Neurobiology

John A. Assad, Ph.D.

Professor of Neurobiology

Our laboratory uses electrophysiological recording techniques in awake, be- having monkeys to explore mechanisms underlying visual perception. Our focus is on "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 its details 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. An example is shown in the figure below. The data are peristimulus-time histograms from a neuron in the posterior partial cortex, an area involved in the analysis of visual motion. In A, a spot of light comes on and moves across the receptive field of the neuron, strongly activating it. In the other two panels, the spot comes on, but then disappears and reappears at a later time, either (B) displaced and moving as if it had been moving during its absence or (C) in the same location and stationary as if it had been stationary during its absence. This neuron, as well as most neurons in the parietal cortex, was more active following the disappearance of the spot when the animal could infer 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. For one, 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.

Peristimulus-time histograms of spike activity from a neuron in the macaque posterior parietal cortex. In A, a spot of light comes on and begins to move at the time indicated by the vertical line. In B and C, the spot comes on but disappears (vertical line) and then reappears, either moving (B) or stationary (C), at the time indicated by the tick marks. Until the time of reappearance, the visual stimulus is identical in B and C; therefore these trials are presented to the animal in distinct blocks.