Tracking Vocal Pitch through Noise: Neural Correlates in Nonprimary Auditory Cortex

In natural environments, a sound can be heard as stable despite the presence of other occasionally louder sounds. For example, when a portion in a voice is replaced by masking noise, the interrupted voice may still appear illusorily continuous. Previous research found that continuity illusions of simple interrupted sounds, such as tones, are accompanied by weaker activity in the primary auditory cortex (PAC) during the interruption than veridical discontinuity percepts of these sounds. Here, we studied whether continuity illusions of more natural and more complex sounds also emerge from this mechanism. We used psychophysics and functional magnetic resonance imaging in humans to measure simultaneously continuity ratings and blood oxygenation level-dependent activity to vowels that were partially replaced by masking noise. Consistent with previous results on tone continuity illusions, we found listeners' reports of more salient vowel continuity illusions associated with weaker activity in auditory cortex (compared with reports of veridical discontinuity percepts of physically identical stimuli). In contrast to the reduced activity to tone continuity illusions in PAC, this reduction was localized in the right anterolateral Heschl's gyrus, a region that corresponds more to the non-PAC. Our findings suggest that the ability to hear differently complex sounds as stable during other louder sounds may be attributable to a common suppressive mechanism that operates at different levels of sound representation in auditory cortex.