Categorization of and Adaptation to Human Voice and Musical Instruments: A Passive Listening EEG Study

The human voice is a highly socially relevant auditory stimulus, which has been shown to have a special status, both perceptually and neurally. Perceptual studies have revealed adaptation effects in the behavioral categorization of sounds as either human voice or musical instruments. The current study measured evoked responses using EEG to voice and instrument sounds under passive listening to explore the neural underpinnings of both categorization and context effects. In Experiment 1, vowel utterances (/a/, /o/, /u/, and /i/) and instrumental tones (bassoon, horn, saxophone, and viola) were presented with equal probability in a random sequence. The two sound categories were found to produce reliably distinguishable EEG responses at latencies of between 70 and 280 msec. In Experiment 2, an MMN paradigm resulted in mixed evidence for early neural categorization, with an MMN observed for rare instrumental tones embedded in a random sequence of four different vowels, but no significant MMN for a rare vowel embedded in a random sequence of four different instrumental tones. In Experiment 3, ambiguous voice-instrument morphs were used to show that brain responses could be used to predict the context (voice or instrument sound) in which the morphed sounds were presented. The results show that neural correlates of both categorization (voice vs. nonvoice) and context effects can be observed in EEG responses under passive listening conditions.