Recent studies (Wolff & Schröger, 2001; Horvath et al., 2001; Näätänen & Rinne, 2002) indicate that occasional repetitions in a sequence of random frequency tones elicit a frontally negative event related potential (ERP) deflection in the latency range of 100-250 ms from stimulus onset.
The characteristics of this ERP are very similar to that of the mismatch negativity (MMN) ERP component. MMN is elicited when some regular feature of a sound sequence is violated. Therefore, if the component elicited by occasional repetitions in an ever-changing sound sequence is truly MMN, it can be assumed that the auditory system pre-attentively extracts a "change regularity", that is, an abstract notion of change itself.
However, it was not investigated, whether the repetition per se elicits the component.
In the current study we investigated whether the elicitation of the repetition-related ERP component is dependent on the probability of repetitions in the sequence. If the component is truly an MMN, then a higher repetition probability would result in a decreased ERP amplitude. In two experiments, tones varying on five distinct pitch levels were presented in random order. The probability of tone repetition was 5%, 20% and 50% (Conditions 1,2 and 3 respectively). In Experiment 1, pure sinusoidal tones were presented, whereas in Experiment 2 spectrally rich tones were used to enhance discriminability. Results showed that the elicitation of the repetition-related ERP component is probability-dependent, i.e. higher probabilities resulted in smaller amplitudes.