Cochlear Tuning in the Gerbil: A Comparison of Responses to Sinusoidal Amplitude Modulation and Difference Tone Stimuli

Abstract

Vocalizations often have periodic variations of their acoustic waveform envelope. Two simultaneously presented frequencies have an envelope fluctuation with a frequency equal to their difference tone (DT = F₂-F₁). Sinusoidal amplitude modulation (SAM) of a carrier frequency also produces an envelope fluctuation. Electrical ensemble responses to DT and SAM stimuli were recorded from the gerbil's round window. The predominant frequency of the response to the DT stimuli is F₂–F₁; to the SAM stimuli, it is the modulation frequency. Both responses are spectrally, temporally, and dynamically non-linear. Forward masking of a low-frequency DT response produced a tuning curve (TC) with a tip at the high-stimulus frequency. Forward masker TCs of a low-frequency SAM ensemble response had tips at the high frequency of the carrier. Tip thresholds and sharpness of tuning of DT and SAM TCs are quite similar, with cochlear neurons having high characteristic frequencies providing sharply tuned information about low frequency acoustic envelope periodicities.

Key Words:

• Periodicity pitch
• Phase locking
• Cochlea
• Envelope response
• Amplitude modulation
• Gerbil