PROJECT SUMMARY/ABSTRACT Stimuli presented too briefly to be noticed can nonetheless facilitate the perceptual processing of the same stimuli many minutes later. Whereas the phenomenon of subliminal priming has been known for decades, whether and how sensory information is encoded in the brain in the absence of awareness in a way that influences subsequent sensory processing across neuronal circuits remains a mystery. We will answer these questions for the first time by examining, at single cell resolution, whether exposure to subliminal stimuli influences perceptual performance and the coding of information across visual cortical populations. To accomplish this goal, we will use multiple-electrode recording of single-unit activity in macaque early and mid- level visual cortical areas (V1 and V4) and behavioral techniques to examine the dynamics and coding in neuronal populations during and after subliminal exposure, and their impact on perceptual performance. Aim 1 will investigate whether exposure to subliminal stimuli increases subsequent perceptual performance and the amount of information encoded in population activity. Our hypothesis is that subliminal exposure improves perceptual discrimination performance when stimuli are subsequently presented above the detectability threshold, and increases the amount of information extracted from the population response. Aim 2 will examine the mechanism of the improvement in neuronal and behavioral performance after subliminal exposure. We will first focus on causal experiments involving optogenetic inactivation which will test whether suppressing neuronal activity in visual cortex during the presentation of subliminal stimuli reduces the strength of subliminal priming. Cross-correlation analysis will subsequently test whether improved network and perceptual performance after subliminal exposure is consistent with a Hebbian mechanism underlying the increase in functional connectivity specifically for the neurons activated by subthreshold stimuli. Aim 3 will examine the impact of attention on the relationship between subliminal priming and neuronal and perceptual performance. We will test the novel hypothesis that spatial attention reduces the efficacy of subliminal priming – exposure to unattended subliminal stimulation will be associated with a larger improvement in network coding and perceptual performance compared to exposure to attended information. In contrast, we expect that attention will increase the strength of supraliminal priming, i.e., exposure to attended subliminal stimulation will be associated with a larger improvement in network coding and perceptual performance compared to exposure to unattended information. Taken together, our proposed experiments can potentially advance our understanding of information coding in visual cortex by testing the limits of sensory experience and its relationship with perception, which will help develop effective therapies to treat the brain-based aspects ...