MIT Encyclopedia of the Cognitive Sciences - Cryptome

lx Neurosciences
a convergence of inputs from contrast-detecting neurons at earlier stages (Hubel and
Wiesel 1962). At successively higher stages of processing, information is combined to
form representations of even greater complexity, such that, for example, at the pinnacle
of the pathway for visual pattern processing—a visual area known as inferior temporal
(IT) cortex—individual neurons encode complex, behaviorally significant
objects, such as faces (see FACE RECOGNITION).
Parallel Processing In addition to multiple serial processing stages, the visual system
is known to be organized in parallel streams. Incoming information of different
types is channeled through a variety of VISUAL PROCESSING STREAMS, such that the
output of each serves a unique function. This type of channeling occurs on several
scales, the grossest of which is manifested as multiple retinal projections (typically
six) to different brain regions. As we have noted, it is the geniculostriate projection
that serves pattern vision in mammals. The similarly massive retinal projection to the
midbrain superior colliculus (the “tectofugal” pathway) is known to play a role in orienting
responses, OCULOMOTOR CONTROL, and MULTISENSORY INTEGRATION. Other
pathways include a retinal projection to the hypothalamus, which contributes to the
entrainment of circadian rhythms by natural light cycles.
Finer scale channeling of visual information is also known to exist, particularly in
the case of the geniculostriate pathway (Shapley 1990). Both anatomical and physiological
evidence (Perry, Oehler, and Cowey 1984; Kaplan and Shapley 1986) from
early stages of visual processing support the existence of at least three subdivisions of
this pathway, known as parvocellular, magnocellular, and the more recently identified
koniocellular (Hendry and Yoshioka 1994). Each of these subdivisions is known to
convey a unique spectrum of retinal image information and to maintain that information
in a largely segregated form at least as far into the system as primary visual cortex
(Livingstone and Hubel 1988).
Beyond V1, the ascending anatomical projections fall into two distinct streams,
one of which descends ventrally into the temporal lobe, while the other courses dorsally
to the parietal lobe. Analyses of the behavioral effects of lesions, as well as
electrophysiological studies of neuronal response properties, have led to the hypothesis
(Ungerleider and Mishkin 1982) that the ventral stream represents information
about form and the properties of visual surfaces (such as their color or TEXTURE)—
and is thus termed the “what” pathway—while the dorsal stream represents information
regarding motion, distance, and the spatial relations between environmental
surfaces—the so-called “where” pathway. The precise relationship, if any, between
the early-stage channels (magno, parvo, and konio) and these higher cortical
streams has been a rich source of debate and controversy over the past decade, and
the answers remain far from clear (Livingstone and Hubel 1988; Merigan and
Maunsell 1993).
See also COLOR, NEUROPHYSIOLOGY OF; COLOR VISION; COLUMNS AND MODULES;
COMPUTATIONAL NEUROANATOMY; COMPUTATIONAL NEUROSCIENCE; FACE RECOG-
NITION; MULTISENSORY INTEGRATION; OCULOMOTOR CONTROL; RETINA; TEXTURE;
THALAMUS; VISUAL ANATOMY AND PHYSIOLOGY; VISUAL PROCESSING STREAMS
Perception
Perception reflects the ability to derive meaning from sensory experience, in the form
of information about structure and causality in the perceiver's environment, and of the
sort necessary to guide behavior. Operationally, we can distinguish sensation from
perception by the nature of the internal representations: the former encode the physical
properties of the proximal sensory stimulus (the retinal image, in the case of
vision), and the latter reflect the world that likely gave rise to the sensory stimulus (the
visual scene). Because the mapping between sensory and perceptual events is never
unique—multiple scenes can cause the same retinal image—perception is necessarily
an inference about the probable causes of sensation.
As we have seen, the standard approach to understanding the information represented
by sensory neurons, which has evolved over the past fifty years, is to measure