269
Plasticity in visual perception and physiology
Charles D Gilbert
Many factors influence our perception
What
of local features.
we see is not strictly a reflection of the physical
characteristics
of a scene,
on the processes
but instead is highly dependent
by which our brain attempts
the scene. As a result, our percepts
context within which local features
previous visual experiences
are presented,
(operating
of time scales), and by our expectations
be before us. The substrate
the lateral interactions
to interpret
are shaped
by our
over a wide range
of what is likely to
for these influences
operating
by the
is found in
within individual areas of
the cerebral cortex and in the feedback
from higher to lower
order cortical areas.
Address
The Rockefeller University, 1230 York Avenue, New York, New York
10021-6399, USA
e-mail: gilbert@rockvax.rockefeller.edu
Abbreviation
Vl
primary visual cortex
Current Opinion in Neurobiology
Instead, a given input can have a very different effect
depending on the other inputs that are concurrently active,
and the combined
effect of two inputs acting together
could be very different than the sum of their individual
effects. Such nonlinear
behavior could be achieved by
membrane
properties such as voltage-dependent
sodium
conductances.
On the other hand, alteration in a cell’s
response to the same input, as a result of a previous
period of conditioning,
can involve an alteration
in
synaptic weights. The precise mechanism
in which a
particular pathway is altered depends partly on the time
course over which the change takes places. A rapid
facilitation of a pathway can be accomplished by a direct
facilitation of excitatory connections
or by a depression
of inhibitory connections.
Facilitation
taking place over
longer time periods can involve axonal sprouting
and
synaptic proliferation.
Contextual
1996, 6~269-274
0 Current Biology Ltd ISSN 0959-4366
Introduction
Even at early stages in the visual pathway, cells are far
more flexible in their functional properties than previously
thought. It had long been assumed that cells in primary
visual cortex (Vl) had fixed properties, passing along the
product of a stereotyped
operation to the next stage in
the visual pathway. Any plasticity dependent
on visual
experience was thought to be restricted to a period early
in the life of the animal, the critical period. Furthermore,
the assembly of contours and surfaces into unified percepts
was assumed to take place at high levels in the visual
pathway, such that the receptive fields of cells in Vl
represented
very small windows on the visual scene.
These concepts of spatial integration and plasticity were
radically modified in the past few years when it was
learned that, contrary to the classical view, even at
the earliest stages in the cortical processing of visual
information, cells were highly mutable in their functional
properties, and were capable of integrating
information
over a much larger part of visual space than originally
believed.
The modifiability of receptive field properties is referred
to variously as plasticity or dynamics, but the different
manifestations
of cortical dynamics
can involve very
different mechanisms
and be unrelated.
For example,
altering a cell’s response by changing the context does
not necessarily involve a change in synaptic strengths.
influences
Although the receptive fields of cells may seem small
when mapped with simple visual stimuli such as short
line segments
of a particular orientation,
the way in
which a cell responds to such a stimulus is dependent
on the context within which that stimulus is presented.
The perceptual consequences
of context dependency
are
probably related to the process of linkage of boundary
contours and fill-in of surfaces. The association of contours
and surfaces belonging to a single object also requires the
dissociation
of these features from their background
or
from the contours and surfaces belonging to other objects.
This is referred to as segmentation.
Remarkably, the context dependency
required for these
processes is seen even in striate cortex. It can be observed
by the facilitatory effects of a line placed outside the
receptive field that, by itself, would not cause the cell
to respond, but when presented
in conjunction
with a
line segment placed within the receptive field can boost
the cell’s response severalfold. This shows a dependency
on relative position and orientation
of the two lines,
similar to psychophysical
experiments.
