Physiological Evidence for the Masking of Low Frequencies by High

Abstract

In guinea pigs with intracochlear electrodes, successive sound waves at 500 cps set off easily detectable, well-synchronized volleys of action potential (AP) and also the familiar cochlear microphonic (CM). At low levels of stimulation, a high-frequency band of noise centering around 7000 cps and having no appreciable acoustic energy in the 500-cps region will not mask these responses, but as the intensity of the high-frequency band is raised to moderately high levels, two results are seen. First, the 500-cps CM in the first turn is, as we might expect, overlaid with the high frequency. Unexpectedly, however, the AP is reduced and blurred. This is best explained as the elimination of that part of the AP to 500 cps that originates in the first turn. Second, at still higher intensities, the AP and CM in the third turn to a 500-cps pip are masked just as they would be by a lower level low-frequency band. A random, low-frequency CM now appears in the apical region. Apparently the ear responds, nonlinearly to the high-level noise and “detects” its envelope as random low frequencies of random amplitude. This anomalous masking is produced by noise but not by pure tones. [This work was supported under Contract N6onr-272 with the Office of Naval Research.]