Surface auditory evoked potentials in the unrestrained rat: component definition

Auditory evoked potentials (AEPs) to click and pure tone stimuli were recorded in unrestrained, unanesthetized rats. The middle latency rat AEPs (N17, P23, N38) had midline scalp distributions similar to human MAEPs and were recorded to within 15 dB above BAEP threshold. In contrast to human MAEPs, rat MAEPs were decreased in amplitude at high stimulation rates and only the N17 component was unaltered by slow wave sleep. The longer latency N50, N80 and P130 components had several response properties comparable to human N100-P200 vertex potentials. These included restricted midline fronto-central scalp distributions, progressive increases in amplitude at ISIs up to 4-8 sec and marked attenuation during slow wave sleep. The frequency sensitivity of the rat AEP revealed a decreased response to pure tones below 4 kHz but robust responses for stimuli up to at least 45 kHz. There was a notch in the rat audiogram with decremented component amplitudes to pure tone stimuli centered at 35 kHz. When equated for intensity, click and pure tone stimuli in the range of the rats maximal audiometric sensitivity (8-20 kHz) generated comparable AEP components. These results provide normative data on rat surface recorded AEPs. It is suggested that these surface recorded rat AEPs are generated by subcortical neural systems involved in the detection of auditory transients.