Functional differentiation in the human auditory and language areas revealed by a dichotic listening task.

The human auditory cortex plays a special role in speech recognition. It is therefore necessary to clarify the functional roles of individual auditory areas. We applied functional magnetic resonance imaging (fMRI) to examine cortical responses to speech sounds, which were presented under the dichotic and diotic (binaural) listening conditions. We found two different response patterns in multiple auditory areas and language-related areas. In the auditory cortex, the medial portion of the secondary auditory area (A2), as well as a part of the planum temporale (PT) and the superior temporal gyrus and sulcus (ST), showed greater responses under the dichotic condition than under the diotic condition. This dichotic selectivity may reflect acoustic differences and attention-related factors such as spatial attention and selective attention to targets. In contrast, other parts of the auditory cortex showed comparable responses to the dichotic and diotic conditions. We found similar functional differentiation in the inferior frontal (IF) cortex. These results suggest that multiple auditory and language areas may play a pivotal role in integrating the functional differentiation for speech recognition.