| |||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||
Items in cart [0]
| [ Top of Page ] [ Home ] [ Contact Us ] [ Help ] [ The National Academies Home ] | ||
|
||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
Introduction
RANDOLPH BLAKE AND ROBERT SHAPLEY
This conference on the modularity of vision is devoted to contempo-
rary developments surrounding one of the oldest problems in science and
medicine: the localization of function in the brain. This volume contains
a collection of transcribed presentations by scientists from a diverse array
of disciplines—anatomy, neurophysiology, and psychology. Taken together,
these presentations critically explore the idea that the processing of visual
information is carried out in different visual areas of the brain. This idea
has been fostered, in part, by exciting developments in the technology for
studying the brain as well as by new ways of thinking about the brain as a
sophisticated device for computing.
HISTORICAL CONTEXT
The idea that the brain can be subdivided into regions with specialized
functions is an old one. As early as the second century ~D., the Egyptians
theorized that the brain was divided into regions, each region having a
particular function—the emotions, for example, resided in the anterior
ventricle and memories in the posterior ventricle. In the seventeenth
century, the anatomist Thomas Willis (whose name is associated with the
circle of Willis) realized that the brain tissue, not the fluid-filled spaces
within the tissue, was the functional part of the brain. He localized memory
in the convolutions of the cortex, imagination in the corpus callosum, and
emotions in the base of the cerebrum. Of course, as we all know, Descartes
went so far as to localize the soul in the pineal gland.
All these early guesses at functional localization were based on philo-
sophical speculation, not empirical proof. The nineteenth century anatomist
1
OCR for page 2
2
RANDOLPH BLARE AND ROBERT SHAP~F:Y
Franz Joseph Gall provided the first empirically based theory of localiza-
tion. Gall divided the brain into functional units based on what he believed
was the brain's topography, as evidenced by the bumps on the outside of
the head. His method, known as phrenology, postulated that the bumps on
the exterior surface of the skull corresponded to regions of the brain that
were particularly well developed. He assigned specific cognitive processes
and even personality traits to each area. The scientific establishment ques-
tioned Gall's methods, pointing out that the exterior of the skull is a poor
representation of the structure of the brain within. Phrenology was wrong
in its details but, as a method, it played a role in advancing the notion that
mind is divisible into faculties spatially distributed throughout the brain.
Gall's reputation as a distinguished anatomist meant that his beliefs were
not to be dismissed lightly.
Following Gall, the question of localization of function within the brain
drew more and more attention. Gradually evidence accumulated in favor
of the idea that the brain consists of an assembly of discrete, specialized
machinery designed to accomplish particular tasks. nercentual. cognitive
and motor. The evidence took several forms.
~ ~ , ~
Lesion experiments by the David Ferrier in England demonstrated
specific behavioral deficits produced by carefully placed, localized lesions.
At more or less the same time, anatomists were beginning to subdivide the
brain into histologically distinct areas; probably the most famous of these
brain cartographers was Korbinian Brodmann, whose areas are still part of
the neuroanatomical landscape. Interesting evidence was also cropping up
in the hospital clinics. Neurologists were beginning to describe syndromes
wherein localized damage to the human brain produced rather specific
functional losses. Perhaps the most famous case is the one described by
Paul Broca, the neurologist who localized speech within the temporal lobe
of the left hemisphere of the brain.
--r
By the early twentieth century, the question of broad functional seg-
regation was settled: there was no doubt that the brain was divisible into
discrete sensory and motor areas, and by the 1940s it was realized that there
existed even finer subdivisions within these two broad functional categories.
However, it wasn't until the late l95Os that we began to get a closer glimpse
of the actual neural machinery housed within these specialized sensory ar-
eas. The Nobel prizewinning work of lbrster Wiesel and David Hubel gave
entirely new meaning to the notion of function specificity, particularly as it
applied to the collation of neurons with similar receptive field properties.
