The modulation of the sensitivity of the mammalian cochlea by low frequency tones. I. Primary afferent activity.

The neural activity of single cells within the spiral ganglion of the first turn of the guinea pig cochlea has been recorded. Period histograms of the spike activity have been collected. The histograms were synchronised with a loud low frequency tone (40 Hz) presented at a constant intensity below the threshold of the cells to phase locking. The activity evoked by a continuous high frequency tone within the tip of the frequency threshold curve was modulated by the presence of a subthreshold low frequency tone. Suppression of this evoked activity was observed for low frequency displacements of the cochlear partition towards scala tympani, and potentiation was observed for moderate displacements towards scala vestibuli. At higher intensities of the low frequency tone suppression is also observed for peak displacements of the partition towards scala vestibuli. The changes to these histograms with increases in the intensity of the high frequency tone are described. Hysteresis, flattening of the histograms at high intensities and nonmonotonic growth of firing rate at certain phases of the low frequency tone were observed. These changes are believed to be due to fast adaptation of the neural response rather than similar changes to the d.c. receptor potential within the inner hair cells that the neurons innervate.