Effects of omission probability on omitted stimulus potentials

Predictions of upcoming sensory events allow the brain to minimize surprises and optimize behavior. When an expected event fails to happen, omitted stimulus potentials (OSPs) occur, consisting of early negative components (oN1 and oN2) and a later positive component (oP3). These OSPs are thought to reflect hierarchical stages of prediction-error signaling, but the functional significance of each component remains unclear. To better characterize these components, this study examined how omission probabilities affect the amplitudes of these components. Participants repeatedly pressed a button at approximately 1-s intervals to trigger a 1000 Hz pure tone. The tone was occasionally omitted, with probabilities of 10%, 20%, 30%, 40%, and 50%. The oN1, oN2, and oP3 components were clearly elicited by low-probability omissions, and their amplitudes decreased systematically as omission probability increased. oN1 and oN2 were largest up to the 20% omission and disappeared at 50%, whereas oP3 remained present across all probability conditions. These results suggest that rarer omissions elicit stronger prediction error responses: oN1 and oN2 appear to index prediction error signaling. In contrast, oP3 is likely to be more closely associated with updating the stimulus context, such as action-effect associations. The present findings provide a useful reference for future OSP research by clarifying how omission probability influences the emergence and magnitude of omission responses.