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Abstract:
Our vocal tone—the prosody—contributes a lot to the meaning of speech beyond the actual words. Indeed, the hesitant tone of a ‘yes’ may be more telling than its affirmative lexical meaning. The human brain contains dorsal and ventral processing streams in the left hemisphere that underlie core linguistic abilities such as phonology, syntax and semantics. Whether or not prosody—a reportedly right-hemispheric faculty—involves analogous processing streams is a matter of debate. Functional connectivity studies on prosody leave no doubt about the existence of such streams, but opinions diverge on whether information travels along dorsal or ventral pathways, or both. Here we show, in a novel paradigm using audio morphing of prosody combined with functional/diffusion-weighted neuroimaging (fMRI/DWI; Experiment 1) and transcranial magnetic stimulation (TMS; Experiment 2), that prosody perception takes dual routes along dorsal and ventral pathways in the right hemisphere. In Experiment 1, categorization of speech stimuli that gradually varied in their prosodic pitch contour (between statement and question) involved (i) an auditory ventral pathway along the middle longitudinal fascicle in the superior temporal lobe, and (ii) an auditory-motor dorsal pathway connecting posterior temporal and laryngeal premotor/inferior frontal areas via the arcuate/superior longitudinal fascicle. In Experiment 2, 15 minutes of inhibitory repetitive TMS of right (but not left) laryngeal premotor cortex as a key node of the dorsal pathway decreased participants’ performance in prosody categorization (but not in a control task), arguing for a motor involvement in prosody perception. Following prevailing dual-stream models of language, we propose that prosody perception resides on complementary mechanisms implemented in ventral and dorsal streams in the right hemisphere: While the ventral pathway may extract and integrate auditory features into a time-invariant “prosodic Gestalt” (‘What’) to map prosody to communicative meaning, the dorsal pathway is more likely to map the perceived pitch contour to (subvocal) articulation (‘How’) to enhance the perception of subtle vocal prosodic cues. In sum, our data draw a dual-stream picture of prosodic processing that shows plausible analogies to the established left-hemispheric multi-stream architecture of language, but with relative rightward asymmetry.
