Not etched in stone: Dynamics of the hand map in primary somatosensory cortex
Lipton, Michael Lawrence
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Since its earliest description, the prevailing view of primary somatosensory cortex in primates holds that the hand is discretely and exclusively represented in the contralateral hemisphere. The hand region of Area 3b receives neither direct thalamocortical afferents from the ipsilateral hand nor callosal connections. Integration of inputs from the digits and the two hands should not occur until at least Areas 1 or 2. Yet, careful assessment of the literature reveals a body of evidence that suggests potential substrates that could support integration of inputs to the hands and digits at the first stage of cortical processing. This dissertation builds on this foundational evidence to demonstrate such integration across the hands and digits in Area 3b and suggests potential implications of the findings for understanding paradoxes of somatosensory perception.;The work begins with unexpected evidence of ipsilateral input from the hand to Area 3b, discovered using functional MRI (fMRI) and elucidated using intracortical electrophysiology. This finding was first elicited during electrical stimulation of the median nerve and later confirmed using light tactile stimulation of the digits. Simultaneous bilateral stimulation of the hands elicited a suppressive (subadditive) interaction effect, whether the animal was conscious or under general anesthesia. Thus, integration of inputs from the hands occurs in Area 3b and is likely to be an essential physiologic mechanism, not dependent on attention or cognitive context.;Assessment of the digit map in Area 3b, using tactile stimulation of individual digits of the contralateral hand, unexpectedly revealed overlapping and poorly circumscribed digit "representations"; stimulation of any digit could elicit a response within a single digit "representation". Simultaneous stimulation of multiple digits produced suppressive (subadditive) interaction effects in Area 3b. The interactions indicate that inputs are integrated at this first level in the somatosensory cortical hierarchy, sharply deviating from the concept of a discrete and fixed map of the digits that has been understood to exist in Area 3b.;This series of unexpected findings suggests a new understanding of hand somatotopy in Area 3b. The mechanisms underpinning such a concept and its implications for function and behavior, however, require further study.
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