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Parietal Influence on Surround Inhibition in the Human Motor Cortex


Alexander Shaft Photo

University of Nevada School of Medicine (Nevada)

Alexander Shaft, Nivethida Thirugnanasambandam, Mark Hallett

Abstract

Surround inhibition is a mechanism within our nervous system that allows neural activity to be focused for a specific desired response, while simultaneously inhibiting unwanted extraneous activity.  Surround inhibition has been extensively characterized in the visual pathway, where it is involved in contrast enhancement and edge detection.  In human motor control, surround inhibition acts to suppress movement of muscles not synergistic to the desired movement, with defects leading to unwanted muscle contractions.  This dysfunction of surround inhibition is believed to be involved in the abnormal movements and posturing seen in disorders such as focal dystonias.  Several cortical regions and pathways have been examined as possible contributors to surround inhibition, including the contralateral motor cortex, premotor cortex, and supplemental motor area.  However, the contribution of the parietal cortex to surround inhibition has not been studied yet.  In this study, the objective was to investigate the role of the parietal cortex in human motor surround inhibition.  We used the cortico-cortical paired pulse paradigm of transcranial magnetic stimulation (TMS) to study the impact of a conditioning parietal stimulus on the amplitude of motor evoked potentials (MEPs). A sub-threshold conditioning pulse was applied over the anterior part of the inferior parietal lobule (localized by neuro-navigation) that is known to inhibit MEPs.  Stimuli were given either during rest or movement initiation (when surround inhibition is expected), and the amplitude of MEPs in the surround muscle was determined.  Data from 5 healthy subjects show a trend of enhanced surround inhibition when conditioning stimulus was given to the parietal cortex as compared to motor cortex stimulation alone.  These preliminary results would support the conclusion that the parietal cortex does in fact influence surround inhibition in the human motor cortex, and may be a cortical area of interest in focal dystonia patients.

Last updated November 27, 2013