At which steps of spatial working memory processing do striatofrontal circuits intervene in humans?

Striatofrontal circuits have been implicated in spatial working memory in non-human and human primates. To determine at which steps of information processing (stimulus encoding, storage or response programming) they intervene, we compared 32 levodopa-treated patients with idiopathic Parkinson's disease (PD) and 32 matched control subjects in a visuo-spatial pattern span paradigm. Our testing procedure allowed us to evaluate the influence of: (1) the type of encoding (controlled vs free); (2) the nature of interference during a 10 s delay (spatial vs verbal); and (3) response elaboration (reproduction vs error detection). As expected, the performance of control subjects was significantly better in controlled than in free encoding, in verbal than in spatial interference and in detection than in reproduction, clearly demonstrating the sensitivity of the procedure to these factors. Compared to controls, PD patients were impaired in all conditions and the severity of the deficit was significantly correlated with that observed in tests of executive functions. The global pattern of performance, however, was identical to that of controls. These data confirm the involvement of striatofrontal circuits in spatial working memory in humans and suggest that the executive working memory component intervenes at all steps of working memory processing.