Download Perception of Motion, Depth, and Form

28
Perceptionof Motion, Depth, and Form
The Parvocellularand Magnocellular PathwaysFeed Into
Two ProcessingPathwavsin ExtrastriateCortex
Motion Is Analyzed Primarily in the Dorsal Pathwav to the
ParietalCorter
Motion Is Representedin the Middle Temporal Area
Cells in MT Solve the Aperture Problem
Control of Movement Is SelectivelyImpaired by Lesions
of MT
Perceptionof Motion Is Altered by Lesions and
Microstimulation of MT
D e p t h V i s i o n D e p e n d so n M o n o c u l a rC u e sa n d B i n o c u l a r
Disparity
Monocular Cues CreateFar-FieldDepth Perception
StereoscopicCues CreateNear-Field Depth Perception
Information From the Tu'o Eyes Is First Combined in the
Primary Visual Cortex
Random Dot StereogramsSeparateStereopsisFrom
Object Vision
Obiect Vision Depends on the Ventral Pathwav to the Inferior
Temporal Lobe
Cells in V2 Respond to Both Illusorv and Actual
Contours
Cells in V4 Respond to Form
Recognition of Facesand Other Complex Forms Depends
Upon the Inferior Temporal Cortex
Visual Attention FacilitatesCoordination Between Separate
Visual Pathways
The Binding Problem in the Visual Svstem
An Overall View
N vISIoN,AS IN orHERmental oPerations, we exPerrence the world as a whole. Independent attributesmotion, depth, form, and color-are coordinated
into a single visual image. In the two Previous chapters
we began to consider how two parallel Pathways-the
magnocellular and parvocellular pathways, that extend
from the retina through the lateral geniculate nucleus of
the thalamus to the primary visual (striate) cortexmight procluce a coherent visual image. In this chapter
we examine how the information from these two pathways feeds into multiple higher-order centers of visual
processingin the extrastriate cortex. How do these pathways contribute to our PercePtion of motion, depth,
form, and color?
The magnocellular (M) and parvocellular (P) pathways feed into two extrastriate cortical pathways: a dorsal pathway and a ventral pathway. In this chapter we
examine, in cell-biological terms, the information processingin each of these pathwaYs.
We shall first consider the perception of motion and
depth, mediated in large part by the dorsal pathway to
the posierior parietal cortex. We then consider the perception of contrast and contours, mediated largely by
the ventral pathway extending to the inferior temporal
cortex. This pathway also is concerned with the assessment of color, which r.r,'ewill consider in Chapter 29'
Finally, we shall consider the binding problem in the
visual system: how information conveyed in parallel
but separatepathways is brought together into a coherent perception.
Chapter 28 / Perceptionof Motion, Depth, and Form
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A . S u b r e g i o n isn V ' l ( a r e a1 7 )a n d V 2 ( a r e a1 B ) .T h i s s e c t i o n
f r o m t h e o c c i p i t allo b e o i a s q u i r r e m
l o n k e ya t t h e b o r d e ro f a r '1
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s t r i p e si n V 2 . ( C o u r t e s yo f M L i v i n g s t o n e . )
The Parvocellular and Magnocellular
Feed Into Two Processing Pathways
in Extrastriate Cortex
Pathways
In Chapter 27 we saw that the parallel parvocellular
and magnocellular pathways remain segregatedeven in
the striate cortex. What happens to these P and M pathways beyond the striate cortex? Early researchon these
pathways indicated tirat the P pathway continues in the
ventral cortical pathway that extends to the inferior
temporal cortex, and that the M pathway becomes the
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dorsal pathway that extends to the posterior parietal
cortex. However, the actual relationships are probably
not so exclusive.
The evidence for separation of function of the dorsal and ventral pathways begins in the primary visual,
or striate, cortex (V1). Staining for the mitochondrial enzyme cytochrome oxidase reveals a precise and repeating pattern of dark, peg-like regions about 0.2 mrn in diameter called blobs. The blobs are especially prominent
in the superficial layers 2 and 3, where they are separated by intervening regions that stain iighter, ti're in-
550
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pathways n severalcorticalareas.The parietalpathwayre-
c e r v e si n p u tf r o m t h e M p a t h w a yb u t o n y t h e t e m p o r a p a t h w a y T e c ev e s i n p u tf r o m b o t h t h e M a n d P p a t h w a y s (. A b b r e v i l r e a ,C I T - c e n t r a i n f e a t i o n s :A I T : a n t e r i o ri n f e r o rt e m p o r a a
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terblob regions. The same stain also reveals alternating thick and thin stripes separated bv interstripes of
little activity (Figure 28-1 in the secondarv visual cortex,
or V2).
Margaret Livingstone and David Hubel identified
the anatomical connections between labeled regions in
V1 and V2 (Figure 28-18). They found that the P and M
pathways remain partially segregated through V2. The
M pathway projects from the magnocellular layers of the
lateral geniculate nucleus to the striate cortex, first to
laver 4Cct and then to layer 48. Cells in layer 48 project
directly to the middle temporal area (MT) and also to the
thick stripes in V2, from which cells also project to MT.
Thus, a clear anatomical pathrt'ay exists from the magnocellular lavers in the lateral geniculate nucleus to MT and
from there to the posterior parietal cortex (Figure 28-2).
Cells in the parvocellular layers of the lateral geniculate nucleus project to layer 4CB in the striate cortex,
from u'hich cells project to the blobs and interblobs of
Vl. The biobs send a strong projection to the thin stripes
in V2, whereas interblobs send a strong projection to the
interstripes in V2. The thin stripe and interstripe areas
of V2 mav in turn project to discrete subregions of Y4,
thus maintaining this separation in the P pathwav into
V4 and possibiy on into the inferior temporal cortex. A
pathway from the P cells in the lateral geniculate nu-
55'1
Chapter 28 / PercePtionof Motion, Depth, ancl Form
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magnocellular layers of the lateral geniculate nucleus
eliminates the responses of many cells in MT and always reduces the responses of the remaining cells;
biocking the parvocellular layers produces a much
weaker effect on cells in MT. In contrast, blocking the activity of either the parvocellular or magnocellular layers
in the lateral geniculate nucleus reduces the activity of
neurons in V4. Thus, the dorsal pathwav to MT seems
primarily to include input from the M pathway,
whereas the ventral pathwav to the inferior temporal
cortex appears to inciude input from both the M and P
pathways. We can now see that there is substantial segregation of the P and M pathways up to V1, probably
mn,iomont
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cleus to the inferior temporal cortex can therefore also
be identified (Figure 28-2).
But are these pathways exclusive of each other?
Several anatomical observations suggest that they are
not. In V1 both the magnocellular and parvocellular
pathways have inputs in the blobs, and local neurons
make extensive connections between the blob and interblob compartments. In V2 cross connections exist between the stripe compartments. Thus, the separation is
not absolute, but whether there is an intermixing of the
M and P contributions or whether the cross connections
allow one cortical pathr,r'ayto modulate activity in the
other is not clear.
Results of experiments that selectively inactivate
the P and M pathways as they pass through the lateral
geniculate nucleus (described in Chapter 27) also erode
the notion of strict segregationbetween the pathways in
V1. Blocking of either pathway affects the responsesof
fewer than half the neurons in V1, which indicates that
most V1 neurons receive physiologically effective inputs from both pathways. Further work has shown that
the responsesof neurons both n'ithin and outside of the
blobs in the superficial lavers of V1 are altered bv blocking only the M pathway. Both observations suggest that
there is incomplete segregation of the M and P pathways in V1.
This selective blocking of the P and M pathways
also reveals the relative contributions of the pathways to
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t h a t t h e v i s u a l s y s t e m a n a l y z e sm o t i o n i n a s e p a r a t ep a t h way.
d s a s e q u e n c eo f v s u a s e n s a A . A c t u a lm o t i o ni s e x p e r i e n c e a
t i o n s ,e a c h r e s ut i n g f r o m t h e m a g ef a l l i n go n a d i f f e r e n tp o s i tionin the retna.
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T h u s ,w h e n t w o g h t s a t p o s i to n s 1 a n d 2 a r e t u r n e do n a n d
o f f a t s u i t a b l el n t e r v a l sw, e p e r c e i v ea s i n ge l i g h tm o v i n gb e t w e e n t h e t w o p o i n t s .T h s p e r c e p t u ai l l u s i o nc a n n o tb e e x p a i n e db y p r o c e s s i n g
t n b a s e do n d i f f e r e n tr e t i n a l
of informao
p o s i to n s a n d i s t h e r e f o r ee v i d e n c ef o r t h e e x i s t e n c eo f a s p e c i a v i s u a ls y s t e mf o r t h e d e t e c t j o no f m o t i o n ( F r o mH o c h b e r g
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PartV/Perception
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M o t i o ns t u d y :
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Motion Is Analyzed Primarily in the Dorsal
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Corona
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F i g u r e2 8 - 5 S e p a r a t eh u m a n b r a i n a r e a sa r e a c t i v a t e db y
motion and color.
M o t i o n s t u d i e s .S r xs u b l e c t sv r e w e da b l a c l a n d w h i t e
r a n d o m - o opta t t e ' ^t h a l m o v e dr n o n e o ' e r g h td r e c t , o r so ' r e
m a i n e ds t a t i o n a r yT.h e f i g u r es h o w s t h e e f f e c to f m o t i o nb e c a u s et h e P E Ts c a n st a k e nw h l e t h e p a t t e r nw a s s t a t i o n a r y
w e r e s u b t r a c t e df r o m t h o s e t a l . e nw h l e t h e p a t t e r nw a s m o v i n g .T h e w h i t e a n d r e d a r e a s s h o w t h e h g h e n d o f a c t i v i t y( i n c r e a s e db l o o df l o w ) T h e a r e a sa r e l o c a t e do n t h e c o n v e x i t yo f
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a random assortment of equally connected areas.There
is substantial evidence for two major processing pathways, a dorsal one to the posterior parietal cortex and a
ventral one to the inferior temporal cortex, but other
pathways may also exist. Second, there is strong evidence that the processingin these two cortical pathways
is hierarchical. Each level has strong projections to the
next level (and projections back), and the type of visual
processing changes systematically from one level to the
next. Third, the functions of cortical areasin the two cortical pathways are strikingly different, as judged both
by the anatomical connections and the cellular activity
considered in this chapter and by the behavioral and
brain imaging evidence discussed in Chapter 25.
