Discrepancy between reaction time and visual evoked magnetic response latency under priming.

We examined effects of the interval between first (S1) and second (S2) visual stimuli, stimulus-onset asynchrony (SOA), on the cortical processing of S2 using magnetoencephalography (MEG). S1 (a circle) and S2 (a cross) were presented at the same location. When the SOA was short (33, 50, and 83 ms), the major deflection of the fields evoked in response to S2 (2M) was difficult to distinguish from that evoked by S1 (1M). However, when the SOA was long (350 and 453 ms), paired stimuli clearly evoked 1M and 2M. At a long SOA, the peak latency of 2M (around 200 ms) was significantly longer than that in the control (S2 alone) condition (around 180 ms). However, in contrast, the reaction time to S2 in all SOA conditions was significantly shortened as compared with that in the control condition, suggesting dissociation between processing in the visual cortex and motor processing.