Visual mismatch negativity: new evidence from the equiprobable paradigm.

In visual oddball studies, deviant compared to standard stimuli elicited a posterior negative ERP at around 100-250 ms. To determine the underlying processes of the negativity, we used the equiprobable sequence in which bar stimuli of five types of orientation were presented with equal probabilities (control 20% each) as well as the oddball sequence in which two stimuli with the closest orientation were presented with different probabilities (deviant 20% and standard 80%). Deviant compared to standard stimuli elicited two negativities at around 100-150 ms with no hemispheric dominance and 200-250 ms with right hemispheric dominance, while deviant compared to control stimuli elicited only a negativity at around 200-250 ms with right hemispheric dominance. These results suggest that the early negativity reflects refractory effect, while the late negativity reflects memory-comparison-based change detection effect (visual mismatch negativity).