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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.

1. The variability in the occurrence probability of the uropod steering response, one of the equilibrium responses, has been analysed quantitatively by a combination of electromyogram techniques and visual observation in the crayfish Procambarus clarkii Girard. 2. Presence of the tilt stimulus alone, whether it might be the body tilt or the substratum tilt, did not necessarily lead to the emergence of uropod steering (occurrence probability = 0.08 and 0.19 in the free condition and in the hanging (fixed) condition respectively). 3. Whenever uropod steering occurred, its direction and magnitude were strictly determined by those of the tilt stimulus. 4. During the abdominal posture movement, the occurrence probability of steering in response to the tilt stimulus was significantly increased (occurrence probability = 0.24 and 0.49). 5. During forward walking in the free-animal observation, the occurrence probability of steering was also significantly higher than during the stance period (occurrence probability = 0.45). 6. During backward walking without the abdominal movement, the occurrence probability of steering showed no significant change. It was increased significantly (0.21), however, when the backward walking was occurring together with the abdominal movement. 7. Turning movements involved the characteristic uropod configuration. The steering response was overridden by turning. 8. The defence reflex also suppressed the steering response. 9. Forward and backward walking, turning, and the defence reflex showed significantly high probabilities of association with the abdominal movement. 10. The uropod steering was preceded by the abdominal movement or the forward walking with a significantly high probability. 11. It was concluded that the association of the uropod steering with the abdominal posture movement or the forward walking was causal: the operation of the abdominal posture system or the forward walking system is the necessary condition for the uropod steering to be released by the tilt stimulus. The uropod steering response thus provides a typical example of postural facilitation. 12. The results of this study clarified the functional organization of the locomotion-related acts including the equilibrium responses in crayfish, showing how they interact with each other to result in a particular behavioural output. The biological significance of such an organization is discussed.

Summary. 1 The stridulation of the bugs Coranus subapterous, Kleidocerys ericae, Kleidocerys resedae, Piesma quadrata and Sehirus bicolor was recorded on magnetic tape and analysed oscillographically. 2 The sounds emitted by Coranus subapterus were elicited only by tactile stimulation, and were irregular in waveform and frequency, varying in and between individuals. It is suggested that stridulation in this insect is a defence reflex. 3 In the remaining bugs the sounds were characterized by being of a pulse modulated type, and where one insect emitted more than one type of sound, these were distinguishable by differing pulse repetition frequencies. The structure of the various types of stridulation in the four insects is demonstrated by oscillograms. 4 In the four species stridulation was spontaneous, although tactile stimuli could also elicit sound emission in the two species of Kleidocerys. In Piesma, and probably in both other genera, stridulation only occurred in sexually mature adults. In Piesma and Kleidocerys both sexes emitted the same song, while in Sehirus the males emitted three songs and the females probably a single song of their own. 5 Behaviour associated with the stridulation is described and the hypothesis that the sounds are related to aggregation and sexual behaviour is discussed.