Our examination of the functional organization
r,r,'ithinthese vast regions of extrastriate visual cortex begins with the dorsai cortical pathway and the most intensivelv studied visual attribute, motion. We then examine the processing of depth information in the dorsal
pathway. Finally, we turn to the ventral cortical pathway and consider the processing of information related
to form. Color vision is the subject of the next chapter.
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separation into V2, a likely predon-rinance of tl-re M
input to the dorsal pathway to MT and the parietal
cortex, and a mixture of P and M input into the pathway leading to the inferior temporal lobe (as indicated
by the lines crossinp;between the pathways in Figure
28-2).
Wllat shoulc1we conclude about the organization of
visual processing throughout the multiple areas of the
visual cortex? First, we knor,r'that there are specific serial pathways through the multiple visual areas,not just
We usually think of motion as an object moving in the
visual field, a car or a tennis ball, and we easily distinguish these moving objects from the stationary background. However, we often seeobjectsin motion not because they move on our retina, but because we track
them with eve movements; the image remains stationary on the retina but we perceive movement because
our eyes move (Figure 28-3).
Motion in the visual field is detected by comparing
the position of images recorded at different times. Since
most cells in the visuai svstem are exquisitely sensitive
to retinal position and can resolve events separated in
time by 10 to 20 milliseconds, most cells in the visual
system should, in principle, be able to extract information about motion from the position of the image on the
retina by comparing the previous location of an object
with its current location. What then is the evidence for a
special neural subsystem specialized for motion?
The initial evidence for a special mechanism designed to detect motion independent of retinal position
came from psychophysical observations on appnrerttmotion, an illusion of motion that occurs when lights separated in spaceare turned on and off at appropriate intervals (Figure 28-4). The perception of motion of objects
that in fact have not changed position suggeststhat position and motion are signaled by separatepathways.
Chapter 28 / Perceptionof Motion, Depth, and Form
553
Box 28-1 Optic Flow
-"isual fieid
Optic flon' refers to the perceived motion of the
that results from an individual's own movement through the
environment. With optic flow the entire visual field moves, in
contrast to the local motion of objects.Optic flor'r,'providestwo
types of cues: information about the orgarrization of the environment (near objects will move faster than more distant objects) and information about the control of posture (side-toside patterns induce body sway). Particularlv influential in the
development of ideas about optic flow was the demonstration
bv the experimental psychologist James J' Gibson that optic
flow is critical for indicating the direction of obsen'er movement ("heading"). For example, n'hen an individual moves
forward with eyes and head directed straight ahead, optic
flow expands outward from a point straight ahead in the visual field, a pattern that is frequently used in movies to show
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Motion Is Represented in the Middle Temporal Area
Experiments on monkeys show that neurons in the
retina and lateral geniculate nucleus, as well as many
areas in the striate and extrastriate cortex, respond very
well to a spot of light moving across their receptive
fields. In area V1, however, cells respond to motion in
orredirection, while motion in the opposite direction has
little or no effect on them. This directionalselectiaittlis
prominent among cells in layer 48 of the striate cortex.
Thus, cells in the M pathway provide input to cells in
48, but these input cells themselves do not show directional selectivity.They simply provide the raw input for
the directionally selective cortical cells.
In monkeys one area at the edge of the parietal cortex, the middle temporal area (MT), appears to be devoted to motion processingbecausealmost all of the cells
are directionally selectiveand the activity of onlv a small
fraction of these cells is substaniially aitered by ihe shape
or the color of the moving stimulus. Like V1, MT has a
retinotopic map of the contralateral visual fieid, but the
receptive fields of cells within this map are about 10
times wider than those of cells in the striate cortex. Cells
with similar directional specificity are organized into
vertical columns running from the surface of the cortex
to the white matter. Each part of the visual field is represented by a set of columns in which cells respond to different directions of motion in that part of the visual fieid.
This columnar organization is similar to that seenin V1.
Cells in MT respond to motion of spots or bars of
Where is optic flon' represented in the brain? Neurons in
one region of the medial superior temporal area of the parietal
cortex in monkevs respond in rvays that rvould make these
cells ideal candidates to analvze optic flor'r'.These neurons respond selectively to motion, have recePtive fields that cover
large parts of the visual field, and respond preferer-rtiallyto
large-field motion in the'n'isualfield. Additionally, the neurons
are sensitive to shifts in the origin of full-field motion and to
differences in speed between the center and peripherv of the
field. The neurons also receive input related to eye movement,
r,r,'hichis particularly significant becausefonvard movement is
typically accompanied by eve and head movement. Finally,
electrical stimulation of this area alters the abilitv of the monkev to locate the point of origin of field motion, providing further evidence that the superior temporal area of the parietal
cortex is important for optic flor,l'.
light by detecting contrasts in luminance. Some cells in
MT also respond to moving forms that are not defined
by differences in luminance but by differences only in
texture or color. While tl-resecells are not selective for
color itself, they nonethelessdetect motion by responding to an edge defined by color. Thus, even though MT
and the dorsal pathway to the parietal cortex may be
devoted to the analysis of motion, the cells are sensitive
to stimuli (color) that were thought to be analyzed primarily by cells in the ventral pathrt'ay. Stimulus information on motion, form, and color therefore is not
processedexclusively in separatefunctional pathways.
This description of motion processingis based on researchon the MT area in monkeys. In the human brain an
areadevoted to motion has been identified at the junction
of the parietal,temporal, and occipital cortices.Figure 28-5
shows ciranges in blood flow in this area in PET scans
made while the subjectviewed a pattern of dots in motion.
A cortical area adjacent to MT, the medial superior
temporal area (MST), also has neurons that are responsive to visual motion and these neurons may process a
type of global motion in the visual field called optic
flow, n'hich is important for a person's oll'n movements
through an environment (Box 28-1).
Cells in MT Solve the Aperture Problem
We have considered the response of MT neurons to the
motion of simple stimulus like an edge or a line. How-
trtr.A
PartY / Perception
A Apertureprollem
B P r o p o s esdo l u to n
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F i g u r e2 8 - 6 T h e a p e r t u r eP r o b l e m .
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s a m e d i r e c to n . T h r e ep a t t e r n sa r e s h o w n m o v l n g n t i . r r e ed r e c t i o n sW h e n s e e n t h r o u g ha s m a a p e r t u r et h e t h r e e g r a t i n g sa p p e a rt o m o v e i n t h e s a m e d i r e c t i o nd, o w n w a r da n d t o
t h e r i g h t .T h i sf a i l u r et o a c c u r a t e l dy e t e c tt h e t r u e d r e c to n o f
m o t i o ni s c a l l e dt h e a p e r t u r ep r o b l e m .( A d a p t e df r o m M o v s h o n
et al 1985.)
B . A f o r m a ls o l u to n t o t h e a p e r t u r ep r o be m . W h e n m o t i o ni n
d i f f e r e n td i r e c t i o n sd, o w n w a r d( 1 )o r r r g h t w a r d( 2 ) , s s e e n
t h r o u g ha s m a l la p e r t u r et,h e m o t i o no f t h e e d g e s e e nt h r o u g h
t h e a p e r t u r ed o e s n o t i n d l c a t et h e t r u e d i r e c t i o no f t h e e n t i r e
p a t t e r nA
. s s u m e n o w t h a t t h e a p e r t u r er e p r e s e n t st h e r e c e p t i v e f i e l do f a n e u r o n ,a n d t h a t t h e r ea r e t w o a p e r t u r e sr a t h e r
t h a n o n e ( 3 ) T h i s r e p r e s e n t st h e s l t u a t i o nl n w h i c h t w o o r
ur t o
m o r e c e l l st h a t r e s p o n dt o s p e cf i c d i r e c t i o n sp e r p e n d i c a
t h e i ra x i so f o r e n t a t i o na r e a c t i v a t e db y d i f f e r e n te d g e sm o v i n g
i n d i f f e r e n td i r e c t i o n sA. h g h e r - o r d ecr e l lt h a t i n t e g r a t e tsh e
s i g n as f r o m t h e J o w e r - o r d ec re l s c o u l de n c o d et h e m o t i o no f
t h e e n t i r eo b l e c t ( A d a p t e df r o m M o v s h o n1 9 9 0 . )
ever, in the everyday world complex two- and threedimensional patterns often give rise to ambiguous or iilusory perception. Consider the example in Figure
28-6A, u'hich shows three gratings moving in three directions. When viewed through a small circular aperture, all three gratings appear to mol'e in the same direction. The observer only reports the component of
motion that is perpendicular to the orientation of the
bars in the gratings. This phenomenon, known as the
apertttreproblem,applies to the studv of neurons as rt'ell
as percepiion. Since most neurons in V1 and MT have
relatively small receptive fields, they confront the aperture problem r,r'henan object larger than their receptive
field moves acrossthe visual field.
To solve the aperture problem, neurons may extract
information about motion in the visual field in tn'o
stages.In the initial stage neurons that respond to a specific axis of orientation signal motion of components
perpendicular to their axis of orientation. The second
stage is concerned with establishing the direction of
motion of the entire pattern. In this stage higher-order
neurons integrate the local comPonents of motion analyzed by neurons in the initial stage.
