Interhemispheric interactions were studied with
functional brain mapping of visual processing. Children
performed a reaction time task with uni- and bilateral
targets and nontargets. The visual evoked potential (VEP)
was segmented into P1a, P1b, and N1 microstates using map
rather than channel features. Map latencies, amplitudes
and sources were tested for bilateral interactions. Bilateral
targets yielded shorter VEP map latencies but later response
onsets than unilateral ones. Source analyses of the unilateral
VEPs indicated a transition from contra- (P1a) to ipsilateral
(P1b) visual cortex activation (interhemispheric transfer).
Bilateral VEPs were smaller than the summed unilateral
VEPs in all microstates, indicating that interhemispheric
interactions both precede and follow interhemispheric transfer.
Brain mapping of uni- and bilateral VEPs in children thus
revealed several distinct forms of interhemispheric interactions
in the same, early time range.