Load-dependent roles of frontal brain regions in the maintenance of working memory

Brain imaging studies have suggested a critical role for prefrontal cortex in working memory (WM) tasks that require both maintainenance and manipulat ion of information over time in delayed-response WM tasks. In the present s tudy, functional magnetic resonance imaging (fMRI) was used to examine whet her prefrontal areas are activated when only maintenance is required in a d elayed-response WM task, without the overt requirement to manipulate the st ored information, In two scans, six subjects performed WM tasks in which, o n each trial, they (1) encoded 1, 3, or 6 to-be remembered letters, (2) mai ntained these letters across a 5-second unfilled delay, and (3) determined whether a single probe letter was or was not part of the memory set. Activa tion of left caudal inferior frontal gyrus was observed, relative to the 1- letter task, when subjects were required to maintain 3 letters in WM. When subjects were required to maintain 6 letters in WM, additional prefrontal a reas, most notably middle and superior frontal gyri, were activated bilater ally. Thus, increasing the amount of to-be-maintained information, without any overt manipulation requirement, resulted in the recruitment of wide-spr ead frontal-lobe regions. Inferior frontal gyrus activation was left-hemisp here dominant in both the 3- and 6-letter conditions, suggesting that such activation reflected material-specific verbal processes. Activation in midd le and superior frontal gyri appeared only in the 8-letter condition and wa s right-hemisphere dominant, suggesting that such activation reflected mate rial-independent executive processes, (C) 1999 Academic Press.