Conditioning with masked stimuli affects the timecourse of skin conductance responses

Abstract

In Pavlovian fear conditioning, an aversive unconditional stimulus (UCS) is repeatedly paired with a neutral conditional stimulus (CS). As a consequence, the subject begins to show conditional responses (CRs) to the CS that indicate expectation and fear. There are currently two general models competing to explain the role of subjective awareness in fear conditioning. Proponents of the single-process model assert that a single propositional learning process mediates CR expression and UCS expectancy. Proponents of a dual-process model assert that these behavioral responses are expressions of two independent learning processes. We used backward masking to block perception of our visual CSs and measured the effect of this training on subsequent unmasked performance. In two separate experiments we show a dissociation between CR expression and UCS expectancy following differential delay conditioning with masked CSs. In Experiment I, we show that masked training facilitates CR expression when the same CSs are presented during a subsequent unmasked reacquisition task. In Experiment II we show that masked training retards learning when the CS+ is presented as part of a compound CS during a subsequent unmasked blocking task. Our results suggest that multiple memory systems operate in a parallel, independent manner to encode emotional memories.

Figure 1

We manipulated awareness in a fear conditioning task, and assessed the effects of conditioning without awareness on subsequent learning. (a) During Phase I, we presented 20 differential conditioning trials. We presented 15-ms CSs that were masked by a 7.985-s masking stimulus to block CS₁ perception. We presented an immediate shock UCS on CS₁+ trials. (b) During Phase II, we split participants into two groups that underwent differential acquisition and extinction with 8-s unmasked CSs. We exposed one group to the same CSs (CS_S) and another to different CSs (CS_D). For both groups, the UCS coterminated with the CS+.

Figure 2

Groups show different patterns of UCS expectancy and SCR expression. (a) Participants shown the previously reinforced stimulus (n = 5) show similar levels of UCS expectancy as those shown the previously unreinforced stimulus (n = 5) and those shown novel stimuli (D+, n = 6; D−, n = 4) during the first trial of Phase II. Bars represent M + SEM. (b) In contrast, participants shown the previously reinforced stimulus (n = 5) show larger magnitude SCRs than those shown the previously unreinforced stimulus (n = 5) or those shown novel stimuli (D+, n = 6; D−, n = 4) during the first trial of Phase II. Graph represents the peak SCR value during the CS period. Bars represent M + SEM. For SCR time course data see Supplemental Figure 1 and Supplemental Table 2.

Figure 3

Groups show similar patterns of UCS expectancy during Phase II. (a, b) Average UCS expectancy across all trials of Phase II. Group SAME (a; n = 10), showed a similar pattern of UCS expectancy as Group DIFFERENT (b; n = 10). Lines represent M ± SEM.

Figure 4

Participants in Group SAME seem to show savings in the production of Phase II differential SCRs, compared to Group DIFFERENT. Group SAME (a; n = 10) shows a differential SCR that is that is longer in duration than that of Group DIFFERENT (b; n = 10). Horizontal lines at top of graph indicate p values <0.05. Plots represent M ± SEM.

Figure 5

As in Experiment I, we manipulated awareness in a fear conditioning task and assessed the effects of conditioning without awareness on subsequent learning. (a) During Phase I, we presented 20 differential conditioning trials. We presented 15-ms CSs that were masked by a 7.985-s masking stimulus to block CS₁ perception. We presented an immediate shock UCS on CS₁+ trials. (b) During Phase II, we split participants into two groups that underwent differential acquisition with 7-s unmasked novel CSs. These CSs were paired with one of the Phase I stimuli. Group BLOCK CSs were paired with CS₁+. Group CONTROL CSs were paired with CS₁−. The blocking stimulus was presented during the last second of all trials. The UCS coterminated with the blocking stimulus on all CS+ trials.

Figure 6

As in Experiment I, Groups show similar patterns of UCS expectancy during Phase II. Group BLOCK (a; n = 11), showed a similar pattern of UCS expectancy as Group CONTROL (b; n = 10). Lines represent M ± SEM.

Figure 7

Training during Phase I affects SCR expression during Phase II when the previous CS+ is used as a blocking stimulus. (a) Group BLOCK (n = 11) does not show differential SCR production. In contrast, Group CONTROL (n = 10) shows robust differential SCR production. Horizontal lines at top of graph indicate p values <0.05. Plots represent M ± SEM.