Ultrasonic Evoked Responses in Rat Cochlear Nucleus.

Numerous studies have reported auditory brainstem responses evoked by stimuli within the "normal" hearing range of rats, with maximum sensitivity peaking around 16 kHz. Yet rats also emit and respond to sounds in the ultrasonic (US) frequency range (30-100 kHz). However, very few electrophysiological studies have recorded auditory brainstem responses using US stimuli, and none have exceeded 70 kHz. We report here short-latency (1-3 ms) evoked potentials recorded in rat cochlear nucleus (CN) to US stimuli ranging from 40 to 90 kHz. Robust responses were recorded in 33 of 36 CN recording sites to stimuli ranging from 40 to 60 kHz; and twenty-eight of these sites continued to yield well-defined responses out to 90 kHz. Latencies systematically increased and overall amplitudes decreased with increasing US frequency. Amplitudes differed significantly in the three CN subnuclei, being largest in posterior-ventral (PVCN) and smallest in anterior-ventral (AVCN). The fact that well-defined responses can be recorded to stimuli as high as 90 kHz significantly extends the recorded upper frequency range of neural activity in the brainstem auditory pathway of the rat. These evoked potential results agree with the well-documented behavioral repertoire of rats in the US frequency range.