Modulation of responses of spiral ganglion cells in the guinea pig cochlea by low frequency sound.

Period histograms were generated from single unit data obtained from the spiral ganglion of the first turn of the guinea pig cochlea in response to continuous tones between 40 and 500 Hz. With the lowest intensities used, spontaneous activity was suppressed during basilar membrane displacement (inferred from cochlear microphonic phase) towards scala vestibula and activity was enhanced during displacement towards scala tympani. At higher intensities the response changed to excitation during maximal basilar membrane velocity towards scala vestibuli. These patterns were delayed by about 0.5 ms producing large phase delays at the higher frequencies. We postulate that the displacement response is produced by cochlear microphonic originating from the outer hair cells acting on the inner hair cell membrane. In contrast, the velocity response is produced by the inner hair cell receptor potential. The effect of a 40 Hz tone on activity evoked by tones above, at, and below the characteristic frequency was investigated by generating period histograms synchronous with the 40 Hz tone. We found that activity evoked by tones around the characteristic frequency of the cell was suppressed during displacement of the basilar membrane towards scala tympani and enhanced in the opposite direction at 40 Hz intensities that had no effect on spontaneous activity. Further increase in the 40 Hz intensity produced suppression during scala vestibuli displacement with activity remaining only during the zero crossings. Still further increase produces the 40 Hz tone alone response. Activity evoked by tones in the low frequency 'tails' of the frequency threshold curve was not similarly modulated. This phenomenon is though to be related to basilar membrane nonlinearity for frequencies close to the cut-off. Investigation of the effect of a 40 Hz tone on the threshold of the compound action potential confirmed data obtained from single units.