Characteristics of visual evoked potentials generated by motion coherence onset.

Prior studies have shown that an electrophysiological correlate of visual motion processing can be found in the N2, a transient negativity occurring at about 200 ms in the visual evoked potential (VEP). In most of the studies, N2 was triggered by the onset of a coherent motion. Results of our first experiment revealed that topography of the negative potential can be modified by motion direction information. In contrast to the onset of uncorrelated motion of pixels in a random dot kinematogram (RDK) correlated motion leads to an right hemispheric amplitude advantage. Hemispheric differences can be increased when the negativity is triggered by the onset of a coherent motion direction preceded by uncorrelated motion in RDKs. In a second experiment, we examined whether the negativity elicited by direction is related to the strength of the impression of motion direction measured psychophysically. The latter was modified by varying the percentage of correlated moving pixels in the RDK. Increasing the proportion of these 'direction signals' was associated with both an increase in the strength of the impression of motion direction and an increase in VEP amplitude. Mean correlations of electrophysiological and psychophysical data, which exceeded 0.7, revealed a higher sensitivity of the right hemisphere. The close relationship between the global motion impression and VEP negativity indicate that electrophysiological correlates of processing stages within visual motion analysis can be isolated. The recorded negativity seems to be associated primarily with the process of global motion integration.