Spatial principal components of multichannel maps evoked by lateral visual half-field stimuli

Multichannel records of responses to large and small hemiretinal stimuli were obtained from 6 healthy subjects. Scalp distribution maps were constructed for all conditions at all post-stimulus times and component latencies were objectively determined by computing a reference-independent measure of field power. This procedure identified 2 components (at 100 and 140 msec). The scalp distribution data at these latencies were entered into a spatial principal components analysis which further reduced the data set to three underlying spatial principal components. These components may be regarded as reflecting underlying processes and were related to experimental conditions. A component reflecting lateralized brain activity displayed a significant interaction between size and retinal location of the stimulus with large stimuli showing a more pronounced lateralization over the hemisphere contralateral to the stimulated hemiretina, and the scores on this component were low for upper hemiretinal stimuli. These findings are in agreement with intracranially recorded evoked potential data and theoretical dipole source computations, and confirm a model of cortical neuronal generator processes whose locations and orientations in the hemisphere ipsilateral to the hemiretina stimulated are influenced not only by retinal stimulus location but also by stimulus size.