Directional tuning of human motion adaptation as reflected by the motion VEP

Motion onset evoked visual potentials are dominated by a negativity (N2) at occipital electrodes and a positivity (P2) at the vertex. The degree of tr ue motion processing reflected by N2 and P2 was estimated from the directio n specificity of motion adaptation. Adapting stimuli moved to the right and test stimuli (random dot patterns of 26 degrees diameter; 10% contrast; 10 .5 degrees /s velocity) moved in one of eight directions, which differed by 45 degrees. VEPs were recorded from occipito/temporal and central sites in eight subjects. Two adaptation effects were observed for N2 (P < 0.01): a global amplitude reduction by 47% and a direction-specific reduction by a f urther 28%. For P2, only the global effect (54%; P < 0.01) was observed. Th e global adaptation effect could also be induced by pattern reversal and pa ttern-onset adaptation, i.e. stimuli containing ambiguous or very little mo tion energy, respectively. We conclude that at least 28% of the N2 amplitud e reflects the activity of direction-specific elements, whereas P2 does not at all. (C) 2001 Elsevier Science Ltd. All rights reserved.