Contribution of harmonicity and location to auditory object formation in free field: evidence from event-related brain potentials.

The contribution of location and harmonicity cues in sound segregation was investigated using behavioral reports and source waveforms derived from the scalp-recorded evoked potentials. Participants were presented with sounds composed of multiple harmonics in a free-field environment. The third harmonic was either tuned or mistuned and could be presented from the same or different location from the remaining harmonics. Presenting the third harmonic at a different location than the remaining harmonics increased the likelihood of hearing the tuned or slightly (i.e., 2%) mistuned harmonic as a separate object. Partials mistuned by 16% of their original value "pop out" of the complex and were paralleled by an object-related negativity (ORN) that superimposed the N1 and P2 components. For the 2% mistuned stimuli, the ORN was present only when the mistuned harmonic was presented at a different location than the remaining harmonics. Presenting the tuned harmonic at a different location also yielded changes in neural activity between 150 and 250 ms after sound onset. The behavioral and electrophysiological results indicate that listeners can segregate sounds based on harmonicity or location alone. The results also indicate that a conjunction of harmonicity and location cues contribute to sound segregation primarily when harmonicity is ambiguous.