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Effects of Stimulus Rate and Number on the Early Components of the Averaged Electroencephalic Response

Early components of the AER were studied in relation to rate and number of clicks presented at 50 dB SL in a total of 18 subjects. Twelve subjects were presented 1024 stimuli at 15.0/sec, 9.5/sec, and 7.5/sec; 512 stimuli at 4.7/sec; 256 stimuli at 1.9/sec; and 128 stimuli at 1.0/sec. Two trials were run with rates randomized within each trial. Responses were also fractionated off-line into smaller groups. Three subjects were presented 10,240 stimuli at a rate of 9.5/sec, and three subjects were presented 1024 stimuli at 4.7/sec, 1.9/sec, and 1.0/sec.

Mean latency measures for 1024 clicks at 9.5/sec were: P_o 13.2 msec, N_a 22.7 msec, P_a 34.3 msec, and N_b 49.1 msec, with the largest mean peak-to-peak amplitude, N_a–P_a, of 0.73 µv. Peak-to-peak amplitudes were smaller and responses were less "noisy" for greater numbers of stimuli with no apparent effect of rate. Amplitude reduction as a function of number appears to be complete within the first 1024 stimuli. Clinical judgments of response at suprathreshold levels can be made quickly and consistently with as few as 512 stimuli at rates of 9.5/sec and 15.0/sec.