This paper addresses the question of the most appropriate theoretical account of the phenomena of orienting and habituation. Several lines of evidence are reviewed. First, it is argued that the effects of stimulus omission require a comparator theory in which it is asserted that responses to iterated events result from a comparison between predicted and actual stimulus input. Second, the data from studies in which paired stimulus events are employed seem, at least at first sight, to be best explained in terms of a comparator theory in which a key role is ascribed to associative processes. Third, secondary task probe reaction time data indicate that events that elicit orienting also command processing resources, and that habituation involves changes in the manner in which events are processed. Finally, recent data on the effects of intermodality change indicate that electrodermal responses are larger on the change trial than on the first habituation training trial; these results seem problematical for noncomparator theories. However, other data on the context-specificity of habituation and on the effects of stimulus miscuing cast doubt on the usefulness of an associative analysis as a general account of habituation phenomena. Nevertheless, the weight of evidence seems to indicate that an adequate theory of human habituation must include a comparison process and must acknowledge that orienting and habituation involve a re-allocation of attentional resources.

Potentiation of blink startle during aversive and nonaversive Pavlovian single-cue conditioning was assessed in human Ss. In Experiment 1 (N = 89), the conditioning group received paired presentations of a visual conditioned stimulus (CS) and an unconditioned stimulus (US), whereas the control group was presented with a random sequence. The US was an electric shock for half the Ss and a nonaversive reaction time task for the other half. Electrodermal conditioning was evident regardless of the nature of the US, but blink potentiation was found only in the conditioning group that had been trained with the aversive US. These results were replicated in Experiment 2 (N = 65), in which a nonaversive US of increased motivational significance was used. Thus, only aversive conditioning seems to affect the affective valence of the CS, at least as reflected by changes in a skeletal reflex.

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Human conditioning research has revealed an apparent resistance to extinction of aversive conditioning to pictures of fear-relevant stimuli such as snakes and spiders, supporting M. E. P. Seligman's (1971) preparedness theory of fears and phobias. This article examines an alternative account based on activation of preexisting response tendencies under threat (selective sensitization). Two experiments demonstrate that selective sensitization of electrodermal responses is attenuated when a fear-relevant stimulus serves as a negative conditioned stimulus (CS-), but is maintained when it serves as a positive conditioned stimulus (CS+). Previous extinction results may therefore be due to preservation of initial responding to CS+ but not CS-. Selective sensitization offers a model for the nonassociative activation of fears and phobias to prepotent stimuli under conditions of stress or threat. Possible genetic and cognitive mechanisms are discussed.

Participants in Experiments 1 and 2 performed a discrimination and counting task to assess the effect of lead stimulus modality on attentional modification of the acoustic startle reflex. Modality of the discrimination stimuli was changed across subjects. Electrodermal responses were larger during task-relevant stimuli than during task-irrelevant stimuli in all conditions. Larger blink magnitude facilitation was found during auditory and visual task-relevant stimuli, but not for tactile stimuli. Experiment 3 used acoustic, visual, and tactile conditioned stimuli (CSs) in differential conditioning with an aversive unconditioned stimulus (US). Startle magnitude facilitation and electrodermal responses were larger during a CS that preceded the US than during a CS that was presented alone regardless of lead stimulus modality. Although not unequivocal, the present data pose problems for attentional accounts of blink modification that emphasize the importance of lead stimulus modality.

Two experiments examined the effects of visually presented threat and nonthreat word lead stimuli on blink modification among unselected young adults (Experiment 1, N = 35) and participants selected for low and high trait anxiety (Experiment 2, N = 60). The blink reflex was elicited by a white noise probe of 105 dB. Lead stimulus intervals of 60, 120, 240, and 2000 ms were used in both experiments. Prepulse inhibition was observed at the 240-ms interval and prepulse facilitation was observed at the 60-ms interval in both experiments. Also, greater facilitation was found in both experiments during threat words at the 60-ms interval and greater inhibition during threat words at the 240-ms interval. Experiment 2 provided some evidence that the greater facilitation during threat words than during nonthreat words at the 60-ms probe interval may be found in high trait anxious participants, but not in low trait anxious participants. The results are discussed in relation to contemporary information processing theories of anxiety.

The effects of tone intensity (60, 75, or 90 dB) on the amplitude and habituation of the pupillary dilation response were investigated using a between‐groups design (N = 72). Although pupillary dilation responses were elicited in all intensity conditions, neither response amplitude nor habituation was influenced by differences in stimulus intensity, and the response was insensitive to changes in intensity following the habituation series. Eighteen subjects in the 90dB group were retested and the reliability of the response was found to be low. In Experiment 2 (N = 30), the variability of pupillary activity was reduced with a low level of background illumination, but overall test‐retest reliability of the dilation response to 90dB tones was low. The insensitivity of the response to intensity manipulations, the rapid development of habituation, and the low test‐retest reliability of the response are clear limitations on the usefulness of the pupillary dilation response as an index of the orienting reflex to nonsignal stimulation.

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