Auditory evoked responses in the rat: transverse mastoid needle electrodes register before cochlear nucleus and do not reflect later inferior colliculus activity.

A previously described technique putatively differentiates short-latency auditory evoked potentials in peripheral and central neural pathways of the mouse and rat [Galbraith G, Waschek J, Armstrong B, Edmond J, Lopez I, Liu W, et al. Murine auditory brainstem evoked response: putative two-channel differentiation of peripheral and central neural pathways. J Neurosci Methods 2006;153:214-20]. This technique involves recording from orthogonally oriented subdermal needle electrode pairs, using fast sample rates (100k/s) to accurately measure differences in neural timing and waveform morphology. Electrodes oriented in a transverse plane (mastoid-to-mastoid) register an initial positive-going peak earlier than peaks recorded from electrodes oriented along the scalp midline (anterior and posterior to the interaural line). The absolute latency of the early mastoid component is consistent with an origin in the primary auditory nerve, while delayed midline latencies implicate activity in central neural pathways. We report here the results of recording simultaneously from transverse mastoid (M) needle electrodes and electrodes acutely implanted in cochlear nucleus (CN) and inferior colliculus (IC). The results show a highly consistent pattern in which the initial mastoid component leads CN by an average of 0.16 ms, suggesting an obligatory neural site of origin of the mastoid response that is distal to IC, namely the auditory nerve. Moreover, later IC components (beyond approximately 3.5 ms) are completely absent in mastoid recordings, indicating that the transverse mastoid recordings provide a relatively isolated measure of early auditory neural activity.