Physiological Evidence for the Masking of Low Frequencies by High

Abstract

Observations were made on guinea pigs using intracochlear electrodes. Successive waves of low frequency stimulation set off well-synchronized volleys of action potentials as well as cochlear microphonics. The phenomenon of masking of a 500 cps tone pip is observed when a low-frequency band of noise, centered on 500 cps, obscures the cochlear microphonic and masks the action potentials that result from the pip. At comparable levels of stimulation, a high-frequency band of noise centered on 6950 cps and having no appreciable energy in the 500 cps region will not mask. But, as the intensity of the high-frequency sound is raised to high levels, masking occurs. A random, low-frequency cochlear microphonic now appears in the apical region in addition to the expected high-frequency cochlear response in the basal turn. We infer that the ear responds non-linearly to the high-level noise and "detects" its envelope as random low frequencies of random amplitude. This anomalous masking is produced both by a band of noise and by a high-frequency tone which is amplitude modulated by a low frequency.