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We present a schema-based model of a classicneuropsychological task, the Wisconsin Card Sorting Task(WCST), where competition between motor and cognitiveschemas is resolved using a variation of a neuroanatomicallydetailed model of the basal ganglia (Gurney et al., 2001). Weshow that the model achieves a good fit with existing data atthe group level, and correctly identifies two distinct cognitivemechanisms held to underlie two distinct types of error.However, at the individual level, the correlations amongstother error types produced by the model differ from thoseobserved in the human data. To address this, we clusterparticipant performance into distinct groups and show, byfitting each group separately, how the model can account forthe empirically observed correlations between error types.Methodologically, this demonstrates the importance ofmodelling participant performance at the sub-group orindividual level, rather than modelling group performance.We also discuss implications of the model for the WCSTperformance of elderly participants and Parkinson’s patients.