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Trott, Alexander

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Trott, Alexander

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    Contextual Modulation of Early Visual Processing and the Role of Cortico-Cortical Feedback
    (2016-05-14) Trott, Alexander; Kreiman, Gabriel; Assad, John; Andermann, Mark; Sheinberg, David
    How does the brain form a useful representation of its external environment from the astoundingly complex patterns of sensory signals it receives? This question has motivated the field of systems neuroscience for decades. Our current understanding stems from the discovery that sensory cortex is a hierarchical system, where the behaviorally relevant information that is only implicit in sensory inputs is made explicit through sequential stages of processing. The connections between cortical areas that create this processing sequence are abundantly reciprocal, such that information flows from lower- to higher-order areas through feedforward connections and in the reverse direction through feedback connections. Despite the anatomical abundance of feedback, its role in computation is poorly understood. Recent evidence suggests that the role of feedback relates to contextual modulation, a process by which neural responses to sensory information are shaped by the behavioral or stimulus setting in which that information arrives. A canonical example of contextual modulation is surround suppression in primary visual cortex (V1). Here, we demonstrate the richness and specificity with which the stimulus surround tailors the representations of context embedded stimuli in V1 of awake macaque monkeys. In addition, we reversibly inactivated cortical areas V2/V3 and their feedback projections to V1 to identify a possible role of feedback in mediating these sophisticated forms of modulation. The effects of feedback inactivation were observed to differ considerably between subjects, but our overall findings suggest that feedback contributes to divisive modulation in a manner that, as shown by previous theoretical work, reflects optimal adaptation to the statistics of natural stimuli. In addition, we observed that noise correlations in V1 were restructured from day to day in a stimulus-dependent and feedback-dependent manner. We use these findings to propose future experiments that will allow us to better understand the diversity of inactivation effects across subjects as well as how the precise network-level signatures of contextual modulation depend on feedback. We expect that these details will provide critical insights towards an understanding of the role of cortico-cortical feedback in sensory processing and cortical computation.