Apparent motion confounds early vernier visual evoked potentials

Human scalp potentials evoked by vernier stimuli have been recorded for offsets less than the diameter of a foveal cone, but always for abruptly moving stimuli. Those evoked potentials were related to the magnitude of vernier offset. Here we report results for stimuli containing no apparent motion confound, using multichannel recordings and multivariate analysis methods which stress the concept of a sampling of the scalp field. We found evidence of cortical activity dependent on the direction of vernier offset, at much shorter latencies (ca. 75 ms) than previously reported, but no evidence of early cortical activity related to the magnitude of offset. Repeating the experiments and analysis using a stimulus containing apparent motion, we found evidence of cortical activity dependent on the magnitude of offset at both 75 and 200 ms, but none related to direction of offset. These findings suggest that previous studies which contained the apparent motion confound might not have obtained visual evoked potentials entirely due to vernier offset.