Pain predictability reverses valence ratings of a relief-associated stimulus

Abstract

Relief from pain is positively valenced and entails reward-like properties. Notably, stimuli that became associated with pain relief elicit reward-like implicit responses too, but are explicitly evaluated by humans as aversive. Since the unpredictability of pain makes pain more aversive, this study examined the hypotheses that the predictability of pain also modulates the valence of relief-associated stimuli. In two studies, we presented one conditioned stimulus (FORWARDCS+) before a painful unconditioned stimulus (US), another stimulus (BACKWARDCS+) after the painful US, and a third stimulus (CS-) was never associated with the US. In Study 1, FORWARDCS+ predicted half of the USs while the other half was delivered unwarned and followed by BACKWARDCS+. In Study 2, all USs were predicted by FORWARDCS+ and followed by BACKWARDCS+. In Study 1 both FORWARDCS+ and BACKWARDCS+ were rated as negatively valenced and high arousing after conditioning, while BACKWARDCS+ in Study 2 acquired positive valence and low arousal. Startle amplitude was significantly attenuated to BACKWARDCS+ compared to FORWARDCS+ in Study 2, but did not differ among CSs in Study 1. In summary, predictability of aversive events reverses the explicit valence of a relief-associated stimulus.

Keywords: backward conditioning; forward conditioning; implicit and explicit responses; pain relief; threat unpredictability.

Figure 1

Conditioning trials. Three out of four yellow geometrical shapes were presented during conditioning as conditioned stimuli (CS). One shape (_FORWARDCS+) was presented before a painful electric shock (unconditioned stimulus, US), one shape (_BACKWARDCS+) was presented after the US, and another shape (CS−) was never associated with the US. In Study 1 (A) 16 USs out of 32 were predicted by the _FORWARDCS+, whereas the other 16 USs were delivered unwarned before the _BACKWARDCS+. In Study 2 (B) all USs were preceded by the _FORWARDCS+ and followed by the _BACKWARDCS+.

Figure 2

Ratings of valence (A) and arousal (B) for the visual stimuli. Participants rated the valence (upper panel) and the arousal (botton panel) of the _FORWARDCS+ (light gray bars), the _BACKWARDCS+ (black bars), the CS− (white bars), and the NEW (striped bars) before and after the conditioning as well as after the test phase. In Study 1 (left panels), both the _FORWARDCS+ and the _BACKWARDCS+ were rated as negatively valenced and arousing. On the contrary, in Study 2 (right panels) the _FORWARDCS+ was rated as negatively valenced and high arousing, whereas the _BACKWARDCS+ as positively valenced and low arousing (⁺p < 0.06; ^*p < 0.05; ^***p < 0.001).

Figure 3

Startle amplitudes to the visual stimuli during the test phase. Bars (with standard errors) depict the startle amplitude in z scores in response to the _FORWARDCS+ (light gray), the _BACKWARDCS+ (black), the CS− (white), and the NEW (striped). Startle responses did not differ among CSs after conditioning in Study 1 (A). On the contrary, startle response was significantly attenuated (i.e., reward-like conditioned responses) by the _BACKWARDCS+ as compared to the _FORWARDCS+ and the NEW after conditioning in Study 2 (B) (^*p < 0.05; ^**p < 0.01).

Figure 4

Skin conductance responses (SCRs) to the visual stimuli. Bars (with standard errors) depict the SCR (sqrt transformed) in response to the _FORWARDCS+ (light gray), the _BACKWARDCS+ (black), the CS− (white) and the NEW (striped) either during the conditioning or during the test phase. In both Study 1 (A) and Study 2 (B) the SCR to the _BACKWARDCS+ was significantly lower compared to the _FORWARDCS+ and the CS− (^**p < 0.01; ^***p < 0.001).