Investigating the long-lasting residual effect of a set shift on frontostriatal activity.

Previous studies have shown the involvement of the ventrolateral prefrontal cortex (PFC) and the caudate nucleus when performing a set shift. However, the effect of set shifting on the frontostriatal activity observed during the later trials within a series of same-set classifications has yet to be determined. Here, young healthy adults underwent the functional magnetic resonance imaging while performing a card-sorting task in which the classification rule was provided prior to each trial. We observed a significant activation in the dorsolateral PFC, regardless of whether a set shift occurred or not. By contrast, the ventrolateral PFC and caudate nucleus showed an increased activity in both the shifting trials versus the control and in trials where the same rule was applied for a few trials before a set shift occurred, unlike trials where the same rule was applied for a longer period. Finally, decreased activity in the caudate nucleus correlated with an increasing trial position in trials where no set shift occurred, suggesting that the more a rule is executed, the better it is established. We argue that a new rule needs to be performed multiple times until the brain areas usually associated with the set shifting are no longer significantly required anymore.