John Assad
Department of Neurobiology
Harvard Medical School
Boston, MA 02115
Telephone: 617-432-2804; fax 617 432-2811
E-mail: jassad@hms.harvard.edu
Our laboratory utilizes electrophysiological recording techniques in
awake, behaving monkeys to explore mechanisms underlying visual perception.
The general issue that we have focused on is "how does what we know influence
what we see?" Visual physiologists have traditionally attempted to understand
higher-level perception as an elaboration of low-level processes - an approach
that has certainly yielded valuable insight. Yet psychology has long recognized
the reciprocal notion that perceptual context can influence our judgment
of low-level visual attributes. We are clearly not passive observers of
our surroundings; rather, we exploit learned consistencies of nature to
draw inferences about the structure of the environment. Our global understanding
of the visual scene exerts a powerful influence over how we perceive details
of the scene such as brightness and motion, and allows us to infer the
structure of objects from incomplete low-level information such as when
objects are partially occluded. Perception thus must involve a dynamic
interplay of bottom-up and top-down processing. Our basic experimental
approach has been to examine the responses of neurons to identical visual
stimuli in different perceptual contexts. For example, we have identified
neurons in the parietal cortex that are more active following the disappearance
of the spot when the animal could infer from context that the spot was
moving rather than stationary even though the visual stimulus was identical
in the two cases. This difference in activity is thus non-visual in origin.
It may reflect or underlie the animal's inference of motion. Much remains
to be learned about these "extraretinal" influences on the visual system.
It will be of interest to examine the extent of these effects with respect
to the stage of visual processing. It will also be important to examine
the generality of the effect with respect to visual task demands. For example,
we have observed similar responses in the context of a visually-guided
hand motion raising the question of whether the same population of cells
can subserve other visual guidance tasks such as eye movements. If so,
it would provide additional evidence that these neurons encode an "abstract"
representation of target disposition.
Selected Publications:
Assad JA and Maunsell JHR (1995) Neuronal correlates of inferred motion
in primate posterior parietal cortex. Nature 373:518-521.