Division of labor between left and right human auditory cortices during the processing of intensity and duration.

Intensity and duration are important parameters for the processing of speech and music. Neuroimaging results on the processing of these parameters in tasks involving the discrimination of stimuli based on these parameters are controversial. Depending on the experimental approach, varying hypotheses on the involvement of the left and right auditory cortices (ACs) have been put forward. The aim of the present functional magnetic resonance imaging (fMRI) study was to find differences and commonalities in location and strength of brain activity during the processing of intensity and duration when the same stimuli have to be actively categorized according to these two parameters. For this we used a recently introduced method to determine lateralized processing in the AC with contralateral noise. Harmonic frequency modulated (FM) tone complexes were presented monaurally without and with contralateral noise. During categorization of the tones according to their intensity, contralateral noise increased activity mainly in the left AC, suggesting a special role for the left AC in this task. During categorization of tones according to their duration, contralateral noise increased activity in both the left and the right AC. This suggests that active categorization of FM tones according to their duration does not involve only the left AC as has been suggested, but also the right AC to a substantial degree. The area around Heschl's sulcus seems to be the most strongly involved during both intensity and duration categorization, albeit with different lateralization. Altogether the results of the present study support the view that the lateralized processing of the same stimuli in the human AC is strongly modulated by the given task (top-down effect).