The hypothesis that motion information in ihe visual system is processed in two stages was tested by
Tony Movshon and his colleagues, who recorded the
responsesof cells in V1 and MT to a moving plaid pattern. The neurons of Vl as well as the majority of neurons in MT responded only to the components of the
plaid. Each cell responded best when the lines in the
plaid moved in the direction preferred by the cell. Cells
did not respond to the direction of motion of the entire
plaid. Movshon therefore called these neurons compltleurlns. In contrast, about 20%
nent directiort-selectiue
of the neurons in MT responded onlv to motion of
the plaid pattern. These cells, called pattern directiott-
Chapter 28 / Perceptionof Motion, Depth, and Form
A C o m p o n e n t so f s t m u l u s
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l{
F i g u r e2 8 - 7 N e u r o n s i n t h e m i d d l e t e m p o r a l a r e a o f c o r t e x
i n m o n k e v s a r e s e n s i t i v et o t h e m o t i o n o f a n e n t i r e o b j e c t
i n t h e v i s u a lf i e l d .
A . S t i m u l iu s e dt o a c t i v a t ec e l l si n V 1 a n d M T P a i r so f g r a t i n g s
a r e o r i e n t e da t a 9 0 oa n g l e( t o p )o r 1 3 5 "a n g l e( b o t t o m )t o e a c h
o t h e r .W h e n e a c hp a i ri s s u p e rm p o s e dd u r i n gm o v e m e n t ,t h e
r e s u l t i n gp l a i dp a t t e r na p p e a r st o m o v e d i r e c t l yt o t h e r i g h t .T h e
m o t r o no f e a c hc o m p o n e n tg r a t i n gi s p e r p e n d i c u l at or t h e o r l e n t a t i o no f i t s b a r s .T h e m o v e m e n to f e i t h e rc o m p o n e n ta l o n e
s h o ud s t i m u l a t ef i r s t - s t a g en e u r o n st h a t p r e f e rt h e d r e c t i o no f
m o t i o no f t h e o n e g r a tn g . W h e n t h e t w o g r a t i n g sa r e s u p e rm ee
) urons
p o s e dt o f o r m a m o v i n gp a i d ,o t h e r ( s e c o n d - s t a g n
s h o u l db e a c t i v a t e d .
B . P o l a rp o t s i u s t r a t et h e m o t o n s i g n ae d b y f r s t - s t a g en e u r o n s n V 1 . T h e p l o t ss h o w t h e r e s p o n s eo f a n e u r o nt o t h e d r e c t i o n( 0 t o 3 6 0 ' ) o f m o t i o no f i n d i v i d u ag r a t i n g s( 1 )a n d t h e
p l ad ( 2 ) .T h e r e s p o n s eo f t h e n e u r o nt o m o t o n n e a c hd i r e c t i o n i s n d c a t e db y t h e d i s t a n c eo f t h e p o i n tf r o m t h e c e n t e ro f
t h e o l o t .T h e c r c l ea t t h e c e n t e ri n d i c a t e st h e n e u r o n ' sa c t l v t y
when no st mu us is presented.
1 . T h i s n e u r o nr e s p o n d sb e s t w h e n t h e m o t i o no f a g r a t i n gi s
d o w n w a r da n d t o t h e r i g h t( b l u e a r r o w )
2 . W h e n p r e s e n t e dw i t h t h e m o v i n gp l ad , t h e n e u r o nr e s p o n d s
t o t h e m o t i o no f e a c hc o m p o n e n tg r a t i n g( s o l i dc o l o r ) r a t h e r
t h a n t o t h e r r g h t w a r dm o t o n o f t h e p a i d . T h e r e s p o n s eo f t h e
c e l lt o t h e g r a t i n gc o m p o n e n t si s e x p e c t e dt o h a v et h e t w o l o b e dc o n fi g u r a to n n d i c a t e db y t h e d a s h e d l i n e s .N e u r o n s
t h a t r e s p o n do n l yt o t h e m o t o n o f t h e c o m p o n e n t so f t h e p l ad
a r e r e f e r r e dt o a s c o m p o n e n td r r e c t i o n - s e l e c t inveeu r o n s .
t h e m o t o n s i g n a l e db y a h g h e r
C . T h e s ep o a r p l o t si l l u s t r a t e
i r e a( M T ) .
o r d e rn e u r o nl n t h e m i d d l et e m p o r a a
1 . A s w i t h t h e o w e r - o r d ecr e l l r n V 1, t h s c e l li n M T r e s p o n d st o
r n o to n d o w n w a r da n d t o t h e r i g h t .
2 . W h e n p r e s e n t e dw i t h t h e p a i d ,t h e n e u r o nr e s p o n d st o t h e
d r e c t o n o f m o t o n o f t h e p l a i d( s o l i dc o l o r ) ,n o t t o t h e d r e c t i o n so f t h e c o m p o n e n tg r a tn g s ( d a s h e dl i n e ) .T h s i n d i c a t e s
t h a t t h e n e u r o nh a s p r o c e s s e dt h e c o m p o n e n ts i g n a l so f V 1
i n t o a m o r e a c c u r a t ep e r c e p t i o no f t h e m o v e m e n to f t h e o b j e c t ,a n d t h e n e u r o ni s r e f e r r e dt o a s a p a t t e r nd r e c to n ']985.)
s e n s i tv e n e u r o n .( M o d f i e d f r o m M o v s h o ne t a l .
556
l' art V / Perception
sertsitiueneurons, receive input from the component
direction-selective celis (Figure 28-7). Thus, as suggested by the two-stage hypothesis, the g/obal rnotiort
of an object is computed by pattern-selectiveneurons in
MT based on the inputs of the component directionselectirreneurons in V1 and MT.
Control of Movement Is Selectively Impaired
by Lesions of MT
T a T o e tm o ' e r e 1 t
/.
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F i g u r e2 8 - 8 C o r t i c a ll e s i o n si n m o n k e y s a n d h u m a n s p r o d u c e s i m i l a r d e f i c i t si n s m o o t h - p u r s u i te v e m o v e m e n t .
A . S m o o t hp u r s ut e y e m o v e m e n t so f a m o n k e yb e f o r ea e s i o n
i n t h e f o v e a lr e g i o nm e d i a s u p e r i o tre m p o r a a r e a( M S T )i n t h e
r i g h th e m i s p h e r e( p r e l e s i o n )a n d2 4 t t a f t e rt h e e s o n ( p o s t l e s i o n ) .T h e m o n k e y ' st a s k i s t o t o k e e pt h e m o v i n gt a r g e to n t h e
f o v e ab y m a k n g s m o o t h p u r s ut e y e m o v e m e n t .T h e d o t t e d
l i n e s h o w s t h e p o s i t i o no f t h e t a r g e to v e rt i m e T h e s t m u l u s s
t u r n e do f f w h e r e t h e m o n k e yi s f x a t i n ga n d t h e n t u r n e do n
a g a i na s i t m o v e ss m o o t h l ya t 1 5 ' p e r s e c o n dt o t h e r i g h t .T h e
d y e m o v e m e n t sa s t h e m o n s o l i d l i n e ss h o w s u p e r i m p o s ee
k e y p u r s u e st h e t a r g e to n 10 s e p a r a t et r i as N o t e t h a t b e f o r e
t h e i e s i o nt h e m o n k e yn e a r y m a t c h e st h e t a r g e tm o t i o nw i t h
s m o o t h - p u r s uei ty e m o v e m e n t ,b u t a f t e rt h e e s o n i t f a i s t o d o
s o . l n s t e a dt m a k e sf r e q u e n ts a c c a d i ce y e m o v e m e n t s( t h es e r i e so f s m a 1s t e p s i n t h e e y e m o v e m e n tr e c o r d t) o c a t c hu p
w i t h t h e t a r g e t .( F r o mD u r s t e l e er t a l 1 9 8 7 . )
B . S m o o t h - p u r s uei ty e m o v e n n e n t sn a h u m a ns u b l e c tw t t h a
r g h t o c c r p i t a l - p a r i e teasli o n T h e p a t i e n ta t t e m p t st o f o o w a
t a r g e tm o v i n g2 0 ' p e r s e c o n dt o t h e r i g h t ,b u t t h e e y e m o v e m e n t s d o n o t k e e p u p w r t h t h e t a r g e tm o t i o n .A s w i t h t h e m o n I e y . l r e s u o . e c tr s e s a s e r i e so +c a t c h - - ps a c c a d e sT oc o - p e n s a t ef o r t h e s l o w p u r s ut I n b o t h h u m a n sa n d m o n k e y st h e
p u r s u i td e f i ct i s m o s t p r o m i n e n tw h e n t h e t a r g e t s m o v i n gt o '
w a r d t h e s i d eo f t h e b r an c o n t an i n gt h e l e s i o n( i nt h e s e c a s e s ,
r i g h tb r a i nl e s i o n sa n d d e f i c i t sw t h r g h t w a r dp u r s u i t )T h e h u man subjectw
, i t h a l a r g el e s o n t h a t m u s t n c l u d em u t i p e
b r a i na r e a s ,h a s a d e fi c i t n s m o o t h - p u r s uei ty e m o v e m e n t s
v e r y s i m ia f t o t h e d e fi c t s e e n r n t h e m o n k e yw i t h a l e s i o nl i m
a r e a s ( F r o mM o r r o w a n d
i t e d t o s m a l la n d i d e n t i fe d v i s u a L
S h a r p e '919 3 )
These correlations of neuronal activity and visual perception raise the question, Is the activity of directionselecti"'e cells in MT causallv related to the visual perception of motion and the control of motion-dependent
movement? The question whether direction-selective
cells in MT directly affect the control of movement was
first addressed in an experiment that examined the relationsirip of these cells to smooth-pursuit eye movements, the movements that keep a moving target in the
fovea (seeFigure 28-3).When discrete focal chemical lesions were made within different regions of the retinotopic map in MT of a monkey, the speed of the moving
target could no longer be estimated correctly in the region of the visual field monitored by the damaged MT
area. In contrast, the lesions did not affect pursuit of targets in other regions of the visual field nor did they affect eve movements to stationary targets. Thus, visuai
processing in MT is selective for motion of the visual
stimulus; lesions produce a blind spot, or a scotoma, for
motron.
