Electrophysiologic Indices of Inter- and Intra-Hemispheric Visuo-Spatial Integration.

Abstract:
Although visual space is perceived as a unitary whole, its cortical representation is initially divided between hemispheres and quadrants. The neurophysiology subserving the integration of visual space is not well understood. We conducted an experiment to identify the dynamics of cortical processing for presentations, either restricted to a single visual field unilaterally or divided between them bilaterally. Psychophysical evidence directly speaks to this integration. The redundant target effect (RTE) is the speeded reaction time on sensory detection tasks to displays of 2 identical stimuli versus displays containing a single stimulus. Electrophysiologic investigation of the RTE to bilateral displays has shown latency shifts in the visual evoked potential (VEP) compared to summed VEPs from constitutive single-stimulus trials. We investigated whether or not identical electrophysiologic effects occur following unilaterally redundant presentations. Data from a 64-channel array reveal shortened VEP latencies, which are indicative of nonlinear cortical processing. This nonlinearity is retinotopically independent to the extent that it occurs following both intrahemispheric and interhemispheric cortical processing.