The ERP omitted stimulus response to "no-stim" events and its implications for fast-rate event-related fMRI designs.

A major difficulty in fast-rate event-related fMRI experiments is the extensive overlap from adjacent trials in the stimulus sequence. One approach to address this problem is to include "no-stim" or "null" events as a trial type. These are randomized as if they were true stimulus events but no stimulus actually occurs. Assuming that no response is elicited by the null events, their time-locked average reflects only the averaged overlap. Thus, contrasting the averages for the other trial types versus the null-event average subtracts out the overlap, enabling the extraction of the response functions for these other trial types. ERP studies, however, have indicated that an endogenous brain response, the omitted stimulus response (OSR), can be evoked by a missing event in a stream of regularly occurring stimuli. To the extent that this response is elicited by null events in an event-related fMRI experiment, the null-event subtraction or contrast would falsely introduce the inverse of the OSR into the averaged responses to the other trial types. Using high-density ERP recordings, we investigated the effect of different percentages of omitted stimuli (11, 22, and 33%) on the auditory OSR at stimulus rates of one event per second or one event per 2 s. Significant OSRs were found for each percentage in the 1-s condition as well as in the 11% 2-s condition. The responses consisted of an early posterior negative wave (180-280 ms) followed by a larger anterior positive wave. These results have important implications for fast-rate fMRI designs, while also providing new data on the brain's response to omitted stimuli.