The effects of low-pass filtering on the primary cortical auditory potential of the rat

The effects of low-pass filtering on the primary cortical auditory evoked potential (CAEP) were studied using the rat as subject. The CAEP is the potential which is generated by the arrival of the afferent volley in the primary auditory projection area. Recordings were made from animals which were totally awake and those which were anaesthetised with pentobarbital. A total of 9 recordings were obtained from each subject. The high-pass (low-frequency) filter remained fixed at 3.2 Hz while the low-pass (high-frequency) filter was set at 32, 80, 160, 320, 800 Hz and 1.6, 3.2, 8 and 16 kHz. The CAEP recorded from the awake animal consisted of a primary positivity (P1) followed by a later secondary positivity (P2). In the anaesthetised subjects, only the P1 potential was present. As the bandpass was progressively opened, there was at first a quite steep decline in latency associated with a gradual increase in amplitude. After the low-pass filter setting had been raised to 320 Hz, the amplitude of components P1 and P2 when awake and of P1 when anaesthetised had stabilized and thereafter there was no additional increase. Likewise, the latency of P2 for the awake subjects subsequently remained constant. In contrast, the latency of P1 recorded from both awake and anaesthetised subjects showed a continuing small decline as the bandpass was extended to 3.2-16 kHz. It is probable that this phenomenon did not represent a further genuine decrease in the latency of P1 but was more likely an artefact caused by the distorting effects of a cluster of late high-frequency components of the brainstem auditory evoked potential generated temporally contiguous to P1. It was concluded that a bandpass of 3.2-320 Hz is optimal for recording both early and late components of the CAEP and that low-pass filtering had an essentially uniform effect on the waveform irrespective of the subject's state of arousal.