When a series of
randomly placed and oriented lines are surrounding
the
receptive field, this will inhibit the cell’s response, but
the response can be disinhibited
by colinear, isoriented
lines placed outside the receptive field. As a consequence,
cells respond not just when the local contour is of the
appropriate orientation,
but also when the line segments
within the global image have the appropriate geometry
([lo]; M Ito, MK Kapadia, CD Gilbert, G Westheimer,
Sot NeurosGi Abstr 1995, 21:771). The phenomenon
of
contour saliency is illustrated in Figure 1, which shows
that contours made up of similarly oriented, colinear line
segments are more readily visible than ones constructed of
lines with large orientation differences. This phenomenon
270
Cognitive neuroscience
Figure 1
is reflected in the responses of cortical cells: even when
exposed to the same stimulus within the receptive field,
the cell will only respond when the local stimulus is
embedded within a global pattern of linked components,
obeying the same geometric constraints
as perceptual
saliency (Figure 1).
In addition to the linkage and segmentation
of contours,
the process of surface fill-in may find its basis in processes
occurring as early as area Vl. Perceptual fill-in has been
studied with the use of artificial scotoma, a gray area
within a pattern of moving lines or twinkling dots [Z],
and with textured fields within a region of a contrasting
texture. When a cell’s receptive field is placed within an
artificial scotoma, the field is induced to expand beyond
its original limits [3-6]. It has been suggested that the
change is primarily an increase in responsiveness
[7]. Some
of the disagreement
may be more a semantic one that
centers on how receptive field dimensions
are defined.
Further experiments have shown, however, that there is a
genuine increase in receptive field area (even when one
normalizes for peak response), with cells responding
to
parts of the visual field where stimuli elicited no response
before conditioning
[6]. In addition to perceptual fill-in,
the consequences
of the expansion may be manifest as
a distortion of spatial perception, such that objects lying
near the boundary of the artificial scotoma appear to be
pulled in towards its center [8]. This effect occurs much
more rapidly than perceptual fill-in (which takes about 15
seconds), suggesting that receptive field plasticity may be
observed within as little as one second.
Context dependency of cortical receptive fields and its relationship
to perceptual linkage. Contours placed within a noisy background
of randomly placed and oriented lines are more salient (i.e. they
‘pop-out’ from a noisy background) when they are composed of
similar oriented, colinear line segments (top row) than when they
consist of lines of widely differing orientation (second row). The
cortical substrate of this is apparent in the responses of cells in Vl :
when a line segment of the appropriate orientation is placed within
their receptive fields (small black squares in the four bottom frames),
these cells will respond, but when the visual field surrounding the
receptive field is filled with a pattern of randomly oriented and placed
lines, their response is greatly suppressed (bottom half of figure, top
left frame). As iso-oriented, colinear lines are progressively added
outside the receptive field, however, the cells responses are greatly
facilitated, thus showing a dependency of cell response on the global
pattern within which local features are embedded (bottom half of
figure, top left to bottom right frames). Top half of figure adapted from
[441; bottom half reproduced, with permission, from (1.1.
When a cell’s receptive field is located within an area of a
textured pattern where there is a surrounding contrasting
texture [9], the cell can respond even when the texture
boundary is located well outside the classical receptive
field [lo’]. The response of the cell to the texture contrast
is seen not in its initial burst, but in the subsequent
tonic
response. Cells were capable of responding
to patterns
with a central area up to 12” in diameter (K Zipser,
TS Lee, VAF Lamme, PH Schiller, Sod Neurosci Ah1994,20:1477). These experiments might represent a more
general case of perceptual fill-in, where the central area is
filled with texture rather than a uniform gray. The time
course of the response did not depend on the diameter
of the central area, however, which is different from the
process of perceptual fill-in [ll].
It has been suggested that some of these phenomena
might arise from long-range horizontal connections within
Vl (for a review, see [12]). These connections
were
initially explored with anatomical techniques,
and their
functional nature has been explored further by combining
optical recording with electrophysiology.
These studies
have confirmed the earlier findings of the relationship of
intrinsic horizontal connections
with cortical orientation
columns.
