Measuring human trace fear conditioning

Trace fear conditioning is an important research paradigm to model aversive learning in biological or clinical scenarios, where predictors (conditioned stimuli, CS) and aversive outcomes (unconditioned stimuli, US) are separated in time. The optimal measurement of human trace fear conditioning, and in particular of memory retention after consolidation, is currently unclear. We conducted two identical experiments (N₁  = 28, N₂  = 28) with a 15-s trace interval and a recall test 1 week after acquisition, while recording several psychophysiological observables. In a calibration approach, we explored which learning and memory measures distinguished CS+ and CS- in the first experiment and confirmed the most sensitive measures in the second experiment. We found that in the recall test without reinforcement, only fear-potentiated startle but not skin conductance, pupil size, heart period, or respiration amplitude, differentiated CS+ and CS-. During acquisition without startle probes, skin conductance responses and pupil size responses but not heart period or respiration amplitude differentiated CS+ and CS-. As a side finding, there was no evidence for extinction of fear-potentiated startle over 30 trials without reinforcement. These results may be useful to inform future substantive research using human trace fear conditioning protocols.