Late Waves In The Response Recorded From The Intracranial Portion Of The Auditory Nerve In Humans
We showed in previous studies that the click-evoked responses from the exposed eighth nerve in some patients contain quasiperiodic components that appear in the interval between 4 and 16 milliseconds after the click stimulus, and that the phase of these oscillatory components reverses when the click polarity is reversed. When the responses to clicks of opposite polarity are subtracted from each other, these late waves appear as a prolonged quasiperiodic oscillation with a frequency around 700 Hz. These late components appear more frequently in patients with high-frequency hearing losses than in patients with normal hearing. We have attributed these components to prolonged and in-phase oscillations of a large portion of the basilar membrane, possibly caused by the generation of standing waves on the basilar membrane. The results of the present study show that these oscillations correspond closely in both frequency and phase to the late oscillations that are seen in the cross-correlograms of the responses from the exposed eighth nerve to pseudorandom noise. The finding that similar quasiperiodic components can be retrieved from the responses from the exposed eighth nerve to both transient and continuous sounds is taken as an indication that the neural activity that these components represent reflects some general property of the cochlear frequency analyzer. We also found that the speech discrimination is not noticeably different in patients with such waves compared with what it is in patients with similar hearing loss who do not have such waves. This is taken as an indication that spectral analysis in the cochlea is less important in speech discrimination than previously assumed. The importance of timing of auditory nerve activity for speech perception is indicated by the finding that surgical trauma of the intracranial portion of the auditory nerve that only causes a moderate degree of deterioration of the pure tone threshold reduces speech discrimination scores to zero.