Plasticity in visual perception and physiology Gilbert
One way of visualizing
the functional
nature of the
horizontal connections
is to measure the area of cortical
activation resulting either from a point stimulus, known
as the cortical point spread, or from a short, oriented line
segment. The area of activation with a point stimulus
shows a central area of spiking activity surrounded
by
a much larger area of subthreshold
activation, with the
area of spiking activity representing
only 5% of the
total activated area measured
optically [13*,14*]. The
orientation
specificity
of the larger area supports the
idea that the lateral interactions
run between columns of
similar orientation specificity 114.1. Optical recording has
also revealed how multiple
stimuli lead to interactions
at the cortical level. Line segments
lying outside the
classical receptive field suppress the response of cells to
a line segment lying within the receptive field, and do so
particularly when they are of the same orientation [13*].
These findings may reveal the mechanisms
underlying
the modulatory effects arising from outside the classical
receptive field.
The functional specificity of the horizontal connections
was supported by combined
optical recording and slice
experiments,
which show that both the excitatory and
inhibitory long-range interactions are between columns of
similar orientation
specificity [15]. The existing framework of horizontal connections
can mediate increases in
effective connectivity
between
cortical neurons whose
receptive fields are caused to expand within a cortical
scotoma [S]. Thus, these connections
can be boosted
in strength from a subthreshold,
modulatory role under
some conditions to a suprathreshold,
driving role under
others. The mechanism
underlying
the alteration
in
effective connectivity
is not known, but it could involve
a potentiation
of excitatory connections,
or a reduction
in inhibitory
inputs,
which would then unmask
the
long-range excitatory inputs [ 161.
The cellular mechanisms
of perceptual fill-in have been
explored outside of area Vl as well. Cells in area V3
respond to an artificial scotoma within seconds, even
when the classical receptive field is contained within the
blank area. The time course of this response corresponds
to that of the psychophysical
measurements
of fill-in
(17.1, and the gradual build up of the response was
referred to as ‘climbing activity’. The response of cells
to this pattern was not observed in Vl by the DeWeerd
et al. [17’] group, although, as discussed above, it was
seen in the earlier studies [3,5]. The time course of
receptive field expansion in Vl has not yet been measured,
so it is not yet possible to determine
its relationship
to that of perceptual
fill-in. In dealing
with these
phenomena,
there is a potential for confusion between
the meaning of receptive
field expansion
as measured
by simple stimuli and a cell’s response
to complex
patterns outside the classical receptive field. The two
phenomena may represent more of a continuum:
initially,
there are subthreshold,
modulatory facilitatory influences
271
from outside the receptive field, possibly mediated by
intrinsic long-range horizontal connections,
which cannot
be seen with a single-line stimulus, but can be seen by
the conjunction
of stimuli inside and outside the receptive
field (such as that observed by Kapadia eta/. [I’]). As these
influences are facilitated, either by direct potentiation
of
the horizontal connections
or by unmasking via adapting
out inhibition,
they can be elevated to a suprathreshold
influence.
To a degree, the suprathreshold
effects may
require multiple stimuli, such as the pattern of lines used
in an artificial scotoma, or, if elevated sufficiently, may
even be seen with the original simple stimuli used to
define the classical receptive field, such as a single line,
and, therefore, may be more clearly defined as a receptive
field expansion.
It has been variously argued that the contextual effects
involve lateral interactions
at lower levels or feedback
interactions
from higher order cortical areas. It has been
found, for example, that grouping operations
are lost
in animals with V2 lesions, and that this area may
therefore be involved in figure-ground
segregation [18].
Facilitation
produced
by linked contours observed in
area Vl might be mediated,
then, by feedback
from
area V2, by the horizontal connections
within Vl, or
by both. The characteristics
of the lateral interactions
of Vl, however, suggest that they are well suited to
mediate these effects. An alternative hypothesis is that the
feedback connections
modulate the surround interactions
that are specified by horizontal connections,
with a joint
role of the two. This idea might further explain why
there would be a difference between the explicit receptive
field properties seen by summation within the receptive
field and the non-classical
interactions from outside the
receptive field. The latter would allow the non-classical
properties to be more dynamic, set up by feedforward
interactions
mediated
by horizontal
connections,
but
gated by attentional
mechanisms mediated by feedback
connections.
