NEURAL MECHANISMS OF VISUAL PERCEPTION

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We proposed three experiments that test hypotheses about the neural mechanisms that underlie the brain's ability to make decisions. Our experiments build on knowledge about how the brain represents information about the movement of objects in vision. Our experiments examine the conversion of sensory evidence in visual cortex to a decision about the direction of motion. We showed that neurons in the association cortex of the macaque represent the accumulation of this evidence for one choice and against an alternative. This year, we demonstrated that the neural computations resemble integration, that is, the accumulation as a function of time of the input firing rates (Huk et al, 2005). We also discovered that these neurons are capable of representing the passage of time (Janssen and Shadlen, 2005), and we now believe that time and evidence and prior biases about the decision outcome are combined by neurons in the association cortex into a single coherent quantity. We are currently testing these ideas using a combination of approaches: psychophysics, neural recording and stimulation, and computational modeling. We have recently extended our experiments in two important ways. We examine the process of decision making when there are more than two options, and we have developed a probabilistic classification task to directly test the brain's ability to 'reason' with probabilities. These studies have been reported in abstracts (Yang and Shadlen, 2004; Churchland et al, 2005). Decision-making bridges the gap between sensation and behavior. Nearly all non-reflexive behaviors require the brain to draw upon its sensory cortex to guide future behavior. Thus, the neural mechanisms for the simple decisions we study are likely to lend insight into more complex cognitive strategies. Our experiments ultimately furnish new insights into the causes and treatments of mental disorders affecting perception, planning and reasoning.