Human patients with lesions of parietal cortex also
sometimes have these deficits in smooth-pursuit eye
movements, but the most frequent behavioral deficit is
cluite different from that seen after lesions of MT. The
neurologist Gordon Holmes originally reported that
these patients were unable to follow a target when it
was moving toward the side of the brain that had the lesion. For example, a patient with a lesioned right hemisphere has difficulty pursuing a target moving toward
the right (Figure 28-88). Later experiments on monkeys
showed that lesions centered on the medial superior
temporal area (MST), the next le"'el of processing for visual motion, produced just such a deficit (Figure 28-8A).
Perception of Motion Is Altered by Lesions
and Microstimulation of MT
The question r,l'hetherMT cells contribute to the perception of visual motion was addressed in an experiment in
which monkeys were trained to report the direction of
motion in a display of moving dots. The experimenter
varied the proportion of dots that moved in the same
clirection. At zero correlation the motion of ail dots was
Chapter 28 / I'erception of Motion, Depth, and Fornr
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D i s p l a c e m e n( rt n r na r c )
F i g u r e2 8 - 9 A m o n k e y w i t h a n M T l e s i o n a n d a h u m a n p a t i e n t w i t h d a m a g e t o e x t r a s t r i a t ev i s u a l c o r t e x h a v e s i m i l a r
d e f i c i t si n m o t i o n p e r c e P t i o n .
A . D i s p l a v su s e dt o s t u d yt h e p e r c e p to n o f m o t l o n l n t h e d i s p l a yo n t h e l e f t t h e r e i s n o c o r r e l aot n b e t w e e nt h e d r e c t t o n s
o f m o v e m e n to f s e v e r a dl o t s ,a n d t h u s n o n e t m o t l o n n t h e
d i s p l a yI.n t h e d i s p a y o n t h e r g h t a l l t h e d o t s m o v e l n t h e
s a m e d i r e c t i o n( 1 0 0 % c o r r e l a t l o nA) .n i n t e r m e d i a t ce a s ei s t n
t h e c e n t e r ;5 0 % o f t h e d o t s m o v e n t h e s a m e d r e c to n w h l e
t l l yn o r s e
t h e o t h e r 5 0 % m o v e i n r a n d o md i r e c t i o n s( e s s e n a
a d d e dt o t h e s i g n a l )(.F r o mN e w s o m ea n d P a r e1 9 B B)
random and at 100c/ccorrelation the motion of all dots
was in one direction (Figure 28-9A). While normal monkeys could perform the task with less than 10% of the
dots moving in the same direction, monkeys u'ith a lesion in MT required nearlv 100c/ccoherenceto perform
as well (Figure 28-98). A human patient with bilateral
brain damage also lost the perception of motion when
tested on the same task (Figure 28-98).In both the monkeys and the human subject,visual acuiiy for stationary
stimuli rvas not affected by the brain damage.
Norma
'1.0
T0
D i s pa c e m e n t( m n a r c )
o f a m o n k e yb e f o r ea n d a f t e ra n M T l e s r o n
B. Theperformance
(left)T
. he performance
o f a h u m a ns u b j e c tw i t h b i l a t e r ab r an
d a m a g ei s c o m p a r e dt o t w o n o r m a s u b j e c t s( r i g h t ) T h e o r d i n a t eo f t h e g r a p hs h o w s t h e p e r c e n tc o r r e l a t i o inn t h e d r r e c t o n s o f a l l m o v n g d o t s ( a si n p a r tA ) r e q ur e d f o r t h e m o n k e y
t o p i c ko u t t h e o n e c o m m o n d r r e c to n . T h e a b s c i s s an d l c a t e s
t h e s z e o f t h e d s p l a c e m e not f t h e d o t a n d t h u s t h e d e g r e eo f
a p p a r e nm
t o t i o n N o t e t h e g e n e r a sl m i l a r i t yb e t w e e nt h e p e r f o r m a n c eo f t h e h u m a n sa n d t h a t o f t h e m o n k e ya n d t h e d e v
a f t e rt h e c o r t c a l l e s o n s . ( F r o m
a s t a to n t o t h i s p e r f o r m a n c e
N e w s o m ea n d P a r e1 9 8 8 ,B a k e re t a l . 1 9 9 1)
Thus damage to MT reduces the abiiity of monkeys
to detect motion in the visual field, as indicated by disruptions in the pursuit of moving objects and perception of the direction of motion. However, monkeys with
MT lesions quickly recover these functions. Directionallv selective cells in other areas of cerebral cortex, such
as MST, apparentlv can take over the function performed bv MT. Recovery of function is greatly slor't'ed
when the lesion affects not only MT but also MST and
o t h e r e r t r a s i r i ai e a r e a s .
558
Pari V / Perception
whole population of MT neurons (Figure 2B-10).Thus,
the firing of a relatively small population of motionsensitive neurons in MT directlv contributes to perception.
0
F i g u r e2 8 - 1 0 A l t e r a t i o no f p e r c e i v e dd i r e c t i o no f m o t i o n b y
s t i m u l a t i o no f M T n e u r o n s .A m o n k e yw a s s h o w n a d i s pa y
o f m o v i n gd o t s w i t h a r e l a t i v e yl o w c o r r e l a t o no f 2 5 . 6 % ( s e e
F g u r e2 B - 9 A a
) n d i n s t r u c t e dt o i n d i c a t en w h i c h o f e i g h td r e c
t o n s t h e d o t s a p p e a r e dt o b e m o v i n g .T h e o p e n c i r c l e ss h o w
t h e n r n n n r t i o no f d e ei s i n n Sm a d ef o r e a c hd r e c t i o no f m o t i o n a b o u te q u a c h o i c ef o r a d i r e c t i o n sE. e c t rc c u r [ e n tw a s
p a s s e dt h r o u g ha m i c r o e l e c t r o dpeo s i to n e d a m o n gc e l l st h a t
r e s p o n d e db e s t t o s t i m u l u sm o t o n n o n e d t r e c t i o n2, 2 5 ' o a
t h e p o l a rp l o t .T h e m r c r o s t i m u l aotn w a s a p p l i e df o r 1 s , b e g i n n i n ga n d e n d i n gw t h t h e o n s e ta n d o f f s e t o f t h e v i s u a ls t i m u l u s . F i l l e dc i r c l e ss h o w t h e r e s p o n s eo f t h e m o n k e yw h e n t h e
M T c e i l sw e r e s t i m u l a t e da t t h e s a m e t m e t h e v i s u a s t i m u l u s
w a s p r e s e n t e dS t m u a t i o n n c r e a s e dt h e l i k e l i h o o tdh a t t h e
m o n k e yw o u d i n d i c a t es e e n g m o t r o ni n t h e d i r e c t i o np r e f e r r e d
hri thc q-i-rr:ted
N , 4 -e p , ; t . ) F , a t r A d : n
cn'"nm
Sa Zma'r a10
N e w s o m e1 9 9 4 . )
If cells in MT are directlv involved in the analysis of
motion, the firing patterns of these neurons should affect perceptual judgments about motion. How r,r'elldoes
the firing pattern of tl-reseneuror-ls actuallv correlate
r,r'ithbehavior? To address this question, William Newsome and Movshon recorded the activity of directionselectiveneurons in MT r,t'hilethe monkevs reported the
direction of motion in a random-dot display. Firing of
the neurons correlated extremelv u'ell with performance. Thus the directional information encoded bv the
neurons of MT cells is sufficient to account for the monkev's judgment of motion.
If this inference is correct, then modifying the firing
rates of the MT neurons should alter the monkev's
perception of motion. In fact, Newsome found that
stimulating clusters of neurons in a single column of
cells sensitive to one direction of motion biasesthe monkey's judgment towarcl that direction of motion. The
electrical stimulation acts as if a constant visual motion signal r,r'ereaclcled to the signal conveved by the
Depth Vision Dependson MonocularCues
and BinocularDisparity
One of the major tasks of the visual system is to convert
a two-dimensional retinal image into three dimensions.
How is this transformation achieved? How do we tell
how far one thing is from another? How do we estimate
the relative depth of a three-dimensional object in the
visual field? Psl,chophysical studies indicate that the
shift from tr.r,'oto three dimensions relies on two types of
clues: monocular cues for depth and stereoscopiccues
for binocular disparity.
Monocular Cues Create Far-Field Depth Perception
At distances greater than about 100 feet the retinal images seen bv each eye are almost identical, so that looking at a distance we are essentially one-eyed. Nevertheless lr,e can perceive depth r,t'ithone eve by reiying on a
variety of tricks called monocular deptl-rcues.Several of
these monocular cues were appreciated by the artists of
antiquitv, rediscovered during the Renaissance, and
codified early in the sixteenth century bv Leonardo da
Vinci.
1 . Fnrnilior size. If we know from experience something about the size of a person, we can judge the
person'sdistance(Figure 28-11A).
2 . Occlusiort.If one person is partly hiding another
person, we assume the person in front is closer
(Figure 28-11A).
3 . Linenr perspecfizre.
