Habituation and the orienting response in the auditory cortical evoked potential.

Habituation of the auditory cortical evoked potential, the GSR, and heart rate was measured in 100 male subjects. Stimuli were 31 tones of 1 sec duration with an IS1 of 33 see. All stimuli were sinusoidal, at a frequency of lo00 Hz, and at an intensity of 95 dB (re 20 N/cmz). The EEG was measured from bipolar electrode placement to the C, and T3 scalp locations. Evoked potentials were averaged over 3 successive blocks of 10 stimuli. The GSR habituation scores were the regression coeSicients over trials of the response amplitude in square root conductance. Signifkant habituation of ali evoked potential amplitude components was found. This was a rapid process occurring between the first and the second 10 sthnuli. The habituation score for the most significant evoked potential variable (P200) was found to correlate significantly with the GSR and heart rate habituation scow. These results were interpreted as suggesting that the amplitude of the evoked potential was danced by the orienting reaction. DESCRIPTORS: Evoked potential, GSR, Heart rate, Habituation, Orienting response. Ritter, Vaughan, and Costa (1%8), using a novel experimental design, identified several types of habituation phenomena within the auditory cortical evoked potential. Stimuli were presented in successive blocks of 30 with short interstimulus intervals (ISIS) of 2 sec within blocks and with long ISIs ( 5 min) between blocks. With this design they were able to average the stimuli both within a block and over the same stimulus positions of the different blocks. They found that the major habituation effects took place in the amplitude of the P200 component. Within the blocks, habituation of this component was initially very rapid for the first one or two stimuli, but was nonsignificant thereafter. Ritter et al. suggested that this habituation was an artifact produced by the short IS1 within blocks. The effects of IS1 on the evoked potential are well known (Davis, Mast, Yoshie, & Zerlin, 1966) and within this type of design the first stimulus is necessarily preceded by a long interval. Habituation of this same P200 component across blocks was much slower. Ritter et al. argued that this slow habituation effect was similar to that found in the visual evoked potential by Bogacz, Vanzulli, Handler, and Garcia-Austt (1960) , Walter (1W) , This work was supported by a grant from the Medical Research Council of the United Kingdom. Address requests for reprints to: Dr. J. Rust, Department of Psychology, Institute of Psychiatry, De Crespigny Park, Denmark Hill, London, SE5 8AF, England. and Perry and Copenhaver (1965). They further argued that, although its determinants were obscure, it could not represent orienting response habituation since it took too long to develop. In other experiments, Ritter et al. interpolated unexpected stimuli into the stimulus sequence and observed a new positive component in the evoked potential at 300 msec. They argued that this P300 component was an aspect of the orienting response. They observed that there was no change in the P200 component in response to unexpected stimuli and therefore concluded that this component was independent of orienting behavior. One way of investigating orienting components in the evoked potential would be to compare them with indices of orienting from other physiological response systems. The GSR response is probably the most widely used and validated measure of orienting. One problem, however, with relating the evoked potential to the GSR is that whereas averaging of many responses is necessary to produce reliable measures in the evoked potential , the GSR produces only a limited number of responses and more transient effects. It also needs longer interstimulus intervals to allow responses from previous stimuli to recover. For these reasons, a simple replication of the Ritter design would not work for concomitant measurement of GSR and evoked potential. In the present experiment, subjects were presented with auditory stimuli of fairly strong intensity at half min intervals. The strong intensity