PSYCHOPHYSIOLOGY Copyright © 1982 by The Society for Psychophysiological Research. Inc. Vol. 19. No. 1 Primed in U.S.A. BriefReport Interactive EflFects of Stimulus Probability and Significance on the Skin Conductance Response GERSHON BEN-SHAKHAR, ISRAEL LIEBLICH, AND SOL KUGELMASS Department of Psychology, The Hebrew University of Jerusalem ABSTRACT The present study deals with the effects of stimulus significance and stimulus probability on skin conduc- tance responses (SCRs). Eighty subjects ("informed" group) chose a numbered card, and 60 subjects ("unin- formed" group) chose a blank card. All subjects were presented with a series of stimuli in the form of questions about numbers. The relative frequencies of the relevant (chosen) numbers were manipulated. The results showed expected effects of stimulus relevance and stimulus probability on SCRs. A more interesting result was an interaction of stimulus significance and relative frequency, meaning that the probability effect (greater responsivity to the less frequent stimulus) was stronger when the series of stimuli was comprised of relevant and neutral stimuli, than when two different neutral stimuli were used. This latter result was interpreted in terms of generalization of habituation. It was suggested that the relevance manipulation interferes with the generalization of habituation process. DESCRIPTORS: Skin conductance response. Detection of information. Generalization of habituation. Stimulus significance. Stimulus probability. The roles of stimulus relevance and stimulus rel- ative frequency in determining physiological re- sponsitivity have been studied by many authors in a variety of different paradigms (e.g., Ben-Shakhar, Lieblich, & Kugelmass, 1975; Berlyne, Crow, Sala- patek, & Lewis, 1963; Lieblich, Kugelmass, & Ben- Shakhar, 1970; Sokolov, 1963, 1966). The purpose of the present study was to investigate the possible interaction of the relevance and probability factors. Previous research in our laboratory, which used subjects who were instructed to choose certain stim- uli (relevant stimuli), indicated that differential au- tonomic responsivity to relevant stimuli was nega- tively related to their relative frequencies. We pro- posed the dichotomization theory to account for this result (Ben-Shakhar, 1977). According to this the- ory, persons who choose certain (relevant) stimuli This study was supported by grants from the Research Fund of the Faculty of Social Sciences and the Eshkol Insti- tute, The Hebrew University of Jerusalem. We wish to thank Ms. Michal Ishai for expert assistance and Ms. Yaffa Gold- berg for preparing the computer programs. Address requests for reprints to: Dr. Gershon Ben- Shakhar, Department of Psychology, The Hebrew University of Jerusalem, Jerusalem 91000, Israel. will demonstrate independent habituation proc- esses to the two types of stimuli (relevant and neu- tral). It would be of importance to the theory to add a control group which would be exposed to neutral (non-chosen) stimuli only. According to the dichot- omization theory we would expect persons who did not choose any stimulus to show generalized habitu- ation to all stimuli presented to them. In other words we expect that, in a situation where no rele- vant stimulus is defined, the effect of stimulus prob- ability on psychophysiological reactivity will be very attenuated, and thus an interaction of the relevance and probability factors will be found. Method Subjects One hundred and forty Hebrew University students participated in the experiment. They were paid for their time. Apparatus A constant current (18(xA) SCR apparatus (direct coupled mode) coupled to a Brush II recorder was used. The recording system was placed in a control room sepa- rated by a one-way mirror from the room in which the sub- ject was tested. Beckman electrodes were used for skin re- 112 0048-5772/82/010112-03$0.30/0 © 1982 The Society for Psychophysiological Research, Inc.