Some effects of the stimulus repetition rate on N1 and N2 in transtympanic and surface recordings

The early auditory evoked electrical activity has been recorded in man at the promontory (transtympanic approach) and on the scalp vertex-mastoid derivation), in response to clicks delivered at different rates. Latency and amplitude of the first two peaks (N1 and N2), as a function of the repetition rate, have been measured and compared. The differences between the latencies of transtympanic and surface N1 are very small, at any rate, with a maximum value of 0.08 msec. In the transtympanic recording, the latency difference between N2 and N1 is constant throughout the whole range of rate values, from 3 to 100 clicks per second. In the surface responses, on the contrary, the latency difference between N2 and N1 tends to increase as the rate is increased. The amplitude of the transtympanic N2 is consistently reduced at click rates above 20-50 per second (more markedly than the amplitude of N1), while the amplitude of the surface N2 is much more insensitive to the rate increase. Some implications of these results are discussed with respect to the origin of N2 in the two recording conditions.