The authors theorize that 2 neurocognitive sequence-learning systems can be distinguished in serial reaction time experiments, one dorsal (parietal and supplementary motor cortex) and the other ventral (temporal and lateral prefrontal cortex). Dorsal system learning is implicit and associates noncategorized stimuli within dimensional modules. Ventral system learning can be implicit or explicit It also allows associating events across dimensions and therefore is the basis of cross-task integration or interference, depending on degree of cross-task correlation of signals. Accordingly, lack of correlation rather than limited capacity is responsible for dual-task effects on learning. The theory is relevant to issues of attentional effects on learning; the representational basis of complex, sequential skills; hippocampal-versus basal ganglia-based learning; procedural versus declarative memory; and implicit versus explicit memory.

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We explored shift costs for a dimensionally organized set of tasks. Task dimensions were type of judgement (numerical vs. spatial) and judgement-to-response mapping (compatible vs. incompatible). Shift costs were determined as the dierence in RTs between switch trials and repetitions in a situation in which the kind of task was unpredictable. Shift costs were greatest when the type of judgement was changed. A change in the mapping increased shift costs when the type of judgement remained unchanged, but reduced shift costs when the type of judgement was changed as well. Also, response alternations produced costs when both the type of judgement and SR mapping were unchanged, but reduced shift costs otherwise. In addition to the relation between successive tasks, shift costs were modulated by the speci®cs of the elementary tasks. The major characteristics of the pattern of shift costs could be accounted for by the hypothesis of a dimensionally organized task space in which hierarchically organized, relative switching operations are performed that aect the highest-level task dimension at which a change is required, and all lower-level dimensions in a ®rst step. In a second step, lower-level dimensions are switched back when no change is required, and then the selected control structure is implemented.