Single Neuron Contributions to the Auditory Brainstem EEG

Abstract

The auditory brainstem response (ABR) is an acoustically evoked EEG potential that is an important diagnostic tool for hearing loss, especially in newborns. The ABR originates from the response sequence of auditory nerve and brainstem nuclei, and a click-evoked ABR typically shows three positive peaks (“waves”) within the first six milliseconds. However, an assignment of the waves of the ABR to specific sources is difficult, and a quantification of contributions to the ABR waves is not available. Here, we exploit the large size and physical separation of the barn owl first-order cochlear nucleus magnocellularis (NM) to estimate single-cell contributions to the ABR. We simultaneously recorded NM neurons’ spikes and the EEG in owls of both sexes, and found that ≳ 5000 spontaneous single-cell spikes are necessary to isolate a significant spike-triggered average (STA) response at the EEG electrode. An average single-neuron contribution to the ABR was predicted by convolving the STA with the cell’s peri-stimulus time histogram. Amplitudes of predicted contributions of single NM cells typically reached 32.9 ± 1.1 nV (mean ± SE, range: 2.5–162.7 nV), or 0.07 ± 0.02 % (median ± SE; range from 0.01% to 1%) of the ABR amplitude. The time of the predicted peak coincided best with the peak of the ABR wave II, independent of the click sound level. Our results suggest that individual neurons’ contributions to an EEG can vary widely, and that wave II of the ABR is shaped by NM units.