Over the past few years, we have come to learn that early visual cortex does not act as a passive image processing machine simply devoted to representing bottom up sensory signals. Rather, the bottom-up sensory signals in visual cortex interact with strong top-down signals related to attention and working memory, so that the neural substrates of visual perception involve recurrent interactions among early visual areas including primary visual cortex (V1) and a number of widely separated cortical areas in the temporal parietal and frontal lobes. The proposed experiments aim to measure the neuronal correlates of perception, attention, and working memory, and then to characterize how these three processes interact to drive perceptual decisions.
Aim 1 will use a combination of psychophysics, functional magnetic resonance imaging (fMRI), and evoked potentials to measure and characterize the activity in early visual cortex that is correlated with perception during a visual detection task, and to test the hypothesis that this precept-related activity derives from a combination of bottom-up and top-down signals.
Aim 2 will use psychophysics (contrast detection) and fMRI to study the neuronal correlates of attention in early visual cortex, first to test the hypothesis that the attentional signals in early visual cortex are involved in the maintenance of sustained attention, and second to test the hypothesis that trial-to-trial correlation between performance and cortical activity is driven by trial-to-trial variability in uncertainty.
Aim 3 will use psychophysics (threshold-difficulty contrast and spatial frequency discrimination tasks that involve comparing two stimuli that ate separated in time by a delay period) and fMRI to test the hypothesis that early visual cortex plays a role in visual working memory. We will explore two alternative possible roles for early visual cortex in working memory. First, early visual areas might exhibit sustained delay-period activity implying that visual cortex is involved in holding the memory itself. Second, if these areas do not exhibit sustained delay period activity, they may nonetheless be reactivated at the end of the delay period, suggesting that they are involved in comparing the incoming sensory signals with the memory of a previously presented stimulus. Although these experiments will focus on understanding visual processing n subjects with normal vision, the knowledge gained about the function of human visual cortex and the experimental protocols that will be developed will both be directly applicable to the study of visual deficits including amblyopia.
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