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After repeated associations between two events, E1 and E2, responses to E2 can be facilitated either because participants consciously expect E2 to occur after E1 or because E1 automatically activates the response to E2, or because of both. In this article, the authors report on 4 experiments designed to pit the influence of these 2 factors against each other. The authors found that the fastest responses to a target in a reaction time paradigm occurred when automatic activation was highest and conscious expectancy lowest. These results, when considered together with previous findings indicating that, under most conditions, the relation between expectancy and reaction times is in the opposite direction, are indicative of a reversed association-an interaction pattern that J. C. Dunn and K. Kirsner (1988) demonstrated to be the only one that unambiguously points to the involvement of independent processes.Keywords: learning, conditioning, expectancy, automatism, dissociations When two normatively unrelated events, E1 and E2, are repeatedly displayed in close temporal succession, the presentation of E1 modifies (and generally improves) the behavioral response to E2. This phenomenon has been investigated in several independent areas of research, such as associative priming (where E1 is the prime, and E2 is the target), classical conditioning (where E1 is the conditioned stimulus [CS], and E2 is the unconditioned stimulus [US]), and studies of motor behavior (where E1 is the warning stimulus, and E2 is the imperative stimulus in a reaction time paradigm). In each case, and even though different terminology has often been used, the same distinction between two general classes of interpretations has been proposed. The first focuses on the conscious expectancy for E2 that is initiated by the occurrence of E1. The second class of interpretations posits some form of automatic activation through which the occurrence of E1 facilitates the response to E2 as a mandatory consequence of their having been repeatedly associated in the past. In this context, automatic activation is therefore assumed to reflect previous experience with the association, independent of the agent's conscious expectancy for E2.

In two H 2 15O PET scan experiments, we investigated the cerebral correlates of explicit and implicit knowledge in a serial reaction time (SRT) task. To do so, we used a novel application of the Process Dissociation Procedure, a behavioral paradigm that makes it possible to separately assess conscious and unconscious contributions to performance during a subsequent sequence generation task. To manipulate the extent to which the repeating sequential pattern was learned explicitly, we varied the pace of the choice reaction time task-a variable that is known to have differential effects on the extent to which sensitivity to sequence structure involves implicit or explicit knowledge. Results showed that activity in the striatum subtends the implicit component of performance during recollection of a learned sequence, whereas the anterior cingulate/mesial prefrontal cortex (ACC/MPFC) supports the explicit component. Most importantly, we found that the ACC/MPFC exerts control on the activity of the striatum during retrieval of the sequence after explicit learning, whereas the activity of these regions is uncoupled when learning had been essentially implicit. These data suggest that implicit learning processes can be successfully controlled by conscious knowledge when learning is essentially explicit. They also supply further evidence for a partial dissociation between the neural substrates supporting conscious and nonconscious components of performance during recollection of a learned sequence.