Plasticity in Primary Auditory Cortex of Monkeys with Altered Vocal Production

Response properties of primary auditory cortical neurons in the adult common marmoset monkey (Callithrix jacchus) were modified by extensive exposure to altered vocalizations that were self-generated and rehearsed frequently. A laryngeal apparatus modification procedure permanently lowered the frequency content of the native twitter call, a complex communication vocalization consisting of a series of frequency modulation (FM) sweeps. Monkeys vocalized shortly after this procedure and maintained voicing efforts until physiological evaluation 5-15 months later. The altered twitter calls improved over time, with FM sweeps approaching but never reaching the normal spectral range. Neurons with characteristic frequencies <4.3 kHz that had been weakly activated by native twitter calls were recruited to encode self-uttered altered twitter vocalizations. These neurons showed a decrease in response magnitude and an increase in temporal dispersion of response timing to twitter call and parametric FM stimuli but a normal response profile to pure tone stimuli. Tonotopic maps in voice-modified monkeys were not distorted. These findings suggest a previously unrecognized form of cortical plasticity that is specific to higher-order processes involved in the discrimination of more complex sounds, such as species-specific vocalizations.