Neural contributions to the perstimulus compound action potential: implications for measuring the growth of the auditory nerve spike count as a function of stimulus intensity

The perstimulus compound action potential (PCAP), unlike the more familiar compound action potential (CAP), can be recorded in response to asynchronous as well as synchronous auditory nerve activity. When all neurons contribute equally to the PCAP, the area under the PCAP (the PCAP area) is proportional to the number of action potentials fired by auditory nerve neurons (the auditory nerve spike count). The auditory nerve spike count is one proposed code for stimulus intensity, and our goal is to use the PCAP to test this hypothesis. In this study, two independent tests were developed to measure the contributions of neurons to the PCAP as a function of their characteristic frequency (CF). The test results were verified using a model of the auditory periphery designed to calculate the auditory nerve spike count as a function of pure tone intensity and frequency. In nearly all experiments, neurons having CFs that span contiguous three or four octave bands contribute equally to the PCAP. For pure tones that stimulate only those neurons contributing equally to the PCAP, the PCAP area grows over intensity ranges frequently exceeding 80 dB, and in one case equaling 108 dB. These results demonstrate that the auditory nerve spike count, at least for pure tones, is capable of encoding changes in stimulus intensity over the entire dynamic range of the auditory system.