Their pioneering experiments set the stage for a more refined examination
of visual information processing. From that work, we now know that in
higher mammals Brodmann's areas 18 and 19 in fact consist of multiple
representations of the visual field, varying in their size, internal organiza-
tion, and pattern of connections. In very recent years physiologists have
OCR for page 3
INTRODUCTION
3
begun elucidating functional differences between these areas, and we're
now seeing in the literature complex circuit diagrams. Some believe that
the field has matured to the point at which it is reasonable to speculate
intelligently about the specialized roles played by various extrastriate areas
in visual perception and in visually guided behavior. And it is some of
those speculations that we'll be hearing about today.
But interestingly, there's more to the story than neuroanatomy and
neurophysiology: there have also been important developments in the
fields of computer science and psychophysics. Those developments likewise
speak to the issue of the functional role of multiple visual maps in the
brain.
CURRENT DEVELOPMENTS
This symposium on the modularity of vision is mainly about visual
areas of the cerebral cortex and the role of the cerebral cortex in visual
information processing. 1b those of us who work in what is called "early
vision," that is, stages of visual processing up to and including the primary
visual cortex or striate cortex, this symposium is mainly concerned with
"late vision," that is, areas of the cerebral cortex that are involved in fairly
high levels of visual image processing. The work is therefore very closely
related to perception, and also to algorithms of high-level image processing
by machines.
One of the major issues in late vision—or, indeed, in the entire field
of vision research- is the nature of the processing of the visual signal by
the brain: how is it done, where is it done, and so on. A more detailed
question, one that is very much a current focus of research, is whether the
visual system processes visual information in a serial or parallel manner.
The idea of a strict serial or hierarchical mode of visual signal pro-
cessing was most explicitly stated in the early papers of Hubel and Wiesel,
though it has remained an idea of contemporary appeal since that time.
The idea, in its simplest form, is that visual properties of neurons- the
receptive fields are built up at each level by convergence of neural infor-
mation from the previous stage of the visual pathway. The concept is that
receptive fields of neurons in the primary striate cortex are composed of
the sum of receptive fields in the lateral geniculate nucleus. The concept is
also that the receptive fields of neurons in accessory visual areas—the extra
striate cortex are built up by summing the activity of groups of neurons in
V-1, the primary area, and so on in series from V-2 to V-3 and V-4. The
name serial processing refers to a series of receptive field processing, one
after the other.
The concept of parallel processing, in contrast, is that there are sep-
arate and independent lines of signal flow from the retina to the brain,
OCR for page 4
4
RANDOLPH BLAKE AND ROBERT SHIPLEY
and from one area to another within the brain. The receptive field prop-
erties of neurons in the cerebral cortex are determined not only by what
stage or level of processing they are at, but also by which stream of signal
how they are getting input from at the periphery. Perhaps the boldest
expression of this hypothesis of parallel processing was presented initially
by Jonathan Stone and Bogdan Dreyer in their work on the visual receptive
field properties of neurons in primary visual cortex.
The frontier of vision science represented here today has revealed
that neither an extreme serial nor an extreme parallel view is sufficient to
account for the way our brains are wired. There is evidence of segregation
of parallel channels in the cortex segregated all the way from the eye to
the cortex. There is also clear evidence for hierarchical serial processing
within these parallel streams. The idea that there is strict serial or strict
parallel processing seems to be ruled out: there is evidence for both.
Another issue addressed in this symposium is the existence and the
significance of multiple maps of the visual world inside our heads. Some
rationale for the existence of these multiple maps comes from recent work
on computational vision. Multiple maps of the outside world are found
not only in vision; they are also found in the auditory system. They signify
the importance of having multiple models of the world inside our head for
subsequent processing in order to perform, in the case of vision, significant
tasks about calculating the brightness and the color of objects and their
shapes, for example.
That the existence of spatial maps also shows us that, intrinsically, the
analysis of visual space is done in parallel, by having available to the brain
at any one time activity representing all visual space.
Representative terms from entire chapter:
cerebral cortex