Further observations on the incremental stimulus intensity effect and habituation of the human electrodermal response.

Two studies are reported which examined the relative effectiveness of an incremental series of stimulus intensities for reducing electrodermal responsiveness to a test stimulus. In the first study, employing five pairs of male undergraduates matched on electrodermal responsiveness, habituation to a 100 db (re 20muN/m2) noise burst was compared following two training procedures. In one, stimulation began at 64 db and increased by 4 db up to 96 db whenever two consecutive presentations of the stimulus failed to elicit a response. In the other, an equivalent number of presentations with a 100 db noise burst were provided. While habituation occurred during both training conditions, subsequent habituation to a 100 db test stimulus was more rapid following incremental training. In the second experiment, employing 40 female undergraduates, incremental and fixed conditions of training were compared for effectiveness using more moderate intensities of test stimulus: viz., 60 db and 80 db. A these levels, the fixed training series was superior to the incremental in reducing responsiveness. The data extend previous findings on the incremental stimulus intensity effect with the human electrodermal response but suggest limiting conditions for its demonstration.

Short-latency acceleration of human heart rate as a function of stimulus intensity.

The study examined the effectiveness of three intensities of white noise in evoking short-latency acceleration of human heart rate. 3 groups of 10 female subjects were presented with 1-sec. bursts of white noise with virtually instantaneous rise times at intensities of 100 db (re 20 mu N/m2), 90 db, or 80 db SPL as measured at the headset. 5 bursts were presented at intervals of 45 sec. while the EKG was recorded. An increase in heart rate during poststimulus beats 3 and 4 compared with the mean rate of the three beats prestimulus was reliably evoked on the first two trials in the 100-db and 90-db groups but not in the 80-db group. Apart from a recovery of acceleration on Trial 5 for the 100-db group, the 100-db and 90-db groups did not differ in amplitude or rate of habituation of the response. Poststimulus heart-rate level was not significantly related to prestimulus level as expected in terms of the law of initial values. The results were interpreted as consistent with F.K. Graham's interpretation of short-latency cardiac acceleration as a component of startle.