Psychophysiological Outcome Responses in Human Pavlovian Fear Conditioning: A Prediction Error Analysis

Prediction errors (PE) are thought to drive associative learning. While neural signals consistent with PE encoding have been identified, the expression of PE in psychophysiological indices remains debated. Here, we sought to fill this gap by investigating responses to unconditioned stimulus (US) occurrence and probability in skin conductance responses (SCR), pupil size responses (PSR), heart period responses (HPR), and respiration amplitude responses (RAR). Data set 1 consisted of eight published studies (N₁ = 264) using differential fear conditioning with partial reinforcement (50%), and novel data set 2 (N₂ = 29) parametrically varied US probability (20%/50%/80%). Across both data sets, all modalities showed differential responses to the US compared to US omission. In data set 1, there was evidence for responses to unexpected as compared to expected US omission in all modalities, but no responses were consistent with signed or unsigned PE encoding. Similarly, data set 2 provided no evidence that US or US omission responses monotonically related to outcome probability, which is incompatible with both signed and unsigned PE encoding. In conclusion, all recorded psychophysiological signals responded strongly to US and less strongly to unexpected US omission, with no evidence of either signed or unsigned PE encoding.