The Difference of Perception Mechanisms between Sonic and Ultrasonic Sounds

Abstract

Cochlear nerve action potential (AP) was analyzed in normal and kanamycin (KM) treated guinea pigs. The stimuli were filtered bursts of 2, 4, 8, 12kHz and bone-conducted ultrasonic (98.8kHz) pip.
The input-output function of the burst evoked AP (burst AP) in normal animals showed typical two segmented curve. On the other hand, that of ultrasonic evoked AP (UAP) showed steep and linear growth which closely resembles the burst AP input-output function of the KM poisoned animals.
KM treatment elevated the burst AP threshold about 30-60dB but the UAP threshold elevation was only 10dB or so, although the AP latency was elongated significantly in the latter.
Simultaneous masking of the burst AP by continuous ultrasonic sound was extraordinary effective despite of far higher masker frequency. This AP inhibition showed nonlinear growth as the masker intensity became stronger. And the higher the burst frequency became the more prominent the inhibition was.
These results suggested that the ultrasonic receptor is not limited at the basal end of the cochlear partition but has rather wide distribution in the basal turn cochlea. And the upper frequency limit for the air-conducted sound was thought to be defined as the high frequency end of cochlear frequency tuning function as well as the problem of outer and middle ear sound conductance.