Mismatch negativity to pitch change: varied stimulus proportions in controlling effects of neural refractoriness on human auditory event-related brain potentials.

Based on a memory-comparison process, changes in the pitch of repetitive sounds are pre-attentively detected, reflected by the mismatch negativity (MMN) event-related brain potential (ERP). In such oddball sequences, ERP responses are also affected by differential refractory states of frequency-specific afferent cortical neurons. This contamination of MMN can be controlled using an additional blocked sequence of equiprobable tones. The present study investigated effects of varying the in-sequence probabilities of these control tones from 10 to 45%, respectively. Results showed that an equal distribution of all control-sequence tones is not necessary for efficiently removing neural refractoriness effects on the ERPs. The genuine memory-comparison-based frequency-change MMN can be estimated using sequences, within which the number of equiprobable control tones varies between three and nine.