Revisiting the effects of Parkinson's disease and frontal lobe lesions on task switching: the role of rule reconfiguration

This study investigated the hypothesis that rule reconfiguration in task switching can isolate aspects of intact and impaired control at different stages of Parkinson's disease (PD) by comparing switches between concrete naming rules pertaining to stimulus selection, to switches between abstract rules which allocate categorization responses to these stimuli. Based on previous findings, it was hypothesized that attentional switches, where task set competition emerges at the stimulus but not response set level, highlights striatal dopaminergic function. Conversely, increasing the degree of task set competition to encompass reconfiguration of response set when switching between abstract rules, represents a condition which engages the prefrontal cortex (PFC) and renders this manipulation sensitive to frontal damage. To this end, we investigated task switching with concrete and abstract rules in unilaterally (Hoehn & Yahr stage I) and bilaterally (Hoehn & Yahr stage II) affected PD patients, as well as striatally intact frontal lesion patients. As predicted, frontal lesion patients demonstrated switching deficits only with abstract categorization rules, where switching engendered complete task set reconfiguration and a new response, as did stage II PD patients with presumed frontal cortical pathology. Replicating previous findings, stage I PD patients with relatively circumscribed striatal pathology demonstrated no such impairment. Disease severity also impacted on attentional switching indexed by naming rules, since medicated stage II but not stage I patients demonstrated switching deficits emerging from stimulus set reconfiguration, suggesting that the ameliorative efficacy of dopaminergic medication is inversely related to the severity of the striatal deficit. These findings illustrate that the nature of the rules that are switched, and its implication in terms of reconfiguring different task set elements, highlights different neural characters of cognitive flexibility. These manipulations may help decipher the differential effects of progressive neurodegeneration on parkinsonian cognition, and provide a framework in which to conceptualize the contributions of cortical and subcortical regions to cognitive control.