Auditory stimulus intensity effects on components of the late positive complex

Abstract

Three groups of subjects were tested in experiments in which the late positive complex (LPC) was elicited by 50 msec white noise pips of various intensities. Principal Components Analysis was used to decompose event-related potential wave forms. In the first experiment noise pips were presented every 4 sec at intensities ranging from 55 to 115 dB SPL under 3 attention conditions: counting the stimuli, ignoring them, or performing a visual search task. Higher intensities produced larger blink, P190, P280 and P340 components. Attention affected blink, P190, P340 and slow wave (SW) components. In the second experiment stimulus intensity ranged in one run between 80 and 110 dB, the probability of the 110 dB stimulus being 0.25; in the other run, stimuli were always 110 dB. P310 was sensitive to both intensity and probability, while SW was affected only by probability. In the third experiment the habituation of the LPC and skin conductance response (SCR) to 115 dB noise pips presented at random intervals between 10 and 90 sec was examined over 62 trials. The amplitude of blink, N95 and P265 declined over trials, and SCR increased after an unexpected verbal message. These results show that components of the LPC with latencies around 30 msec have both exogenous and endogenous properties. Only SW was completely independent of stimulus intensity in these experiments.