitive to directional motion but do
respond to image expansion. Sun and Frost (1998) show
three types of looming-sensitive neurons in the nucleus
rotundus: one computing the relative rate of expansion;
the second the absolute rate of expansion; and a third
multiplying size-dependent responses and instantaneous angular velocity of the object in a manner comparable to findings from the locust LGMD (compare
Gabbiani et al., 1999).
What is so remarkable about these various systems is
the apparent evolutionary convergence— both of morphological attributes and of functional properties—suggesting
that common selective pressures, such as the necessity of
avoiding objects during fast movements in three dimensions (water or air), have forced similar adaptations across
phyla. It would be most interesting to study the nucleus
rotundus of hummingbirds because, unlike pigeons, hummingbirds use looming and receding stimuli as cues for
stationary flight. In this they show an exquisite parallel to
sphingid hovering, a behavior that is utterly distinct from
the escape and landing responses of locusts that rely
solely on looming stimuli.
There are similarities between the organization of
control systems for stationary and for forward flight.
During stationary flight the flight motor continuously
makes compensatory fine adjustments to adjust for visual drift—signifying changing position toward or away
from the target (Pfaff and Varju, 1991; Farina et al.,
1994, 1995; Kern and Varju, 1998). In a comparable
manner, compensatory movements stabilize the platform along the pitch, yaw, and roll axes to maintain
visual balance during forward flight (Hausen and
Egelhaaf, 1989). Although it could be argued that for
forward flight one system would suffice to detect progressive and regressive horizontal motion, it seems that
this is insufficient for fine control: several orthogonal
systems have evolved that provide highly accurate information about all extractable features of the visual
flow field (Hausen, 1982; Krapp and Hengstenberg,
1996). The several systems supporting stationary flight
are likewise organized as discrete assemblages of nerve
cells that collaborate in the detection of movement
along the z axis. Even though systems of looming neurons in moths are crudely distinguished as motionindependent and motion-dependent neurons, their four
divisions into looming and antilooming neurons described here may not be the minimum required for
precise control of stationary flight. In this account we
have shown that for any type of looming or antilooming
neuron there exist several distinct morphs or species of
nerve cells that have different field dimensions across
the neuropil. Possibly, many of these morphological
types are required to provide the exquisite accuracy of
motor control manifested by the moth’s behavior. Com-
LOOMING-SENSITIVE NEURONS IN MANDUCA
parative studies of insects that show different grades of
visually guided behaviors may resolve the question of
whether systems of looming and antilooming neurons
derive from an evolutionary elaboration of a simpler
and phylogenetically basal system, like the locust
LGMD.
ACKNOWLEDGMENTS
We thank Albert Brower, BS, for instructions in imaging and digital montaging, Robert Gomez, BS, for technical help, and A.A. Osman, PhD, for rearing moths. We are
grateful to Holly Campbell, PhD, for discussing various
aspects of this work. This account has also profited from
Harvey Karten’s (MD) valuable insights about avian visual systems. N.J.S. received a grant from the NIH National Center for Research Resources (RR08688), and
M.W. received a Feodor Lynen stipend from the A. v.
Humboldt Foundation, as well as small grants from the
University of Arizona’s NSF Integrative Graduate Education and Research Traineeship Program and NSF PlantInsect Interactions Group.
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