The visual
association
cortex
Semir Zeki
University
The concept
of visual
studies,
assorted
cases
speculation.
A review
review
our
Current
College
Opinion
in Neurobiology
The concept of association cortex is essentially a somewhat vague functional concept inferred from fairly precise anatomical studies. In time, philosophical speculation came to make its contribution to the concept, especially in visual cortex. Not surprisingly, this served to
confuse the concept rather than illuminate it. However
that may be, the different approaches reinforced each
other and led to a fundamentally flawed view of the cerebral processes involved in vision. This supposed that the
function of visual association cortex was to ‘understand
what was seen, seeing being a function of the primary
visual cortex, area Vl. Any revision of the concept of vi
sual association cortex thus entails a profound revision
of our views not only of visual cortex and the processes
it undertakes, but also of our philosophical approach to
the problem of vision.
of visual ‘association’
UK
association
cortex
derives
from
early
myelogenetic
of so-called
visual
agnosia
and much
philosophical
of the evidence
suggests
that
it is perhaps
time
concept
of the visual
association
cortex.
Introduction
Definition
London,
1993,
to
3:155-159
scious awareness? Or was association intended to signify
the unification of the representation of different points,
representing different regions of the body surface, in the
topographically organized sensory areas? Neurologists,
assuming them to have thought about the implications
of their terminology, remained vague about what they
meant by the term. Instead, they proposed a definition
that was so general that it applied to all the above categories. They imagined that it was association cortex that
gave visual ‘impressions’ their meaning and hence that
it was visual association cortex which dealt with ‘higher’
functions.
cortex
The human cerebral cortex is not fully differentiated at
birth. Some areas, which Flechsig [1,2] called ‘primordial’ and amongst which he numbered the primary visual
cortex (area Vl), are myelinated at birth, though they
occupy only a small fraction of the total cortical surface
(Fig. 1). They are connected to the peripheral organs and
are separated from each other by other, and much larger,
cortical areas. The latter are not connected to the peripheral organs and become myelinated at various stages after
birth, as if their myelination depends upon the acquisition of experience. Flechsig used the term ‘association’
cortex to describe these latter areas, believing them to be
the Geistige Zentren or Cogitationszentren (mind centres
or psychic centres). Hence, the commonly used altemative term for visual association cortex was the visuo-psychic cortex. Large cerebral areas, whose boundaries can
be defined with fair precision, were thus inferred to have
a function, that of association, though the function itself
was not precisely delined. Was it an association of present
with past records in a given modality, such as vision, or
an association between different modalities that neurologists had in mind? Would it be the kind of association
that dignifies vision with meaning and therefore con@Current
Biology
F&l.
Flechsig’s
diagram
of the medial
view of the human
brain,
to show the primordial
areas (hatched
and cross-hatched)
and
the association
areas (in white).
The visual association
cortex
was
considered
to surround
the primary
visual cortex.
Lissauer [ 31, whose speculations were to have a powerful
influence on visual neurologists, anticipated the present
talk about ‘lower’ and ‘higher’ levels of vision by using Leibniz’s term ‘apperception’ to speak of the function of Vl in high sounding terms. Apperception was
“....the highest degree of perception, in which the consciousness accepts the sensory impression with maximal
intensity” - a view which invests Vl with a critical role in
consciousness. This was followed by the process of ‘association’, of “....connecting other conceptions with the
content of the perception”, thus giving them their meaning. But no one specified what these conceptions may
Ltd ISSN 0959-4388
155
156
Cognitive
neuroscience
be or what underlying neural mechanisms to look for.
It was suficient that lesions within Vl led to blindness,
while those in the ‘visuo-psychic’ cortex led to the syndrome of mind blindness (seelenblindheit), later termed
agnosia, a condition in which a patient was deemed to
be able to ‘see’ but not to ‘understand’ what was seen.
