John Assad, Ph.D.

Professor of Neurobiology

Harvard Medical School
Dept of Neurobiology, WAB 227
200 Longwood Ave
Boston, MA 02115
Telephone: 617-432-2804
Fax: 617 432-2811
E-mail: john_assad@hms.harvard.edu
Predocs: 2 Postdocs: 2 Completed PhD's: 1

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.

References:

Freedman DJ and Assad JA (2006) Experience-dependent representation of visual categories in parietal cortex. Nature 443:85-88.
Maimon G and Assad JA (2006) A parietal signal for the proactive timing of action. Nature Neurosci. 9:948-955.
Padoa-Schioppa C and Assad JA (2006) Neurons in orbitofrontal cortex encode economic value. Nature. 441:223-226.
Lee IH, Seitz AM and Assad JA (2006) Activity of tonically active neurons in the monkey putamen during initiation and withholding of movement. J. Neurophys. 95:2391-2403.
Maimon G and Assad JA (2006) Parietal area 5 and the initiation of self-timed movements vs. simple reactions. J. Neurosci. 26: 2487-2498.