Response characteristics of the rat anterior cingulate cortex to ultrasonic communicative vocalizations.

Emotional communication involves transmitting information on affective states. Rat 50- and 22-kHz ultrasonic vocalizations (USVs) are emotional communication signals that reflect positive or negative states, respectively. In general, emotional communication signals that elicit changes in event-related potentials (ERPs), suggesting a specific mechanism for processing these signals. As this is observed in several communication systems including humans, we hypothesized that rat USVs can also trigger such changes. In this study, we measured rat local field potentials and used them to calculate ERPs from the anterior cingulate cortex (ACC), an area related to motor production of USVs. In Experiment 1, we measured ERPs to pure tones with peak frequencies around 50 or 22 kHz to examine whether the ACC is involved in auditory sensory processing related to pitch. The results showed that pure tones around 50 kHz elicited large amplitude ERPs, whereas those around 22 kHz resulted in smaller ERPs. In Experiment 2, we compared ERPs in response to rat USVs (emotional stimuli) with ERPs to pure tones modeled after USVs (neutral stimuli). The peak frequencies were the same between USVs and neutral sounds, but USVs showed more frequency modulation than neutral sounds. USVs elicited larger changes in the amplitude of ERP components than neutral stimuli did. These experiments suggest that the rat ACC is sensitive to frequencies around 50 and 22 kHz, and is especially tuned to frequency modulation around these frequencies. Taken together, the rat ACC may process the emotional content of communication signals on the basis of pitch and frequency modulation.