A rift between implicit and explicit conditioned valence in human pain relief learning

Abstract

Pain is aversive, but does the cessation of pain ('relief') have a reward-like effect? Indeed, fruitflies avoid an odour previously presented before a painful event, but approach an odour previously presented after a painful event. Thus, event-timing may turn punishment to reward. However, is event-timing also crucial in humans who can have explicit cognitions about associations? Here, we show that stimuli associated with pain-relief acquire positive implicit valence but are explicitly rated as aversive. Specifically, the startle response, an evolutionarily conserved defence reflex, is attenuated by stimuli that had previously followed a painful event, indicating implicit positive valence of the conditioned stimulus; nevertheless, participants explicitly evaluate these stimuli as 'emotionally negative'. These results demonstrate a rift between the implicit and explicit conditioned valence induced by pain relief. They might explain why humans in some cases are attracted by conditioned stimuli despite explicitly judging them as negative.

Figure 1.

Bars represent mean (with standard errors) startle amplitudes in z-scores assessed in the test phase (i.e. extinction) in the presence of visual CS+; the black bar represents the FORWARD group, the grey bar the BACKWARD group, and the white bar the CONTROL group. Positive values indicate startle response potentiation; negative values startle attenuation relative to the mean. CS+ has been reinforced during the preceding training phase (i.e. the acquisition) with a different timing relative to the US (i.e. a mild electric shock). In the FORWARD and the BACKWARD group, the CS+ briefly preceded or followed the US, respectively, and consequently the CS+ had opposite effects on startle response modulation during test. In the CONTROL group, the CS+ preceded the US with a long delay and this did not subsequently affect the startle response.

Figure 2.

Bars represent mean scores (with standard errors) of (a) valence and (b) arousal ratings of the CS+ with the neutral value (i.e. 5) subtracted from the ratings. Hatched bars represent the ratings before the acquisition phase (i.e. conditioning) and filled bars after the acquisition phase. Black fill represents the FORWARD group, grey fill the BACKWARD group and white fill the CONTROL group. Negative values represent negative valence and low arousal, whereas positive values represent positive valence and high arousal. (a) Ratings of valence before training were neutral and after training were consistently negative indicating that the CS+ acquired negative explicit valence, independent of event-timing. (b) Ratings of arousal were in all three groups higher after compared with that before training, indicating that the CS+ became arousing independent of event-timing.