Neural Coding of Visual Stimuli
Neurons in different regions within the visual system carry out different aspects of visual pattern analysis needed for visual perception. This project involves discovering how and where the pieces that make up a visually perceived object are brought together and integrated. The latency before neurons start firing after pattern presentation is one important aspect of the time structure of a neuronal message. We find that the latency indicates how easily the pattern can be seen, and the intensity of firing--the response strength--is a code indicating what the pattern is. Finding that the latency is related to how easily the pattern can be perceived suggests that the features within a pattern are grouped. We construct a model to show how the neuronal signals entering the visual cortex from the lateral geniculate nucleus can be combined to make this feature grouping occur. When inferior temporal cortical neurons respond to a pattern obscured by visual noise, the time it takes for the monkey to respond to the pattern correctly is closely predicted by the time it takes for the neuronal messages to indicate which pattern is present. Our results at both ends of the visual cortical streams suggest there is a valid stimulus pattern template that gates these neuronal responses. All of these findings taken together suggest that the visual system is transmitting signals that describe messages that encode stimuli as complex, integrated objects from the earliest cortical stages, and this integration becomes more general as the signals pass to later stages of visual processing.
Neural Mechanisms of Motivation and Reward
In our studies of motivation, we have found that monkeys work faster and with fewer errors when a cue indicates that a reward will be delivered immediately after the next correct response than when the cue indicates that additional trials will follow. Single neurons in the ventral striatum signal the rewarded trial when it follows one or more unrewarded trials, thus providing a neural signal that could reinforce complex behavior. The neuronal responses are directly related to the associative learning of the meaning of the cue in a complex behavioral task. Specifically, the neurons track the animal's chronological course of a behavioral sequence that ultimately leads to a reward. Neurons located 3-4 mm away in area TE of the temporal cortex fail to show this effect. Neurons in the perirhinal cortex are gated by the motivational state of the animal. Thus we hypothesize that dopaminergic input provides signals sensitive to long-term progress through a planned or expected series of tasks which culminate in reward. Disorders of motivation accompany many serious psychiatric and neurological disorders. Also, pharmacological agents that interfere with normal motivational processes have a devastating effect on individual behavior. This project studies the mechanisms that underlie motivational behavior with the goal of designing strategies for more effective treatments for those disorders that adversely affect normal motivation.