This view, approved of by both Henschen and Holmes,
was well summarized by Campbell [4] in 1905 when
he spoke of two areas “ ...one specialized for the primary reception of visual sensations, and the other constituted for the final elaboration and interpretation of these
sensations”. Flechsig had believed, though without compelling evidence, that the role of visual association cortex
was to associate visual signals with signals derived from
other sources, and endowed it with a certain level of
consciousness. Other neurologists, including Campbell;
Holmes and Henschen, had believed, again without much
evidence, that it would associate the received visual ‘impressions’ with similar past ‘impressions’, thus leading to
‘understanding’. By the time neurophysiologists got hold
of the idea, they perpetuated the earlier views without
considering the evidence, which was in any case scant.
Thus, Clare and Bishop [5] studied an area well removed from the primary visual cortex in the cat and
“inferred [it] to comprise an association area relating optic and acoustic activity” although no acoustic activity was
studied there. In summary, these views - however much
they may have differed in detail - separated the process
of ‘seeing’ from that of ‘understanding’ and attributed a
separate cortical seat to each. It is this fundamental concept that more recent work on visual ‘association’ cortex
has challenged.
The challenge
cortex
to the concept
of association
Perhaps the first step in the revision of this view came
from the demonstration that the visual association cortex, far from being the single area which its uniform
architecture and relatively late myelination had implied,
consists in fact of multiple visual areas (for a review,
see [6] ). This demonstration raised the possibility that
the cerebral processes involved in vision are much more
complex than the one implied in the dual concept of the
early neurologists. The fundamental turning point came,
however, with the demonstration that the visual areas of
the ‘association’ cortex undertake different tasks, not the
same task at ever-increasing levels of complexity, as was
implicit in the earlier doctrine of exclusive hierarchies
[7,8]. Thus, area V5 is specialized for visual motion [9],
while area V4 is specialized for cblour and form in association with colour [l&14]. V3, by contrast, is specialized
for dynamic form [ 15,161. A functional specialization is
also characteristic of the prestriate cortex of man [ 171,
but this is not to imply that the specializations enumerated above are the only functions of these areas. That
the areas of the visual association cortex (now better referred to as the prestriate cortex) receive parallel inputs
from Vl [ 111, not only served to emphasize the fact that
the cortex undertakes several visual operations in parallel to construct the visual image in the brain, raising
the fundamental question of how the specialized visual
areas interact to provide the unitary visual image in the
brain, but strongly suggested that the functional segregation evident in prestriate cortex would be mirrored
somehow in area Vl itself [ 111, even if the compelling
evidence for such a supposition was to come only several years later [ 181. No one has been able to show that
the cells in the above prestriate areas are influenced to
any extent by stimuli belonging to another modality, say,
olfactory or auditory. Equally, no one has yet been able to
show that the memory of past visual experiences or stimulation is crucial for the activation of cells in these areas,
which is not the same thing as saying that such influences
may not be found to be crucial in the future. Thus, the
speculations of the early neurologists, whether of Flechsig or of Holmes, concerning the ‘understanding’ cortex
do not gain much support from the current physiological
profile of the visual areas of prestriate cortex.
Does visual association
fundamentally
different
cortex use a
strategy?
What is it that distinguishes the visual areas of the prestriate cortex from area Vl? Is it a qualitative difference
or a quantitative one? The single most striking feature of
prestriate visual areas is their specialization. While anyone
wanting to explore the functional organization of area Vl
or area V2 (which surrounds it) with an electrode will
encounter cells with many different properties [l’$-211,
even if cells dealing with a given attribute are grouped
together, the properties of cells in individual prestriate
areas are more homogeneous. The cells of area V5,
for example, are overwhelmingly directionally selective
and uninterested in colour, whereas those of area V4
are overwhelmingly wavelength selective [ 12-15,21,22].
The initial temptation therefore would be to suppose that
the role of prestriate cortex is to segregate or ‘dissociate’
signals rather than associate them. But the segregation of
visual signals belonging to different sub-modalities of vision is not a radically new strategy employed by the presmate cortex, even if it was first demonstrated there. More
recent studies show that visual signals are also segregated
into sub-compartments in area Vl [IS], from which the
specialized areas receive their cortical input, as well as
in area V2, which surrounds area Vl and projects to
the same specialized visual areas [ 23-251. Hence, the
segregation of visual signals in the prestriate cortex is
not a novel strategy but a continuation of the strategy
employed at earlier levels of the visual pathways.
