DISSOCIATING WORKING-MEMORY FROM TASK-DIFFICULTY IN HUMAN PREFRONTAL CORTEX

A functional magnetic resonance imaging (fMRI) study was conducted to determine whether prefrontal cortex (PFC) increases activity in workin g memory (WM) tasks as a specific result of the demands placed on WM, or to other processes affected by the greater difficulty of such tasks . Increased activity in dorsolateral PFC (DLPFC) was observed during t ask conditions that placed demands on active maintenance (long retenti on interval) relative to control conditions matched for difficulty. Fu rthermore, the activity was sustained over the entire retention interv al and did not increase when task difficulty was manipulated independe ntly of WM requirements. This contrasted with the transient increases in activity observed in the anterior cingulate, and other regions of f rontal cortex, in response to increased task difficulty but not WM dem ands. Thus, this study established a double-dissociation between regio ns responsive to WM versus task difficulty, indicating a specific invo lvement of DLPFC and related structures in WM function. (C) 1997 Elsev ier Science Ltd.