Visuo-spatial neural response interactions in early cortical processing during a simple reaction time task: a high-density electrical mapping study

The timecourse and scalp topography of interactions between neural response s to stimuli in different visual quadrants, straddling either the vertical or horizontal meridian, were studied in 15 subjects. Visual evoked potentia ls (VEPs) were recorded From 64 electrodes during a simple reaction time (R T) task. VEPs to single stimuli displayed in different quadrants were summe d ('sum') and compared to the VEP response from simultaneous stimulation of the same two quadrants ('pair'). These responses would be equivalent if th e neural responses to the single stimuli were independent. Divergence betwe en the 'pair' and 'sum' VEPs indicates a neural response interaction. In ea ch visual field. interactions occurred within 72-86 ms post-stimulus over p arieto-occipital brain regions. Independent of visual quadrant, RTs were fa ster for stimulus pairs than single stimuli. This replicates the redundant target effect (RTE) observed for bilateral stimulus pairs and generalizes t he RTE to unilateral stimulus pairs. Using Miller's 'race model inequality (Miller J. Divided attention: evidence for coactivation with redundant sign als, Cognitive Psychology 1982:14:247-79), we found that probability summat ion could fully account for the RTE in each visual field. Although measurem ents from voltage waveforms replicated the observation of earlier peak P1 l atencies for the 'pair' versus 'sum comparison (Miniussi C, Girelli M, Marz i CA. Neural site of the redundant target effect: electrophysiological evid ence. Journal of Cognitive Neuroscience 1998,10:216-30), this did not hold with measurements taken from second derivative (scalp current density) wave forms. Since interaction effects for bilateral stimulus pairs occurred with in 86 ms and require interhemispheric transfer. transcallosal volleys must arrive within 86 ms, which is earlier than previously calculated. Interacti on effects for bilateral conditions were delayed by approximate to 10 ms ve rsus unilateral conditions. consistent with current estimates of interhemis pheric transmission time. Interaction effects place an upper limit on the l ime required for neuronal ensembles to combine inputs from different quadra nts of visual space ( approximate to 72 ms for unilateral and approximate t o 82 ms for bilateral conditions). (C) 2001 Elsevier Science Ltd. All right s reserved.