Cortical representation of natural complex sounds: effects of acoustic features and auditory object category

How the brain processes complex sounds, like voices or musical instrument sounds, is currently not well understood. The features comprising the acoustic profiles of such sounds are thought to be represented by neurons responding to increasing degrees of complexity throughout auditory cortex, with complete auditory "objects" encoded by neurons (or small networks of neurons) in anterior superior temporal regions. Although specialized voice and speech-sound regions have been proposed, it is unclear how other types of complex natural sounds are processed within this object-processing pathway. Using functional magnetic resonance imaging, we sought to demonstrate spatially distinct patterns of category-selective activity in human auditory cortex, independent of semantic content and low-level acoustic features. Category-selective responses were identified in anterior superior temporal regions, consisting of clusters selective for musical instrument sounds and for human speech. An additional subregion was identified that was particularly selective for the acoustic-phonetic content of speech. In contrast, regions along the superior temporal plane closer to primary auditory cortex were not selective for stimulus category, responding instead to specific acoustic features embedded in natural sounds, such as spectral structure and temporal modulation. Our results support a hierarchical organization of the anteroventral auditory-processing stream, with the most anterior regions representing the complete acoustic signature of auditory objects.