Perceptual
learning
Longer term plasticity of receptive field structure and
functional
architecture
in visual cortex may underlie
perceptual learning. Whereas one usually associates visual
learning with acquiring the ability to recognize novel
objects, which, based on neurological literature is thought
to be primarily
based in higher order visual cortical
areas in the temporal lobe, the ability to discriminate
much simpler attributes
may be associated with much
earlier stages in the visual pathway. Perceptual
learning
has been studied for over 100 years (for a review, see
[19]), with improvement
demonstrated
with practice in
various hyperacuity
tasks, orientation
and direction of
movement discrimination.
Recent studies on its specificity
have suggested the involvement of Vl. Below, I summarize
some of the recent work on this topic that has been
published in the past few years.
272
Cognitive neuroscience
Insight into the physiological mechanisms
of perceptual
learning comes from growing evidence that the learning
is specific for the task, with a specificity suggestive of the
involvement
of early stages in visual processing. Attempts
to perform a hyperacuity
task results in substantial
improvement
over days and weeks. Training with lines
of one orientation
does not transfer to doing the task
with lines of the orthogonal orientation [ZO]. On the other
hand, when subjects are trained for vernier acuity and
standard spatial resolution tasks, learning on one task does
transfer to the other, and training in one eye transfers to
the other eye [Zl]. This last study tested vernier acuity
(also known as hyperacuity) and standard resolution acuity,
and found that learning transfers between
tasks. The
findings of transfer raised the issue of whether the training
involved an improvement
in the ability to attend to the
periphery, but other studies suggested
instead a more
specific change in the circuitry involved in the task itself,
such as discrimination
of orientation
or spatial position.
Consistent
with earlier studies, however, learning was
specific for the hemifield in which the training was done.
The specificity of perceptual learning was further refined
in improvement
of orientation
discrimination,
which is
specific for the trained orientation and highly specific for
stimulus position, with little transfer between visual loci
just a few degrees apart [Z&23]. Learning of hyperacuity
also shows specificity for the orientation of the lines used
in the hyperacuity task [ 19,241. The transfer between eyes
suggests that this type of learning takes place perhaps after
the stage where the input arrives to Vl, but the positional
and orientation specificity suggests that it may be localized
to Vl, where the retinotopic order and orientation tuning
are most refined.
One interesting feature of the perceptual learning studies
is that learning is often most clearly seen between sessions,
perhaps allowing for a period of rapid eye movement
(REM) sleep that might be required for consolidation
of
the memory [23,25,26].
Cellular correlates of perceptual
learning in the visual
system have only begun to be explored, though the
results of earlier work on the somatosensory and auditory
system by Merzenich and others (outside the scope of
this review) suggest that the improvements
may involve a
recruitment of cortical territory involved in the task. The
idea that the receptive fields of cells and the functional
architecture
of the cortex can be modified as a result
of visual experience
has received considerable
support
from experiments
involving retinal lesions. If one makes
focal, bilateral retinal lesions, so as to remove functioning
visual input from a restricted
area of Vl, over time
that area recovers visual responses, with the receptive
fields of cells having shifted to positions immediately
outside of the lesioned area of retina [27-301, in effect
producing a remapping of the visual field on the cortical
surface. These changes originate in the cortex, rather
than at antecedent
stages in visual processing,
and
probably involve the long-range horizontal connections,
which appear to strengthen
their influence during the
period of reorganization
by a process of sprouting and
synaptogenesis
[ 140,31*,32]. Although these changes take
months to be completely
developed,
there are striking
changes occurring even within minutes after making such
lesions [Z&33].
