Cerebral processing of timbre and loudness: FMRI evidence for a contribution of Broca's area to basic auditory discrimination

Sound timbre and sound volume processing are basic auditory discrimination processes relevant for human language abilities. Regarding lateralization effects, the prevailing hypotheses ascribe timbre processing to the right hemisphere (RH). Recent experiments also point to a role of the RH for volume discrimination. We investigated the relevance of the RH for timbre and volume processing, aiming at finding possible differences in cerebral representation of these acoustic parameters. Seventeen healthy subjects performed two auditory discrimination tasks on tone pairs, differing either in timbre or volume. FMRI was performed using an EPI-sequence on a 1.5 T scanner. Hemodynamic responses emerged in both tasks within a bilateral network of areas, including cingulate and cerebellum, peaking in primary and secondary auditory cortices (core and belt areas). Laterality analyses revealed a significant leftward dominance at the temporal cortex. Task comparison revealed significant activation within Broca's area during the timbre task and a trend for an increase of right parietal responses during volume processing. These results contribute to a more differentiated view of timbre processing. Additionally to the engagement of the right temporal cortex during processing of musical timbre, there seem to be language related aspects of timbre that are preferentially processed in the left hemisphere. These findings are discussed within the framework of a model of timbre perception comprising two differentially lateralized subprocesses. Processing of spectral cues (harmonic structure) linked to the right hemisphere and processing of temporal cues (i.e. attack-decay dynamics) linked to the left hemisphere. Moreover, activation of Broca's area linked to the timbre task indicates a participation of this area in discriminating phonetic changes of the vowel-like non-speech signals, encouraging the argument that basic acoustic cue processing at a pre- or non-speech level is represented within this "classical language area."