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 ( N 1 = 28, N 2 = 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.

Fear conditioning is a laboratory paradigm commonly used to investigate aversive learning and memory. In context fear conditioning, a configuration of elemental cues (conditioned stimulus [CTX]) predicts an aversive event (unconditioned stimulus [US]). To quantify context fear acquisition in humans, previous work has used startle eyeblink responses (SEBRs), skin conductance responses (SCRs), and verbal reports, but different quantification methods have rarely been compared. Moreover, preclinical intervention studies mandate recall tests several days after acquisition, and it is unclear how to induce and measure context fear memory retention over such a time interval. First, we used a semi-immersive virtual reality paradigm. In two experiments (N= 23 andN= 28), we found successful declarative learning and memory retention over 7 d but no evidence of other conditioned responses. Next, we used a configural fear conditioning paradigm with five static room images as CTXs in two experiments (N= 29 andN= 24). Besides successful declarative learning and memory retention after 7 d, SCR and pupil dilation in response to CTX onset differentiated CTX+/CTX−during acquisition training, and SEBR and pupil dilation differentiated CTX+/CTX−during the recall test, with medium to large effect sizes for the most sensitive indices (SEBR: Hedge'sg= 0.56 andg= 0.69; pupil dilation: Hedge'sg= 0.99 andg= 0.88). Our results demonstrate that with a configural learning paradigm, context fear memory retention can be demonstrated over 7 d, and we provide robust and replicable measurement methods to this end.

Learning to predict threat is of adaptive importance, but aversive memory can also become disadvantageous and burdensome in clinical conditions such as post-traumatic stress disorder. Pavlovian fear conditioning is a laboratory model of aversive memory and thought to rely on structural synaptic reconfiguration involving matrix metalloproteinase (MMP) 9 signalling. It has recently been suggested that the MMP9-inhibiting antibiotic doxycycline, applied before acquisition training in humans, reduces fear memory retention after one week. This previous study used cued delay fear conditioning, in which predictors and outcomes overlap in time. However, temporal separation of predictors and outcomes is common in clinical conditions. Learning the association of temporally separated events requires a partly different neural circuitry, for which the role of MMP9 signalling is not yet known. Here, we investigate the impact of doxycycline on long-interval (15 s) trace fear conditioning in a randomised controlled trial with 101 (50 females) human participants. We find no impact of the drug in our pre-registered analyses. Exploratory post-hoc analyses of memory retention suggested a serum level-dependent effect of doxycycline on trace fear memory retention. However, effect size to distinguish CS+/CS- in the placebo group turned out to be smaller than in previously used delay fear conditioning protocols, which limits the power of statistical tests. Our results suggest that doxycycline effect on trace fear conditioning in healthy individuals is smaller and less robust than anticipated, potentially limiting its clinical application potential.Significance statementThe inhibition of matrix metalloproteinase-9 attenuates memory consolidation and subsequent recall in a delay cue conditioning paradigm. However, it is currently unclear whether this is also the case for other learning scenarios that rely on a different neurocircuitry. We test this hypothesis in human trace fear conditioning which employs remote cues and is additionally dependent on hippocampus involvement. We find that doxycycline does not reduce fear retention in our pre-registered analyses. Exploratory analyse might potentially suggest a weak effect of doxycycline on trace fear memory retention.

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