Parallel lines, sucir as those of
a railroad track, appear to converge with distance. The greater the convergence of lines, the
sreater
is the imnression of distance. The visual
b-.'-".'system interprets the convergence as depth by assuming that parallel lines remain parallel (Figure
28-11A).
4 . Sizt Ptr57tg67itt'.
If two similar objects appear different in size, tire smaller is assumed to be more distant (Figure 28-11A).
5 . Distrihutiottof shndoztsard illtnninsfiorr.Patternsof
light and dark can give the impression of depth.
For example, brighter shades of colors tend to be
seen as nearer. In painting this distribution of light
and shadow is called chiarosctu'0.
Chapter 28 / Perceptionof Motion, Depth, and Form
A
S*at o^:'i
559
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T r a c i n go n p c t u r ep a n e
F i g u r e2 8 - 1 1 M o n o c u l a r d e p t h c u e s p r o v i d e i n f o r m a t i o no n
t h e r e l a t i v ed i s t a n c eo f o b j e c t sa n d h a v e b e e n u s e d b y
p a i n t e r ss i n c e t h e s i x t e e n t hc e n t u r y .
A . T h e u p p e rd r a w i n gs h o w s t h e s i d ev i e w o f a s c e n e .W h e n
- r e s c e . e s t ' a c e d o n a p l a ^ eo +g a s s h e d b e t w e e ^ t h e e y e
a n d t h e s c e n e( l o w e rd r a w i n g )t h e r e s u l t i n gt w o - d i m e n so n a l
du
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OCCIUSiOn:
T h e f a c t t h a t r e c t a n g l e4 n t e r r u p t st h e o u t l i n eo f 5 i n d i c a t e s
w h i c h o b j e c ti s n f r o n t ,b u t n o t h o w m u c h d i s t a n c et h e r e i s b e
: l t h o u g hl i n e s6 - 7 a n d B 9
t w e e n t h e m . L i n e a r p e r s p e c t i v eA
a r e p a r al e l n r e a l i t yt,h e y c o n v e r g ei n t h e p i c t u r ep l a n e .S i z e
p e r s p e c t i v eB
: e c a u s et h e t w o b o y sa r e s i m i la r f l g u r e s t, h e
s m a le r b o v ( 2 ) i s a s s u m e dt o b e m o r e d l s t a n t h a n t h e l a r g e r
6. Motiort (or monocular movement) parallnx.Perhaps
the most important of the monocular cues, this is
not a static pictorial cue and therefore does not
come to us from the studv of painting. As rt'e move
our heads or bodies from side to side, the images
projected by an object in the visual field move
Observer
b o y ( ' 1) i n t h e p i c t u r ep l a n e F a m i l i a rs i z e :T h e m a n ( 3 )a n d t h e
r e a r e s to o y a ' e d ' a w n t o n e a r v t ^ e s a - e s i z e n t h e p i c t l l r e .l '
w e k n o w t h a t t h e m a n i s t a e r t h a n t h e b o y ,w e d e d u c eo n t h e
b a s i so f t h e i r s l z e si n t h e p i c t u r et h a t t h e m a n s m o r e d r s t a n t
t h a n t h e b o y T h i st y p e o f c u e i s w e a k e rt h a n t h e o t h e r s .
( A d a p t e df r o m H o c h b e r g1 9 6 8)
B . M o t o n o f t h e o b s e r v e ro r s i d e w a y sm o v e m e n to f h e a da n d
p r r o qn r o d re e q d c n t h n r e s .l f t h e o b s e r v e rm o v e st o t h e l e f t
w h i e l o o k i n ga t t h e t r e e , o b j e c t sc l o s e rt h a n t h e t r e e m o v e t o
t h e r i g h t ;t h o s e f a r t h e ra w a y m o v e t o t h e l e f t .T h e f u l l j i e l d m o s wn movements referred
t o n t h a t r e s u l t sf r o m t h e o b s e r v e r ' o
t o a s o p t c f o w . ( S e e B o x 2 B ' ' 1) ( A d a p t e df r o m B u s e t t i nel t a l
1996)
v y r J
v
v v v v v e
across the retina. Objects closer than the object
\,\,e are looking at seem to move quickly and in
the direction opposite to our own movement,
whereas more distant objects move more slowlv
and in the same direction as our movement (Figure
28-11B).
560
PartV / Perception
StereoscopicCues Create Near-Field
Depth Perception
The perception of deptl-rat distances less than 100 feet
also depends on monocular cues but in addition is mediated by stereoscopicvision. Stereoscopicvision is possible because the twc.reyes are horizontally separated
(by about 6 cm in humans) so that each eye views the
world from a slightly different position. Thus, objects at
different distances produce slightly different images on
the two retinas. This can be clearly demonstrated by
closing eacir eye in turn. As vision is switched from one
to the other eve, any near object will appear to shift sidewavs.
Understanding stereopsis begins r,t'ith an understanding of the simple geometry of the images falling
on the retina. When we fixate on a point, the image
of this point falls upon corresponding points on the
center of the retina in each eye (Figure 28-12). The
point of focus is called the firntiott point; lhe parallel
(vertical) plane of points on r,r,hichit lies is called the
fixationplarte.Thedistance of an image from the center
of the two eyes allows the visual system to calculate
the distance of the obiect relative to the fixation point.
Any point on the object that is nearer or farther than
the fixation point will project an image at some distance from the center of the retina. Parts of the object
that are closer to us tr,'illbe farther apart on the two retinas in a horizontal direction. Parts of the object that are
farther from us will project closer together on the tl'r,'o
retinas.
Clearly, the difference in position, called bhtocular
dispority, depends on the distance of the object from
ihe fixation plane. Thus points on a three-dimensional
object just outside the fixation plane stimulate different points on each eve, and the multiple disparities
provide cues for stereopsis, the perception of solid
objects.
Surprisingly, not one of the great early students of
optics-Euclid, Archimedes, Leonardo da Vinci, Newton, nor Goethe-understood stereopsis,although each
could readily have discovered it l,r,'iththe methods available to them. Stereoscopic vision was not discovered
until 1838, when the physicisi Charles Wheatstone invented the stereoscope. Trn'o photographs of a scene
60-65 mm apart, one taken from the position of each
eye, are mounted into a binocular-like device such that
the right eye seesonly the picture taken from one position and the left eye seesoniy the other picture. Remarkably, this presentation produces a three-dimensional
scene.
i=,:;.:::::J
p o n t a r er u s e o
niornrat
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Figure 28-12 When we fix our eyes on a point the converg e n c e o f t h e e y e s c a u s e st h a t p o i n t ( t h e f i x a t i o n p o i n t ) t o
f a l l o n i d e n t i c a lp o r t i o n s o f e a c h r e t i n a .C u e sf o r d e p t ha r e
p r o vd e d b y p o i n t sj u s t p r o x i m aol r d i s t a lt o t h e f i x a t i o np o i n t .
T h e s ep o n t s p r o d u c eb l n o c u l adr s p a rt y b y s t i m u l a t i n gs l i g h t l y
d i f f e r e n tp a r t so f t h e r e t r n ao f e a c he y e .W h e n t h e l a c ko f c o r r e s p o n d e n c ies i n t h e h o r i z o n t adl i r e c t i o no n l ya n d i s n o t
g r e a t e rt h a n a b o u tO 6 m m o r 2 " o f a r c ,t h e d i s p a r i t yi s p e r c e i v e da s a s r n ge , s o l i d( t h r e e - d i m e n s i o nsapl )o t .
Information
From the Two Eyes Is First Combined
in the Primary Visual Cortex
How is stereopsisaccomplished?Clearly the brain must
somehow calculate the disparity between the images
seenby the tr.t'oeyesand then estimate distancebased on
simple geometric relations. Howevet this cannot occur
before information from the two eyes comes together,
and cells in the primary visual cortex (V1) are the first in
the visual svstem to receive input from the two eyes
(Chapter 27). Stereopsis,howevet requires that the inputs from the two eyes be slightly dit't'erent-theremust
be a horizontal disparity in the two retinal images (Figure 28-13).The important finding that certain neurons 1n
V1 are actually selective for horizontal disparity was
Chapter 2E / Perceptionof Motion, Depth, and Form
C o r r e s P o nndg
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I--\,.\c i F i g u r e2 8 - 1 3 N e u r o n a l b a s i s o f s t e r e o s c o p i cv i s i o n .
( A d a p t e df r o m O h z a w ae t a . 1 9 9 6 . )
A . W h e n a n o b s e r v e rl o o k sa t p o i n tP t h e i m a g e P ' f a l l so n c o r " e s p o n dn g p o i ' t s o n t h e r e t r a o f e a c l -e y e . T h e s ei m a g e s
c o m p l e t e l yo v e ra p a n d t h e r e f o r eh a v ez e r o b n o c u l a rd i s p a rt y .
, o i n tO , t h e i m W h e n l o o k i n ga t a p o i n tt o t h e l e f t a n d c l o s e r p
a g e O ' i n t h e l e f t e y e f a l l so n t h e s a m e p o i n ta s P ' , b u t t h e i m a g e i n t h e r i g h te y e i s l a t e r a l l dy i s p l a c e dT h e s e m a g e sh a v e
b i n o c ua r d i s p a r i t y .
b r n o c u l ai rn p u t si s m a x i m a l l ya c t B . A c o r t i c anl e u r o nr e c e r v i n g
v a t e dw h e n t h e i n p u t sf r o m t h e t w o e y e s h a v ez e r od l s p a r t y
asat P'.
