Sharply tuned cochlear nerve ensemble periodicity responses to sonic and ultrasonic frequencies

Vertebrates are able to perceive the pitch of a series of harmonics, even when the fundamental frequency has been removed from the acoustic stimulus. Neural periodicity responses corresponding to the "missing fundamental" frequency of sonic stimuli have been observed in the auditory system of several animal species, including our own. This paper examines periodic cochlear neural responses of the gerbil. Periodicity responses to both sonic and ultrasonic stimuli originate within the cochlea of this animal. Acoustic stimuli, consisting of 2-12 successive harmonic frequencies, were used to generate an ensemble cochlear nerve periodicity response that was recorded from the round window of the cochlea. This response had a frequency equal to that of the missing fundamental, and not to those of the harmonic stimuli. Forward masking of the stimuli used to produce the periodicity response was used to generate sharp tuning curves, with tip frequencies corresponding to the harmonics and not to the periodicities. The sharpness of these functions increased as the frequencies of the harmonics increased, up to at least 38 kHz. This property could be related to reception of ultrasonic vocalizations utilized by many rodent species.