Acquisition learning is stronger for aversive than appetitive events

Appetitive and aversive learning are both key building blocks of adaptive behavior, yet knowledge regarding their differences is sparse. Using a capsaicin heat pain model in 36 healthy participants, this study directly compared the acquisition and extinction of conditioned stimuli (CS) predicting pain exacerbation and relief. Valence ratings show stronger acquisition during aversive compared to appetitive learning, but no differences in extinction. Skin conductance responses and contingency ratings confirmed these results. Findings were unrelated to individual differences in pain sensitivity or psychological factors. Our results support the notion of an evolutionarily hardwired preponderance to acquire aversive rather than appetitive cues as is protective for acute aversive states such as pain but may contribute to the development and maintenance of clinical conditions such as chronic pain, depression or anxiety disorders.


Calibrated temperatures, subjective ratings and questionnaire results
Values are given in mean ± standard deviation (M ± SD) in Table 1  *Please note that model calculation with random slopes for the factor time was not possible due to the number of random effects exceeding the number of observations. AIC = Akaike information criterion; AIC of winning models are presented in bold font. CS: conditioned stimulus.

Supplementary Figure 1. Pain ratings over the course of the experiment. Pain intensity (A) and pain (un)pleasantness (B) ratings (raw values)
during all experimental phases for all US types on a 0-100 VAS (A) and a -50-50 VAS (B) in mean ± SEM. Displayed are single ratings for the USincrease and USdecrease during training (train) and acquisition (acq) and ratings for the USmedium during training, acquisition, and extinction (ext).

Influence of contingency awareness on learning slopes (valence ratings)
We further included the potential covariate contingency into the analyses of differential learning to investigate the influence of contingency awareness on the acquisition and extinction of pain-related emotional responses, i.e., valence ratings of CS types.
Regarding the extinction phase, we observed a significant interaction of the factor time and the covariate contingency for the CSincrease (β: -0.08 ± 0.04; t(142.23) = -2.11, p = 0.04, d = -0.35) only, indicating a steeper decrease in negative valence ratings, i.e. enhanced extinction, in those participants with higher contingency ratings after extinction. This result could be influenced by the high valence ratings at the end of the acquisition phase in those subjects with high contingency awareness.
Specifically, subjects who were highly aware of the CS-US contingency during the acquisition phase also showed higher valence learning curves during acquisition and probably also during extinction, i.e., better learning leading to better extinction. Moreover, we cannot rule out that subjects intermixed the contingency rating after the extinction phase with the acquisition phase, as we did not specifically separate the phases.

Skin conductance responses -responder analyses
We performed additional analyses including only SCR responders. For that purpose, participants showing valid US-induced SCRs (i.e., amplitudes > 0.01 μS) in less than 33% of the trials were excluded from the analyses (n=4). Here, by AIC comparison, for the analyses of CS-induced SCR amplitudes, the models with subject-specific random slopes and the random factor CS type best predicted the data (acquisition: ΔAIC = -10, p < 0.001; extinction: ΔAIC = -7.3, p =0.81). For the model of US-induced SCR amplitudes, calculation of the model including random effects was not possible due to a limited number of observations.

Conditioned stimuli
For the responder analysis of the acquisition phase, there were no significant effects in the analysis of neither the raw data nor the differential data (all p > 0.3).
For the extinction phase, we again observed a significant main effect of the factor CS type indicating

Unconditioned stimuli
For the responder analysis, results also revealed significantly higher SCR amplitudes for the USincrease as