The next striking feature of the prestriate areas is that,
compared with the striate cortex, cells in the former
have larger receptive fields. This is almost certainly the
consequence of the need to collect information from
larger parts of the field of view. But this is not a strategy developed in, or unique to, the visual areas of the
prestriate cortex. Indeed it is a hallmark of the visual
pathways in general. The simple cells of the striate cor-
The visual
tex have larger receptive fields than those of the lateral
geniculate nucleus from which they receive input, and the
complex cells have larger fields still [ 71. The strategy is
continued well beyond the prestriate cortex, for cells in
the visual areas of the inferior temporal cortex and the
pati& cortex have yet larger fields (for examples, see
[ 26,271).
There is, next, the question of complexity in the cellular
responses - itself a consequence of the enlargement of
receptive fields and the collecting of information from
large parts of the field of view. For example, the responses of cells in V4 correlate with the perception of
colours, whereas the responses of their counterparts in
~1 do not 1221; the generation of colour is itself a more
complex process than the registering of the presence and
intensity of different wavelengths [ 281, and to this extent
the responses of V4 cells are more complex than those
of Vl. Equally, the cells of area V5, or at least some of
them, respond to the coherent motion of an entire object, whereas their counterparts in Vl (from which V5
receives its input) respond only to the components of
which the whole is made [29]. The consequence of
this is that the relevant cells of Vl may signal a direction of motion which is not identical to the direction of
motion of the entire object. But this complexity is not
a new departure; instead it is the continuation of a process which starts in the retina itself, to the extent that the
photoreceptors have simpler receptive fields and simpler
responses than the ganglion cells into which they feed.
In a continuation of this process, the orientation selective
cells of Vl have more exigent requirements than the cells
of the lateral geniculate nucleus,
The effects of cortical
lesions on vision
If, therefore, the anatomical and functional profile for the
prestriate cortex that we have built up over the past two
decades does not suggest
a radical functional departure
from the kind of functional organization found in area
vl, is there any plausible reason to suppose that seeing
is vested in Vl and understanding in the surrounding cortex, apart from the fact that lesions in Vl lead to nearly
complete blindness, whereas those in visual association
cortex do not? Insights into this problem may be gained
by a renewed study of the effects of cortical lesions on
vision. I do not refer here to the carefully controlled
experimental lesions in monkeys, which have been the
single worst guide to the organization of the visual cortex imaginable, but to the natural uncontrolled lesions
in human brains produced by gunshot wounds or cerebral accidents which, paradoxically, have been a lot more
informative.
That lesions in Vl, and possibly also V2 (see [30] ),
should cause a total blindness is relatively easy to explain - such lesions do not usually spare a given subcompartment, for example, the blobs of Vl or the thin
stripes of V2 in which cells concerned with colour are
concentrated, but instead involve all compartments, The
consequence is that cells dealing with all the attributes
association
cortex
Zeki
of vision are destroyed, leading to blindness. Naturally,
much the same thing would happen if all the specialized
visual areas were to be destroyed. But this could only be
the consequence of a lesion that is so large that it would
amount to an hemispherectomy, assuming it not to have
led to death.
The consequence of the more common type of lesion,
one restricted to one of the specialized visual areas,
is a blindness for the corresponding visual attribute. A
striking example is provided by the syndrome of cerebral achromatopsia, or acquired cortical colour blindness
following lesions in area V4, which is located in the fusiform gyrus (for a review, see [31]). No less specific is
the syndrome of cerebral akinetopsia or motion imperception [17]. This is the consequence of lesions in area
V5 [32], which is located laterally and ventrally in man
and, perhaps surprisingly, in a zone (Feld 16) that Flechsig considered to have been myelinated at birth and
therefore a primordial area. But is the consequence of
such lesions a radically new kind of syndrome, concerned with ‘understanding’ motion or colour, or is it,
as the physiology suggests, a more complex example of
the same phenomenon? Can meaning and understanding
be attached to vision only through the participation of the
visual areas of prestriate cortex, or can areas Vl and V2
contribute explicitly to both seeing and understanding
the visual world?
