Conditioned Diminution of the Unconditioned Skin Conductance Response David C. Knight, Eleanor P. Lewis, and Kimberly H. Wood University of Alabama at Birmingham During Pavlovian conditioning the expression of a conditioned response typically serves as evidence that an association between a conditioned stimulus (CS) and an unconditioned stimulus (UCS) has been learned. However, learning-related changes in the unconditioned response (UCR) produced by a pre- dictable UCS can also develop. In the present study, we investigated learning-related reductions in the magnitude of the unconditioned skin conductance response (SCR). Healthy volunteers participated in a differential conditioning study in which one tone (CS) was paired with a loud white-noise UCS and a second tone (CS) was presented alone. In addition, probe trials that consisted of UCS presentations paired with the CS (CS  UCS) and CS (CS  UCS), as well as presentations of the UCS alone were included to assess UCR diminution. SCR and participants’ expectations of UCS presentation were monitored during conditioning. Greater diminution of the UCR was observed to the UCS when it followed the CS compared to when it followed the CS or was presented alone. Further, UCR amplitude showed an inverse relationship with the participants’ ratings of UCS expectancy. However, conditioned UCR diminution was also observed independent of differential UCS expectancies. Our findings demonstrate conditioned diminution of the unconditioned SCR. Further, these findings suggest that although UCR amplitude is modified by conscious expectations of the UCS, conditioned diminution of the UCR can be expressed independent of learning-related changes in these expectations. Keywords: fear, conditioning, memory, UCR, UCS An important function of Pavlovian conditioning is that it per- mits an individual to anticipate and effectively respond to a threat. Learning the relationship between a threat and the cues that predict it allows us to more effectively avoid, escape, and react to threat- ening conditions (Franchina, 1969; Helmstetter & Bellgowan, 1993; Kamin, 1954). Anticipatory responses to the cues that pre- dict a threat have received significant attention in conditioning research, while relatively less attention has been given to the study of the response produced by the threat itself. From a functional perspective however, it is the change in response to the actual threat that is the most important function of Pavlovian condition- ing (Domjan, 2005). For example, anticipation of an impending aversive event induces hypoalgesia, reducing the pain a noxious stimulus produces (Helmstetter & Bellgowan, 1993; Helmstetter & Tershner, 1994). In the laboratory study of Pavlovian conditioning, a conditioned stimulus (CS) is repeatedly paired with an unconditioned stimulus (UCS). The expression of a conditioned response (CR) to the CS, in anticipation of the UCS, is typically taken as evidence that an association between the CS and UCS has been learned. In contrast to CRs, the unconditioned response (UCR) produced by the UCS is typically considered an unlearned response that is reflexively expressed. However, learning-related changes in the magnitude of the UCR have been observed in prior conditioning research (Bax- ter, 1966; Canli & Donegan, 1995; Dunsmoor, Bandettini, & Knight, 2008; Knight, Waters, King, & Bandettini, 2010; Rust, 1976). This prior work has demonstrated that UCR amplitude can be enhanced or attenuated (see Domjan, 2005, for review). For example, an enhanced response is observed during fear-potentiated startle (Cook, Davis, Hawk, Spence, & Gautier, 1992; Grillon, Ameli, Woods, Merikangas, & Davis, 1991; Vrana, Spence, & Lang, 1988), whereas eyeblink and skin conductance response (SCR) conditioning studies have typically observed an attenuation of the UCR (Baxter, 1966; Canli & Donegan, 1995; Dunsmoor et al., 2008; Kimmel, 1967; Knight et al., 2010; Rust, 1976). This conditioned attenuation of the UCR is often referred to as UCR diminution. UCR diminution has been reported in a number of prior Pav- lovian conditioning studies. Baxter (1966) demonstrated that the magnitude of the UCR decreases as the CS and UCS are repeatedly paired. Further, UCR diminution is greater to paired compared to unpaired presentations of the CS and UCS (Kimmel, 1967). Others have investigated conditioned diminution of unconditioned SCRs using differential conditioning procedures in which one CS was paired with the UCS (CS) while a second CS was presented alone (CS). This work demonstrated greater UCR diminution when the UCS followed the CS compared to when the UCS followed the CS on probe trials (Marcos & Redondo, 1999). These findings indicate the reduction in UCR amplitude during Pavlovian conditioning cannot be solely explained by a simple nonassociative learning process (i.e., habituation). Instead, the findings suggest that presentation of the CS modulates UCR expression during Pavlovian conditioning. Others have suggested that conscious expectations modify UCR expression (Dunsmoor et This article was published Online First June 20, 2011. David C. Knight, Eleanor P. Lewis, and Kimberly H. Wood, Department of Psychology, Civitan International Research Center, University of Ala- bama at Birmingham. Correspondence concerning this article should be addressed to David C. Knight, CIRC 235H, 1530 3rd Avenue, South, Birmingham, AL 35294- 0021. E-mail: knightdc@uab.edu Behavioral Neuroscience © 2011 American Psychological Association 2011, Vol. 125, No. 4, 626 – 631 0735-7044/11/$12.00 DOI: 10.1037/a0024324 626