A temporal component of the auditory evoked response

Abstract

We studied the 75–225 msec portion of the auditory evoked response (AER) in 32 normal adults at vertex (C z) and temporal (T 3 and T 4) placements referred to a balanced, noncephalic reference electrode using a monaural 1 msec click stimulus delivered every 4.7 sec at 60 dB above threshold. The tape-recorded EEG was filtered at 1–25 c/sec, and 128 individual responses were summed, sampling every 0.5 msec for 250 msec post-stimulation. The C z AERs showed the classic vertex response, a negative peak, N 1, at 100 msec, followed by a positive peak, P 2, at 160–200 msec. The T 3 and T 4 AERs were similar to the C z AERs from 0 to 80 msec and from 200 to 250 msec. They differed significantly from the C z AERs from 80 to 200 msec. The difference is best explained by the hypothesis that the C z AERs consisted of N 1P 2, while the T 3 and T 4 AERs consisted of N 1P 2 plus an additional super-imposed component, which we called the T complex, comprising a positive peak, T a, at 105–110 msec, and a negative peak, T b, at 150–160 msec. By computer, the corresponding C z and T 3 or T 4 AERs were normalized to equalize their amplitudes, and the former was subtracted from the latter, thus isolating the T complex. The T a peak was found to occur 1.5±1.6 msec earlier at the electrode contralateral to stimulation, and 2.2±4.0 msec earlier at the T 4 (right) electrode. Both differences were statistically significant. The T complex amplitude was greater at the electrode contralateral to stimulation and at the T 4 electrode. These findings appear to resolve current controversies concerning the form of the temporal AER. While N 1P 2 is apparently a product of widespread areas of cortex, we conclude that the T complex is probably a product of secondary auditory cortex.