C . A n o t h e rc o r t i c anl e u r o nT e c ev i n g b i n o c u l a rn p u t s r e s p o n d s
c e s t w h e n t h e i n p u t sf r o m t h e t w o e y e s a r e s p a t i a l l yd i s p a r a t e
o n t h e t w o r e t i n a s( O ' ) ;i t j s m o s t s e n s i t v et o n e a rs t i m ui .
made in 1968 by Horace Barlow, Colin Blakemore, Peter
that a
Bishop, and Jack Pettigrew. They found
lleuron that prefers an oriented bar of light at one place in
the visual field responds better when that stimulus appears in front of the screen (referred to as a near stimulus)
561
or when the stimulus is beyond the screen (a far stimulus). There is thus an additional level of organizationof
information in the ocular dominancecolumns in V1.
Cells sensitive to binocular disparity are found in
severalcorticalvisual areas.In addition to V1, some celis
in the extrastriate areas V2 and V3 respond to disparity,
and many direction-selectivecells in MT respond best to
stimuli at specificdistances,either at the plane of fixation
or nearer or farther than the plane. Some cells in MST, the
next step in the parietal pathway, fire in responseto combinations of disparity and direction of motion. That is,
the direction of motion preferred by the cell varies r,r'ith
the disparity of the stimulus. For example, a cell that responds to leftward-moving far stimuli might also respond to rightwarci-moving near stimuli. Thesecells can
convey information not onlv about the direction of motion but about the direction of motion at different depths
r,tithin the visual field (asin Figure 28-118).
Studiesof cellsin tl-restriateand extrastriatecortexthat
respond selectivelvto binocular disparitv fall into several
broad categories.Among these,tuned cellsrespond bestto
stimuli at a specificdisparity,frequently on the plane of fixatior-r.Other cellsrespond best to stimuli at a range of disparities either in front of the fixation plane ("near cells") or
bevond tire plane ("far cells") (Figure28-14).
Just as the motion information processed in MT is
used both for the visual guidance of movement and for
visual perception, disparity-sensitive cells in different regions of visual cortex may use disparity information for
different purposes. One use is the perception of depth,
r,r,'hichr,l'ehave already considerecl.Another is in aligning the eyes to focus at a particular depth in the field.
The eyesrotate to.ll,.ardeach other (convergence)to focus
on near objectsand rotate apart (divergence) to focus on
more distant objects.The ability to align the eyes develops in the first fer'r'months of life and disparitv information may play a kev roie in establishing this alignment.
Random Dot Stereograms Separate Stereopsis
From Object Vision
Must the brain recognizean objectbefore it can match the
corresponding points of the object in the tr,l'oeves?Until
1960 this l,r,'asgenerally thought to be so, and stereopsis
therefore was thought to be a late stagein visual processing. In 1960BelaJuleszproved that this idea was wrong
l,r'henhe found that stereoscopicfusion and deptll perception do not require monocular identification of form.
The onlv clue necessaryfor stereopsisis retinal disparity.
To demonstrate this remarkable fact, Julesz created
a pattern of randomly distributed dots in the middle of
which is a square area of dots. He made tu'o copies of
562
Part V / Perception
o
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Figure 28-'14 Different disparity profiles are found in neur o n s i n c o r t i c a lv i s u a l a r e a s o f t h e m o n k e y .T h e c u r v e ss h o w
t n e r e s p o n s e so f s i r o i " e ' e ^ t n e J r o l s t o b r i g t r tb a r so f o p t i - a l
o ' i e n t a ro n ^ r o v n g ^ t h e p r e f e " e d d i r e c t i o na c r o s sr h e r e c e p t v e f i e l d sa t a s e r i e so f h o r i z o n t adl s o a r i t i e sT. h e s ed f f e r e n t
d i s p a r i t yp r o f i l e sh a v eb e e n o b s e r v e dn m a n y a r e a so f t h e
m o n k e yv i s u a lc o r t e x .T h e t u n e d c e l s a r e m o r e c o m m o n i n
' o v e a lr e p r e s e r - a . e a sV 1 a n d\ / 2 , e s p e c r ayl i n t h e r e g . o no f
"near"
"far"
t a t i o n ,a n d t h e
and
c e ls a r e m o r e c o m m o ni n
"l
M S T ( A f t e rP o g g i o 9 9 5 . )
the pattern but in one copy the inner square of dots is
positioned slightly differentlv from the other copv. The
inner square of dots is visible only n'hen the identical
copies of the pattern are viewed in a stereoscope.If one
inner square is displaced so the tlt'o squares are closer
together, in binocular vien'' the square appears to lie in
front of the pattern. If one inner square is shifted so the
two squares are further apart, the perceived square appears to lie behind tire surrounding dots (Figure 28-15).
Bv itself, each random-dot pattern will not produce any
depth clues. Only with stereoscopicvision can one see
the square within the pattern. With this method, Julesz
demonstrated that humans can detect form based
strictly on binocular disparity.
Are there, among the disparity-sensitive neurons in
the visual cortex, individual neurons that respond to a
stereogram that contains no depth clues except retinal
disparity? To answer this question, Cian Poggio first located responsive cells using a bar of light as a stimulus.
He then replacedthe barwith a random-dot pattern stereogram. Many of the neurons that responded to the solid
figure also responded to the random-dot stereogram.
Object Vision Depends on the Ventral Pathway
to the Inferior Temporal Lobe
The ventral cortical pathway extends from V1 through
V2 to V4 and then to the inferior temporal cortex. We
have already noted that V2 has subregions referred to as
thick stripes, thin stripes, and interstripes and that the
thin and interstripe regions project to V4. As we have indicated, the ventral pathway appears to be concerned
lr.ith analysis of form and color. Here we will concentrate on the processing of form in V2, V4, and the inferior temporal cortex.
Cells in V2 Respond to Both Illusory
and Actual Contours
As in V1, cells in V2 are sensitiveto the orientation of
stimuli, to their color, and to their horizontal disparity,
and they continue the analysis of contour begun by cells
explored in experiin V1. Their responseto contours r,r''as
ments in which cells were tested for their sensitivity to
certain illusorv contours of the sort r.t'e considered in
563
Chapier 2E / Perceptionof Motion, Depth, and Form
F i g u r e2 8 - 1 5S t e r e o p s idso e sn o t
d e p e n do n p e r c e p t i o on f f o r m .
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l a r g e ' p a t r e r nf .t h e n n e ' s o u a ' e sa r e
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Chapter 25. Many cells in V2 responded to the illusory
contours just as they responded to edges (Figure 28-16).
In contrast, few cells in V1 responded to the same illusory contours (although other experiments have shown
responsesof V1 cells to more limited illusorv contours).
These results suggest that V2 carries out an analysis of
contours at a level bevond that of V1, and they are further evidence of the progressive abstraction that occurs
in each of the two pathways of tl-revisual system.
Initial observations on cells in V4 indicated that the cells
were selective for color, and it was thought that they
were devoted exclusivelv to color vision. However,
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many of these same cells are also sensitive to the orientation of bars of light and are more responsive to finergrained than to coarse-p;rainedstimuli. Thus, some V4
cells are responsive to combinations of color and form.
Does removal of V4 alter a monkey's responsesto
color more than to form? Experiments show that ablation
of V4 impairs a monkev's ability to discriminate patterns
and shapesbut onlv minimally affectsits ability to distinguish colors n'ith different hues and saturation. In other
experiments ablation of V4 altered only subtle color discriminations, such as the ability to identify colors under
different illumination conditions (colorcotrstanctl).
We have noted that some humans lose color vision
(achromatopsia) after localized damage to the ventral
occipital cortex. PET scansof normal human subjectsre-
PartY / Perception
564
v
e
I
I
human brain is directly comparable to the V4 region in
the monkey, but instead includes more extended regions, including the inferior temporal cortex, the area
we considernext.
o o
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t, e
2
Recognition of Faces and Other Complex Forms
Depends Upon the Inferior Temporal Cortex
3
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l e v e l i n f o r m a t i o n p r o c e s s i n gi n V 2 c e l l s o f t h e m o n k e y .
A . E x a m pe s o f i l l u s o r yc o n t o u r s 1 . A w h i t e t r i a n g l ei s c l e a r l y
s e e n ,a l t h o u g ht i s n o t d e fi n e d i n t h e p c t u r eb y a c o n t i n u o u s
b o r d e r2
. . A v e r t i c a bl a r s s e e n ,a t h o u g ha g a r nt h e r e i s n o c o n t i n u o u sb o r d e r 3. . S l g h t a l t e r a t i o nos b l t e r a t et h e p e r c e p t i o no f
by
t h e b a r s e e n i n 2 . 4 . f h e c u r v e dc o n t o u ri s n o t r e p r e s e n t e d
a n y e d g e so r l i n e s .( F r o mV o n d e r H e y d te t a l . 1 9 8 4 . r
B . A n e u r o ni n V 2 r e s p o n d st o J u s o r yc o n t o u r sT. h e c e l l ' sr e b y a n e l p s e i n t h e d r a w i n g so n t h e
c e p t i v ef i e l di s r e p r e s e n t e d
l e f t . 1 . A c e l r e s p o n d st o a b a r o f l i g h tm o v l n ga c r o s sr t s r e c e p t i v e f i e d E a c hd o t n t h e r e c o r do n t h e r g h t n d i c a t e sa c e d t s c h a r g ea n d s u c c e s sv e l i n e si n d l c a t et h e c e l l ' sr e s p o n s et o s u c c e s s i v em o v e m e n t so f t h e b a r .2 . T h e n e u r o na s o r e s p o n d s
w h e r ^a n i l r * s o r yc o n l o u ro a s s e so v e ' i t s r e c e o t i v ef i e l d
3 . 4 . W h e n o n l v h a l fo f t h e s t i m u l u sm o v e sa c r o s sf f i e c e l s r e c e p t i v ef e l d ,t h e r e s p o n s er e s e m b l e ss p o n t a n e o u as c t i v i t y( 5 )
( A d a p t e df r o m V o n d e r H e y d te t a l . 1 9 8 4)
veal an increase in activitv in the lingual and fusiform
gvri when colored stimuli are presented (see Figure
28-5). The deficits in patients with achromatopsia differ
lesions of V4. The
from those in n-ronkeys witll
human patients cannot discriminate hues but can discriminate shape and texture, rt'hereas the monkeys'
ability to differentiate shapes is markedlv diminished
whiie hue discrimination is only minimally affected. It
therefore seems unlikelv
that the area identified
in the
We are capable of recognizing and remembering an almost infinite variety of shapesindependent of their size
or position on the retina. Clinical work in humans and
experimental studies in monkeys suggest that form
recognition is closely related to processesthat occur in
the inferior temporal cortex.
