[Contribution of cochlear nucleus to 80Hz amplitude-modulation following response]

The steady-state response (SSR) evoked by a sinusoidally amplitude-modulated (SAM) tone is known as an amplitude-modulation following response (AMFR). The amplitude of the SSR which is elicited using clicks or tone bursts at a stimulus rate of 40 Hz, decreases during sleep. The same trend is also observed for AMFR at a modulation rate of 40 Hz. Thus it was difficult to analyzing SSR and AMFR is therefore difficult in young children, since objective audiometry just be performed while the child is asleep. Recent reports, however, have announced that the AMFR can be clearly detected at higher MFs (modulation frequencies), especially at frequencies between 80 and 100 Hz. This finding has proven useful in objective audiometry for young children. Recent reports have also suggested that AMFRs arise from multiple sources, including the auditory cortex and auditory nuclei in the brainstem. However, the exact sources of AMFRs have not been clarified. The purpose of this study is to clarify the contribution of the cochlear nucleus in evoking AMFRs at a modulation frequency of 80 Hz. The near-field potentials elicited with a SAM tone were recorded from the ipsilateral cochlear nucleus and its vicinity in cats. The near-field potential recorded by bipolar electrodes consisted of two different components: a low frequency component similar to the stimulus envelope (modulation), and a high frequency component similar to the actual stimulus tone. A sequence of field potentials was recorded using monopolar electrodes located at different sites within the cochlear nucleus and in its vicinity and on the surface of the cerebellum to confirm that near-field potentials elicited by SAM tones at a MF of 80 Hz in the cochlear nucleus can be recorded at the surface of the cerebellum or at other brain sites. The phase of the 80 Hz frequency component of the potentials elicited by a SAM tone at a MF of 80 Hz was then analyzed using a fast Fourier transformation. A contour map was produced using the means of the Fourier component phases corresponding to the 80 Hz response. The contour lines showed a rapid change in the phases recorded near the cochlear nucleus. These findings suggest that the cochlear nucleus contributes to the generation of scalp recorded AMFR at a MF of 80 Hz.