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Abstract:
Vocal tract resonances (or formants) are acoustic signatures in the voice related to the shape and length of the vocal tract. Formants play a role in both human and monkey vocal communication, allowing listeners to extract important information related to the physical characteristics of the speaker, such as age, sex and/or body size. We investigated the neural representation of this acoustic cue in rhesus monkey auditory cortex using parametric manipulations of synthetic coo calls. The synthesis procedure allowed us to generate 4 different fundamental frequencies and 4 different vocal tract lengths (generating a large set of formant frequencies) independently, allowing us to explore whether auditory cortex is differentially sensitive to one cue or the other. Duration of the call was also a variable, as the original vocalizations were from 4 different monkey callers. Spiking activity (multiunit clusters and single units) and local field potential (LFPs) were recorded from the core (44 sites) and lateral belt fields (33 sites) of two passively listening monkeys. Responses to these coo stimuli were complex, often multi-peaked. As expected for vocal stimuli, response amplitudes were greater in the belt than the core. Using multiple regression, with fundamental frequency, formant frequencies and duration as factors, we analyzed spikes and LFPs pooled across cortical sites in a given auditory area. For both types of neuronal signals, formants and duration explained a significant portion of the variance for belt and core regions. The fundamental frequency explained a much smaller proportion of the variance. The coefficient of regression was higher for the belt region compared to the core region. Removing formants, but not fundamental frequencies, drastically reduced the coefficient, demonstrating that the belt region may respond to the formant frequencies in vocalizations better than the core region. These cortical differences, revealed with the use of parametrically-manipulated, ethologically-relevant sounds, hint at the functional organization of auditory cortex for real world events.
