Pre-attentive categorization of sounds by timbre as revealed by event-related potentials

Infrequent (10%) pure tones were randomly presented among nine different missing-fundamental tones having the same pitch (10% each) to subjects playing a computer game. MMN (an index of pre-attentive change detection) was elicited by timbre-deviant pure tones with 150 and 500 ms stimulus duration. This suggests that the spectral component of timbre is pre-attentively determined from relatively short (150 ms) acoustic samples. Previous research established that resolving the pitch of the same missing-fundamental tones requires longer (> 150 ms) sounds. Consequently, timbre and pitch are probably determined by separate neural processes. The present results also demonstrate pre-attentive categorization of sounds based on timbre as MMN could only be elicited by the pure tones if their timbre was contrasted with the combined group of the nine standard sounds of qualitatively similar rich timbre.