Stimulus omission and recovery of the electrodermal and digital vasoconstrictive components of the orienting response.

Three experiments are reported which investigated Sokolov's (1968) hypothesis that, after a number of stimulus presentations, complete omission of a stimulus leads to increased responsiveness of the orienting response (OR). The skin conductance response (SCR) and finger pulse volume (FPV) response components of the OR were studied. In experiment 1 (N=60), the effect of number of pre-omission training trials on response recovery was investigated, while experiment 2 (N=120) investigated the effects of stimulus intensity (70 or 90 dB) and interstimulus interval (12 or 21 sec) on recovery to stimulus omission following a fixed number of training trials. In experiment 3 (N=40), an attempt was made to control for possible below-zero habituation effects by training each subject to a habituation criterion before stimulus omission. All experiments employed a 1000 Hz tone of 3 sec duration which was presented at a constant interstimulus interval. Although recovery of the SCR did occur under some conditions, the results were largely negative. Manipulation of the number of training trials, training stimulus intensity and interstimulus interval had little effect on response recovery. A consistent finding, however, was that subjects who displayed SCR recovery also displayed significantly more spontaneous fluctuations in skin conductance during the pre-stimulus period and required significantly more training trials to reach the criterion of habituation than did subjects displaying no recovery. Moreover, the SCRs displayed by 'labile' subjects on omission trials were significantly larger than those displayed on either the last training trial or during a control interval just prior to stimulus omission.