The profound changes in cortical functional architecture
that follow retinal lesions may represent the mechanisms
of recovery following lesions of the CNS, but they
may also reveal adaptive mechanisms
associated with
normal processes such as perceptual learning. Evidence
for changes at various stages in the visual pathway
associated with learning itself is beginning
to emerge.
Improvement
in the discrimination
of direction of motion
of a random dot field has been shown to be associated
with a change in the response characteristics of individual
cells in area MT (a visual area in the parietal cortex
specializing
in movement)
[34*]. Clearly, in area IT
(interotemporal
cortex), where it has long been thought
that the representations
of complex forms are stored,
the functional specificities of cells reflect the images on
which animals have been trained [35,36]. It is likely that
the ability to store various forms of visual information,
reflective of the ongoing experiences the animal receives,
is a universal property of the cortex. The advantage of
studying this phenomenon
at early stages in the visual
pathway is that one has a somewhat greater ability to
understand its mechanism in terms of cortical connectivity,
receptive field properties and functional architecture.
Attention
Another potential
influence
on the dynamic properties
of cells at early levels in visual processing is that of
attention or expectation. Although attentional effects had
been seen in area V4 in earlier studies, more current work
is establishing
their presence as far back in the visual
pathway as area Vl. In multiple visual areas, including
Vl, focal attention
to a stimulus can enhance a cell’s
peak response, particularly when the stimulus is in the
presence of multiple competing stimuli [37]. In area V2, a
cell’s non-preferred
stimulus will suppress its response to a
preferred stimulus, but attention to the preferred stimulus
will reduce the inhibitory
effect of the non-preferred
stimulus, and attention to the non-preferred
stimulus will
increase its inhibitory
effect (J Reynolds, L Chelazzi,
S Luck, R Desimone, Sot Neurosci A&F 1994, 20:1054).
In area Vl, attention to a particular position in the visual
field will increase a cell’s response to an oriented bar when
the attended position is close to the cell’s receptive field
(CE Connor, JL Gallant, DC Van Essen, Sot Neurosci A&F
1994,20:1054). In area MT, where cells are highly sensitive
for movement
of objects in a particular direction, cells
will continue to respond when an object moves behind
an occluder, as if the cell is signalling the ‘awareness’ of
the presence of the object, even though for a period the
retina receives no physical stimulus [38*]. To a degree,
Plasticity in visual perception and physiology Gilbert
the responses of cells in area MT require a higher level
segmentation
operation -when
parsing the direction of
movement
of elements
of a complex pattern, the cells
have to differentiate between foreground and background,
suggesting a possible influence of a calculation done in
higher order cortical areas on the response properties of
these cells (RO Duncan, GR Stoner, TD Albright, Sot
Neurosci Abstr 1995, 2 1:664).
In psychophysical
experiments,
discrimination
thresholds
of very simple attributes such as stereo acuity, line length
estimation
and orientation
are strongly influenced
by
attention [39,40]. This is seen when an observer attends
to a particular position in visual space as opposed to being
uncertain as to which of a number of possible positions
in which the stimulus may appear. Similar influence of
positional certainty have been seen with respect to line
orientation
discrimination
thresholds. Attention
also has
an influence
on temporal certaintyknowing when a
test object will appear will decrease the threshold of the
discrimination
relative to being uncertain as to which of
five 500ms time intervals the test will appear [41].
One model of pattern recognition
incorporates
the idea
that feedforward and feedback mechanisms influence cells
at all stages in the visual pathway, and that the pathways
selected for information transfer in an ascending direction
are influenced
by the product of calculations
made in
a descending
direction [42]. Similar models suggest that
higher order cortical areas make abstractions that, via the
feedback connections,
are fit to the more concrete data
represented in the lower order areas-in
effect, a process
for checking hypotheses. The lower areas, in turn, send
information concerning
the errors between the data and
the abstractions
[43]. These ideas generally give to the
lower areas a role of a high resolution buffer that can be
modified or addressed by the higher areas.
cortical area, and the feedback connections
lower order stages in the visual pathway.
References
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reading
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have been highlighted as:
.
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