Event-related perturbations in an electrophysiological measure of auditory function: a measure of sensitivity during orienting?

The effects of salient foreground stimuli in evoked potentials to weak background probe stimuli were examined in situations requiring passive observation or discriminative judgments of foreground tone stimuli. The background probe stimuli consisted of a continual train of weak acoustic stimuli presented at a rate of about 40 stimuli per second. Under such conditions, a 40-Hz steady-state rhythm (SSR) is established, which has been proposed to consist of the algebraic summation of individual middle-latency components evoked by stimuli in the train. The 40-Hz SSR was averaged over trials and extracted from the composite event-related potential signal using narrow-band digital filtering, for continuous examination of latency and amplitude during the course of the period immediately preceding and following the foreground stimulus. The foreground stimulus was followed by a brief period (peaking at about 200 ms) during which the latency of response to the background probe stimuli was reduced. The extent of this latency reduction was in proportion to the magnitude of the simultaneous slow-wave ERP responses and, to a lesser extent, heart rate responses. It is proposed that the results may reflect a transient period of sensitization during orienting, at a presumably early level in the auditory system, and that the method thus offers a means for determining the extent and temporal course of such effects.