Interhemispheric differences in auditory processing revealed by fMRI in awake rhesus monkeys.

Lesion studies in monkeys have suggested a modest left hemisphere dominance for processing species-specific vocalizations, the neural basis of which has thus far remained unclear. We used contrast agent-enhanced functional magnetic resonance imaging to map the regions of the rhesus monkey brain involved in processing conspecific vocalizations as well as human speech and emotional sounds. Control conditions included scrambled versions of all 3 stimuli and silence. Compared with silence, all stimuli activated widespread parts of the auditory cortex and subcortical auditory structures with a right hemispheric bias at the level of the auditory core. However, comparing intact with scrambled sounds revealed a leftward bias in the auditory belt and the parabelt. The left-sided dominance was stronger and more robust for human speech than for rhesus vocalizations and hence does not reflect conspecific call selectivity but rather the processing of complex spectrotemporal patterns, such as those present in human speech and in some of the rhesus monkey vocalizations. This was confirmed by regressing brain activity with a model-derived parameter indexing the prevalence of such patterns. Our results indicate that processing of vocal sounds in the lateral belt and parabelt is asymmetric in monkeys, as predicted from lesion studies.