Examination of the so-called agnosic patients, as well as
patients with akinetopsia and achromatopsia, leads one
to a general theory of residual vision [6]. This supposes that each visual area contributes explicitly to visual perception (that is in a way that requires no further
processing) and that the patient is able to see and to understand in exact relationship to that contribution and
no more. Moreover, the theory supposes that the ability
to see a particular visual attribute, for example motion,
is not dependent upon the integrity of the entire visual
pathway, up to V5 and possibly beyond. Instead, it supposes that, if the latter is destroyed, the patient will be
able to see visual motion in proportion to the direct
and explicit contribution to vision that the intact parts
of the system, including areas Vl and V2 which feed V5,
are able to make. When area V5 is destroyed in an akinetopsic patient, the directionally selective cells of area Vl
that feed it are not; consequently, the patient is aware
of the presence of motion but cannot make much of it
(see [33]). Equally, an achromatopsic patient is able to
discriminate between different wavelengths, but is unable
to combine this information to construct colours (see
[6] ), a deficit remarkably similar to the consequence
of lesions in macaque area Vd [34]. Indeed, the orientation selective cells of Vl, which are able to respond to
pure chromatic borders [35,36], would endow an achromatopsic patient with an intact, or partially intact, Vl, with
the ability to detect the orientation of the boundary between two equiluminant surfaces of different colour, even
though the colours on either side of the boundary remain
identical to him [ 371,
Additional support for this view comes from comparing
the nature of the so-called agnosia in carbon monoxide
157
158
Cognitive
neuroscience
patients, and in patients with large lesions in the prestriate cortex. It is almost certain that, in the former, area Vl
is much a&ted, although of course the areas of the prestriate cortex are probably also compromised. The consequence is a profound defect in form vision, with patients
hardly able to recognize or copy simple geometric figures
such as triangles or squares [38]. The interpretation that
I have given to this syndrome is that even the elementary
kind of integration and association which Vl is responsible for - the generation of cells especially responsive
to straight lines - becomes compromised. The nature of
the agnosia in patients with lesions in the prestriate cortex is markedly different. Now the patients can even draw
complex figures, without being able to recognize the final
product! Yet how is it that these patients draw? There is
good agreement in the literature that the drawing is piecemeal, small segments of the picture, or of its outline segments that the patient can see and understand - being drawn, one after another. Once drawn, the patient can
still only recognize small segments of his drawing and not
its entirety. It is the simple components of a figure that
the patients are able to see and to understand because
the integrative mechanisms necessary to construct simple
forms, such as lines, are intact, while those needed for
more complex forms are compromised. The intact area
Vl makes a direct and explicit contribution to perception, but the destroyed prestriate areas are not able to
perform their function and hence associate the various
elements and organize into a larger whole. The patient
consequently sees, understands, and is able to draw only
in proportion to what his intact Vl allows him to do.
Vl is as important
and understanding
for conscious
Conclusion
I have concentrated in this brief review on the conceptual
doctrine that we have inherited about visual ‘association’
cortex, and hence about vision, and tried to show that
the separation between seeing and understanding what
is seen - a concept deeply tied to that of association cortex - is not easy to achieve. One can make a
fair argument - from ordinary visual perception, from
anatomy and physiology, and from the study of the dam
aged brain - that seeing and understanding are part of
the same process, though no one would wish to deny
that there are many instances in which we see things
that we do not properly understand. The concept of
visual association cortex, in the sense intended by the
early neurologists, is therefore perhaps now best abandoned. But, in doing so, we must acknowledge that those
who originated the concept and speculated about it have
a high and honourable place in the history of our subject.
If the concepts that they fought for with such conviction
and passion have turned about to be false, we must reflect that the concepts that we today fight for, with no
less conviction and passion, may equally turn out to be
ilawed speculations in the vast ocean of the unknowns
that the visual brain still is.
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S Zeki, Department
of Anatomy
College London, Gower Street,
and Developmentai
Biology,
London, WClE GBT, UK.
University