Stimulus omission and the orienting response: latency differences suggest different mechanisms.

Following response habituation to a regularly-presented innocuous stimulus, omission of that stimulus may elicit a response. The missing-stimulus effect has been of some importance in the development of Orienting Response (OR) theory, particularly Sokolov's neuronal model/stimulus comparator mechanism. Western work on this effect has been somewhat equivocal, with only some subjects emitting small responses at stimulus omission. Barry (1984b) proposed that the fragility of the data might reflect the elicitation of a voluntary OR (Maltzman, 1979) rather than the more robust (reflexive) involuntary OR commonly emphasised in OR work. A prediction generated from this hypothesis, that the OR to stimulus omission has a longer latency than that associated with physical stimuli, was tested here. The first experiment found such an effect in the electrodermal responses to simple visual stimuli and their omission. This was replicated in a second experiment with both significant and indifferent visual stimuli. These latency delays (group mean differences ranged from 1.15 to 1.65 s) imply an intervening process in the elicitation of the electrodermal response to stimulus omission, compatible with it being viewed as a voluntary OR. An implication of these results for the conceptualization of other ORs to stimulus change is discussed.