The response properties of cells in the inferior temporai cortex are those we might expect from an area involved in a later stage of pattern recognition. For example, the receptive field of virtually every cell inciudes
the foveal region, where fine discriminations are made.
Unlike cells in the striate cortex and many other extrastriate visual areas,the cells in the inferior temporal area
do not have a clear retinotopic organization, and the receptive fields are very large and occasionally may inciude the entire visual field (both visual hemifields).
Such large fields may be related to positiortinaariance,
the ability to recognize the same feature anywhere in
the visual field. For example, even a small eye movement can easily move an edge stimulus from the recePtive field of one V1 neuron to another' In contrast, such a
movement r,t'ould simply move the edge within the receptive field of one inferior temporal neuron. The larger
receptive field of many extrastriate regions, including
the inferior temporal, may be important in the ability to
recognize the same object regardlessof its location.
The most prominent visual input to the inferior
temporal cortex is from V4, so it would not be surprising to see a continuation of the visual processing observed in V4. Inferior temporal cortex aPPearsto have
functional subregions and, like V4, may have separate
pathways to these regions. Also like V4, inferior temPoral cells are sensitive to both shape and color' Many cells
in inferior temporal cortex respond to a variety of
shapesand colors, although the strength of the response
varies for different combinations of shape and color
(Figure 28-77).Other cells are selectiveonly for shape or
color.
Most interesting is the finding that some inferotemporal cells respond only to specific types of complex
stimuli, such as the hand or face. For cells that respond
to a hand, the individual fingers are a particularly critical visual feature; these cells do not respond when there
are no spacesseparating the fingers' Howevet all orientations of the hand elicit similar responses' Among
Chapier 28 / Perceptionof Motion, Depth, and Form
F i g u r e2 8 - 1 7 M a n y i n f e r i o rt e m p o r a l
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neurons selective for faces,the frontal viel,r,of the face is
the most effective stimulus for some, r,t'hilefor others it
is the side vier,r'.Moreover, whereas some neurons respond preferentially io faces, others respond preferentially to specific facial expressions. Although the
proportion of cells in the inferior temporal cortex responsive to hands or faces is small, their existence, together with the fact that lesions of this region lead to
specific deficits in face recognition (Chapter 25), indicates that the inferior temporal cortex is responsible for
face recognition.
One of the major issues in understanding the
brain's analysis of complex objects is the degree to
which individual cells respond to the simpler components of these objects. Certain critical elements of faces
are sufficient to activate some inferior temporal neurons. For example, instead of a face, two dots and a line
appropriatelv positioned might activate the cell (Figure
28-18). Other experiments suggest that some cells respond to facial dimensions (distance betrt'een the eves)
and others to the familiaritv of the face.There is also evidence that cells respondinp;to simiiar features are organized in columns.
Visual Attention Facilitates Coordination
Between Separate Visual Pathways
The limited capacity of the visual svstem means that at
any given time only a fraction of the information available from the visual scenefalling on the two retinas can
be processed.Thus some information is used to produce
perception and movement while other informaiion is
lost or discarded. Tl-risselective filtering of visual information is achieved by visual attention. As may be appreciated from tl'reevidence presented in this and earlier
chapters, understanding the neuronal mechanisms of
attention and conscious awareness is one of the great
unresolved problems in perception. Can we resolve
these mechanisms and understand tl-reircontribution to
behavior? How does attention alter the processing of visual information?
Invesiigation of spatial attention at the neuronal
level began in the 1970sr,r'ithexploration of the cellular
basis of visual attention in the superior colliculus, the
striate cortex (V1), and the posterior parietal cortex of
ar,r''akeprimates (see Figure 20-15). Michael Goldberg
and Robert Wurtz examined the response of cells to a
spot of light under tlt'o conditions: (1) lr,hen the monkey
looked elsewhere and did r-rotattend to the location of
the spot, and (2) rt'hen the animal'r,r'asrequired to fix its
gaze on the spot of light by making rapid or saccadic
eye movements to tl're spot. Wherl the animal attended
to ihe spot, cells in the superior colliculus responded
more intensely,n'l-rilethe responseof cells in V1 showed
little modulation. Hol'r,'ever,the enhanced response of
the cells in the superior colliculus did not result from selective attention per se but l,r'asdependent upon the initiation of eve movement. In similar tests of the resporrsivenessof cells in the posterior parietal cortex, a region
knolvn frorn clinical stuc'liesto be involved in attention (Chapter 20), the cells' responseswere enhanced
r,r,'hetherthe monkev made an eve movement to the visual target or reachedfor it (Box 28-2).
The effects of attention on cells in V4 and inferior temporal cortex r.rrerenext determined by Robert
PartV/Perception
566
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F i g u r e2 8 - 1 8 R e s p o n s eo f a n e u r o n i n t h e i n f e r i o rt e m p o r a l
c o r t e x t o c o m p l e x s t i m u l i . T h e c e l l r e s p o n d ss t r o n g l yt o t h e
f a c e o f a t o y m o . k e y l A ) . T F e c r i r c a lf e a i u r e sp ' o d - c i r ^ gt h e r e s p o n s ea T er e v e ae d i . a c o ^ f g r r a t i o ^o ' t w o o a c k s p o t sa r d
o n e h o r i z o n t ab l a c kb a r a r r a n g e do n a g r a yd i s k ( B ) .T h e b a r ,
cnntc
a n d e i . e r r : r n . . t l i n e t n n o t h o r \ ^ / a T oa q c a ^ t i a
aS Can be
Desimone and his colleaguesby presenting two stimuii,
both falling in the receptive field of one cell. The experimenters found that they could turn a neuron on or off depending on whether they required the monkey to attend
to one of the stimuli. The stimulus remained the samebetween trials; only the monkey's attention shifted.
Since attention is the selection of one stimulus from
among many, it would be reasonableto expect that the
effect of attention on cell responsivenesswould increase
with the number of stimuli presented. In one experiment a monkey rvas required to focus attention on one
stimulus in a group of six to eight identical stimuli. The
responsesof one-third of the neurons in V4 were altered
when the monkey's attention shifted to one stimulus.
Activity in most of these same neurons was not altered
as much when the monkey had to choosefrom onlv two
identical stimuli. Furthermore, increasing ihe number of
stimuli also enhanced the responsesof neurons in earlier stagesof the visual pathways, in V2 and V1. As the
demands for selection amon€i visual targets increases,
so does the relative effect of attention.
Changes in cellular activity also occur when the focus of attention is a specific object rather than a location.
In one set of experiments a monkey l,u'ascued to select
an object, or the color or shape of an object, and then required to select a similar object from among a set of objects presented either simultaneously or in series. Remarkably, presentation of the matcl-ringobject can have
a greater effect on a neuron's response than the sample
stimulus that is present. In one of these experiments the
cells in V4 responded more vigorously when the color
of the matching object was the same as the cue (Figure
28-19).During the search for matching stimuli the activity of neurons in tl'reventral pathway and inferior tem-
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(
t h e s ef e a t u r e s C , D , E , F ) .T h e c o n t r a s tb e t w e e nt h e i n s i d e
a n d o u t s i d eo f t h e c i r c u l a cr o n t o u rw a s n o t c r i t i c a(l G ) .H o w e v e r ,t h e s p o t sa n d b a r h a d t o b e d a r k e rt h a n t h e b a c k g r o u n d
w i t h i nt h e o u t i n e ( H ) .( i : s p i k e s . () M o d i fe d f r o m K o b a t a [ e
a n d T a n a k aI 9 9 4 . )
poral cortex is modified. In the dorsal pathway the activity of cells in areaTA, MSI and MT is also modified,
particularly when multiple stimuli fall within the receptive field of a cell.
The Binding
Problem in the Visual System
We have seen that information about motion, depth,
form, and color is processedin many different visual areas and organized into at least two cortical pathways.
How can such distributed processing lead to cohesive
perceptions? When we see a red ball we combine into
one perception the sensations of color (red), form
(round), and soliditv (ball). We can equally well combine red with a square box, a pot, or a shirt. The possible
combination of elements is so great that the existenceof
an individual feature-detecting cell for each set of combinations is improbable.
Instead, as we have seen in this chapter, the evidence is strongly in favor of a constructive process by
which complex visual images are built up at succes"final
sively higher processing centers. Is there a
common pathway" where all the elements of a complex percept are brought together? Or do the distributed afferent
pathways interact in some continuous fashion to produce coherent percepts? There is as yet no satisfactory
solution to Ihe binding prohlem, the problem of how consciousnessof an ongoing, coherent experience emerges
from the information processing being conducted independently in different cortical areas.
As described in Chapter 25, Anne Treisman and
Bela Julesz independently showed that the associative
process by n'hich multiple features of one object are
Chapter 2E / Perceptionof Motion, Depth, and Form
567
Ilox 28-2 Parietal Cortex and Movement
The dorsal visual path extends to the posterior parietal cortex,
which, based on clinical obsen'ations of patients with parietal
clamage,is known to be involved in the representation of the
visual world and the planning of movement. Recent studies of
neurons in the parietal cortex of monkevs have revealed several functionallv distinct areas,which may account for the varied deficits following damage to the parietal cortex. The activity in most of these areas is related to the transition from
sensory processing to the generation of mo"'ement.
