Adaptation in visual cortex: Issues of timescale and hierarchy
Patterson, Carlyn A.
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Sensory systems constantly update how they process information. Adaptation is a type of experience-dependent plasticity, in which the stimulus history of the preceding hundreds of milliseconds to minutes can alter neuronal responses and perception. The role of cortical adaptation in visual processing remains unclear, in part because fundamental features of adaptation are poorly understood. First, adaptation is distinguished from other forms of experience-dependent plasticity by the timescale of its effects. Yet the timescales of adaptation span several orders of magnitude, and effects on sensory processing within this range may differ. Second, sensory systems form hierarchically organized networks, but most studies of adaptation focus on a single area or level and it remains unclear how adaptation affects systems as a whole. In this thesis, I explore adaptation effects over different timescales and cortical areas. To accomplish this, I performed extracellular recordings in anesthetized macaque monkeys in two areas involved in visual motion processing: primary visual cortex (V1) and area MT, which receives the majority of its input from V1.;First, I determined that effects of adaptation in V1 differ in both magnitude and quality after adaptation lasting hundreds of milliseconds (the length of a visual fixation) to tens of seconds (often used in perceptual studies). These results provide evidence for multi-timescale adaptation effects, suggesting distinct mechanisms and functions for the plasticity triggered by brief and prolonged exposures.;Second, I aimed to determine how adaptation effects in V1 altered responses downstream in MT. I found that adaptation effects induced with drifting gratings (simple motion stimuli) in MT closely mirror those observed in V1 under a variety of matched stimulus conditions. These findings suggest that MT simply inherits adaptation effects from V1. Further, I found that inheriting these adaptation effects reduced motion integration in MT, a more complex response property not observed in V1.;Taken together, these findings paint a more intricate picture of adaptation than previously described. Indeed, the results presented here indicate a diversity of adaptation effects across different timescales that affects multiple levels of the visual hierarchy.
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