Suppression and omission effects in auditory predictive processing-Two of the same?

Recent theories describe perception as an inferential process based on internal predictive models that are adjusted by prediction violations (prediction error). Two different modulations of the auditory N1 event-related brain potential component are often discussed as an expression of auditory predictive processing. The sound-related N1 component is attenuated for self-generated sounds compared to the N1 elicited by externally generated sounds (N1 suppression). An omission-related component in the N1 time-range is elicited when the self-generated sounds are occasionally omitted (omission N1). Both phenomena were explained by action-related forward modelling, which takes place when the sensory input is predictable: prediction error signals are reduced when predicted sensory input is presented (N1 suppression) and elicited when predicted sensory input is omitted (omission N1). This common theoretical account is appealing but has not yet been directly tested. We manipulated the predictability of a sound in a self-generation paradigm in which, in two conditions, either 80% or 50% of the button presses did generate a sound, inducing a strong or a weak expectation for the occurrence of the sound. Consistent with the forward modelling account, an omission N1 was observed in the 80% but not in the 50% condition. However, N1 suppression was highly similar in both conditions. Thus, our results demonstrate a clear effect of predictability for the omission N1 but not for the N1 suppression. These results imply that the two phenomena rely (at least in part) on different mechanisms and challenge prediction related accounts of N1 suppression.