Dissociation between implicit and explicit responses in postconditioning UCS revaluation after fear conditioning in humans

Abstract

The nature of the relationship between explicit and implicit learning is a topic of considerable debate. To investigate this relationship we conducted two experiments on postconditioning revaluation of the unconditional stimulus (UCS) in human fear conditioning. In Experiment 1, the intensity of the UCS was decreased after acquisition for one group (devaluation) and held constant for another group (control). A subsequent test revealed that even though both groups exhibited similar levels of UCS expectancy, the devaluation group had significantly smaller conditional skin conductance responses. The devaluation effect was not explained by differences in the explicit estimates of UCS probability or explicit knowledge that the UCS intensity had changed. In Experiment 2, the value of the UCS was increased after acquisition for one group (inflation) and held constant for another group (control). Test performance revealed that UCS inflation did not alter expectancy ratings, but the inflation group exhibited larger learned skin conductance responses than the control group. The inflation effect was not explained by differences in the explicit estimates of UCS probability or explicit knowledge that the UCS intensity had changed. The SCR revaluation effect was not dependent on explicit memory processes in either experiment. In both experiments we found differences on an implicit measure of learning in the absence of changes in explicit measures. Together, the differences observed between expectancy measures and skin conductance support the idea that these responses might reflect different types of memory formed during the same training procedure and be supported by separate neural systems.

Figure 1

a) UCS expectancy data for CS+ and CS− trials for each group in the acquisition phase. b) Skin conductance responses for CS+ and CS− trials for each group in the acquisition phase. Error bars show SEM.

Figure 2

Verbal ratings of the UCS intensity following the acquisition phase (left panel) and the revaluation phase (right panel).

Figure 3

a) UCS expectancy data for the first three CS+ trials in the test session. b) Skin conductance data for the first three CS+ trials in the test session.

Figure 4

a) Change in UCS intensity ratings from acquisition to revaluation for subgroups of the devaluation group that were classified as being aware and unaware of this change. b) SCR for the UCS intensity aware and UCS intensity unaware subgroups at test (the control group is depicted for reference).

Figure 5

a) UCS expectancy data for CS+ and CS− trials for each group in the acquisition phase. b) Skin conductance responses for CS+ and CS− trials for each group in the acquisition phase.

Figure 6

Verbal ratings of the UCS intensity following the acquisition phase (left panel) and the revaluation phase (right panel).

Figure 7

a) UCS expectancy data for the first three CS+ trials in the test session. b) Skin conductance data for the first three CS+ trials in the test session.

Figure 8

a) Change in UCS intensity ratings from acquisition to revaluation for subgroups of the inflation group that were classified as being aware and unaware of this change. b) SCR for the UCS intensity aware and UCS intensity unaware subgroups at test (the control group is depicted for reference).

Figure 9

Correlation between UCS intensity awareness and SCR revaluation effect when the experimental groups from Experiment 1 and 2 are combined.