Neurons in one of these subregions, the lateral intraparietal area, fire in connection with saccadic eye movements
(Chapter 39). These neurons fire in response to a visual target,
before a saccadeto the target, and increase their activity just
before the beginning of the saccade,indicating that the activitv
in ihese neurons is related both to the visual input and to the
motor output of the brain. Between the sensory and motor
c.vents,continuing activity in these neurons depends on the
condition under which the saccadeis made, such as whether
the saccadeis made to a visual stimulus or the location of a remembered stimulus. Thus, although activiiv in these neurons
is closelv associatedwith the transition from sensory perception to motor movement, it is not exciusivelv related to one or
the other.
These neurons also clearlv receive informatiot-tmore complex than either pure sensory ancl pure motor information.
Manv neurons respond differently to the same visual stimulus,
depending upon where in space the eves and head are oriented, indicating that they receive input about eve position as
'rvellas the visual stimulus. Such neurons might be involved in
shifting the frame of referencein which sensorv information is
processed(from eve to head to body; see Box 25-1) a shift that
is necessarvto control movements such as reaching. We therefore ha"'e strong evidence that parietal neurons are involved in
putting visual information in the service of the motor systems
and for compensating for the disruption to vision that results
from such movement.
brought together in a coherent percept requires attention. They suggested that different properties are encoded in different feature maps during a preattentive
stage of perception and that attention selects specific
features in these different maps and ties them together
(as illustrated in Figure 25-15).
A related view of the effect of attention on the binding problem recently was advanced by John Reynolds
and Robert Desimone. They based their interpretation
on two observations already described in this chapter:
neurons have larger and larger receptive fields at higher
levels in the cortical visual pathways and attention to
one of several stimuli falling in one of these large receptive fields increasesthe response to that stimulus. They
assume that attention acts to increase the competitive
advantage of the attended stimulus so that the effect of
attention is to shrink the effective size of the field
around the attended stimulus. Now instead of manv
stimuli \,\'ith different characteristics such as color and
form, only the one stimulus is functionally present in
the receptive field. Becausethe effective receptive field
now just includes that one stimulus, all the characteristics of the stimulus are effectively bound together.
Another approach to the binding problem has been
emphasized by Charles Gray and Wolfgang Singer and
Reinhold Eckhorn and their colleagues.They found that
when an object activates a population of neurons in the
visual cortex, the neurons tend to oscillate and fire in
unison. They suggest that these oscillations are indica-
tive of a synchrony among cells and that this synchrony
of firing would bind together the activity of cells responding to different features of the same object. To
combine the visual features (color, form, motion) of the
same object, the synchrony between neurons would, according to this view, extend across neurons in different
cortical areas.
Quite a different solution to the binding problem
was proposed by Lance Optican, Barry Richmond, and
their colleagues. They found that neurons extending
from the laterai geniculate nucleus to the inferior temporal cortex convey more information if the temporal
pattern of their discharge is considered. Instead of measuring the total number of spikes in a time period, they
measured the distribution of the spikes in that time period and found that different stimulus features (eg,
form, contrast, color) tended to be representedby different response patterns of the same cell. They propose
that the pattem of discharge in each cell carries information about different features so that the problem of binding across cells, each representing a different feature, is
eliminated. Ceils in different areas would all convey
some information about a number of stimulus features,
but different cells r,r'ould carry comparatively more or
lessabout each feature.
Thus, while several solutions to the binding problem have been proposed, it still remains one of the central unsolved puzzles in our understanding of the neurobioiogical basesof perception.
568
I' art V / Perception
I n i t i a lf x a t i o n
S t i m uu s p r e s e n t aot n
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F i g u r e2 8 - 1 9 T h e r e s p o n s eo f V 4 n e u r o n st o a n e f f e c t i v ev i s u a l s t i m u l u s i s m o d i f i e d b y s e l e c t i v ea t t e n t r o n '
'ts
A . A m o n l ' e yw a s t ' a i n e dr o s l ' i f t a l T e l to n t o o ' e s e t o f
' t t h e s t a r to f e a c ht r i a l( i n i t i a l
s t i m u l ia s o p p o s e dt o a n o t h e rA
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s c r e e n( t h ed o t i n t h e s q u a r e )A. t t h i s p o i n ti n t h e e x p e r i m e n t
(dotted circle)
the receptivefield of a V4 cell has been located
( u p p e rr o w )
O n a n y g i v e nt r i a lt h e f i x a t i o np o i n tw a s e i t h e rr e d
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f i e l do f t h e c e l l ,a s l n t h e m a t c ht r i a l s( t o p r o w ) ,t h e m o n k e y
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point
i i u l d l f t h o r " s t i m u l rw i t h t h e s a m e c o l o ra s t h e f i x a t i o n
An Overall View
Much like the somatic sensory system the visual system
consists of several parallel pathways not a single serial
pathway. The M and P pathways pass from the retina'
in.ongti the parvocellular and magnocellular layers of
the laLral geniculate nucleus, to layer 4C of the primary
visual .o.t"" (V1), where they feed into parallel pathways extending througl-r the cerebral cortex' A dorsal
puth*uy extends from V1, through areasMT and MST'
io the posterior parietal cortex. A ventral pathway extends fiom V1, through V4, to the inferior temporal cortex. The parietal pathway appears to be dominated by
the M input, but the inferior temporal pathway depends
upon both the P and the M inPut'
Several factors lead to the conclusion that these
pathways serve different functions much as do ihe submodalities for somaesthesis: The anatomical connections along these two pathways, differences in neuronal
activity, the behaviclral deficits occurrinS;after damaS;e
(bottom
e o u t s r d et h e r e c e p t i v ef i e l d ,a s i n t h e n o n m a t c ht r i a l s
r o w ) ,t h e m o n k e yp r e s u m a b l yr s a t t e n d i n gt o t h e s t i m u l io u t f i e l d .T h e r e s p o n s e so f t h e V 4 n e u T o nt o
s i d et h e r e c e p t r v e
t h e s e m a t c ha n d n o n m a t c ht r i a l sw e r e c o m p a r e d N o t e t h a t
t h e s t i m u l u sf a l l l n go n t h e r e c e p t i v ef i e l do f t h e c e l l i s t h e s a m e
i n t h e m a t c ha n d n o n m a t c ht r i a l sw e r e c o m p a r e do, n l y i t s s i g n i f c a n c ef o r t h e m o n k e yh a s c h a n g e d l n t h e l a s tp h a s eo f a
on,andtne mont r a l ( d i s c r i m i n a t i o no) n l yt w o s t i m u l iT e m a r n
key, in order to obtaina reward,must indicatewh-etherthe
m a t c h e ds t i m u l u si s t i l t e dt o t h e r i g h to r t o t h e l e t t
B . l n c r e a s e dr e s p o n s et o t h e s a m e v i s u a ls t i m u l u sf a l l i n go n
( u p p e rr e c o r d )
t h e r e c e p t i v ef e l d o f a V 4 c e l l d u r i n gt h e m a t c h
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. h i l et h e m a t c ha n d n o n m a t c ht r i a l sa r e
h a v i o r arle s p o n s eW
s h o w n s e p a r a t e l yt h, e y w e r e i n t e r l e a v e d u r l n gt h e e x p e r r m e n t . ( A f t e rM o t t e r ' 19 9 4 . )
to the terminal areas of the pathways in both humans
and monkeys, and the activity detecied in the human
brain during tasks that should differentially activate the
two pathways. One view is that the dorsal or posterior
parietal pathway is concerned with determining where
an objeci is, wiiereas the ventral or inferior temporai
pathway is involved in recognizing what the object is'
Anothei view is that the dorsal pathway leads to action'
the ventrai to perception' But all agree that the function
of the two pathwaYs is different.
We have concentrated on the neuronal mechanisms
mediating motion and depth information in the dorsal
posterior parietal pathway and on form perception in
ihe ventril inferior temporal pathway' Both pathways
represent hierarchies for visual processing that lead to
er-eaterabstraction at successivelevels' Neurons in MT
ierporld to the motion of a patterned stimulus, whereas
celis in V1 respond only to motion of the elements of a
pattern. Neurons in inferior temporai cortex respond to
a glven shape at any position in larp;eareasof the visual
Chapter 28 / Perceptionof Motion, Depth, and Form
field, whereas simple cells in V1 respond only when an
edge is positioned at one location in the field. In addition, cellular responsesalonp; the pathways tend to become increasingly dependent on the stimulus characteristic selectedfor attention. The effect of the remembered
object in a visual search can have more effect on the response of neurons in V4 and the inferior temporal cortex tl'ranthe stimulus that is present.
While we have considered separately the visual
processing for motion, depth, and form, these parallel
pathways may not be mutually exclusive pathways.
Some processing combines the activity of the pathways.
For example, form can be seen when the only cue is the
coherent motion of components of the scene (which is
regarded as the purview of the parietal pathway). Likewise, some MT cells respond to the motion of an edge
defined only by color (a property that should be conveyed by the inferior temporal pathr,r,ay).Thus, at both
a perceptual and a physiological level, cross talk between the posterior parietal and inferior temporal pathways must occur as is aiso indicated by the anatomical
e.videncefor cross connections.
We know the outline of the steps the brain takes in
constructing complex visual images from the pattern of
Iight and dark falling on the retina-tlle early processing along the M and P pathways and the later, more abstract processing in the dorsal posterior parietal and
ventral inferior temporal pathways. But these steps remain only an outline. Many cortical areas must still be
erplored, and critical details of visual processing are
only beginning to be understood.
RobertH. Wurtz
Eric R. Kandel
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