P.C.Fletcher and R.N.A.Henson Frontal function in working memory tasks The term 'working memory'is ger rally used to refer to the currently maintain ability to maintain information on-line,often in the service as he onitoring and h her-level planning of a particular be illustrated or goa ever,the term has diffe om often used to describe the ability of an animal to remembe simply require that the subject decides whether a prob a stimulus for a short period after it is removed (in orde matches one of a set to perform e.g.delayed matching-to-sample tasks).In th cognitive stimulus at a time fror ature.on th the set presented previously.such that,over trials,every sources of information are order to perform timulus has been selected once (without repetition). Thi requires that the wo perspectives etore consi revious responses.According to ol stigation eas a sel tent with thi defici tasks but not typi ally on delayed-matchin Perspectives from animal studies:domain in patients which typical lly include (Petrides nal specializ 98 19 0).Fur theories. These theories co entrate in r particular on dis with DLFC lesions w re imnaired on simple verhal o sociations between ventral and dorsal regions of lateral FC span tasks that require only maintenance of a stimulus on-line ry. hou ymanipula on)(D'Esposito oppo Moreover,the fo mainte prec n the is 0 largely on electrophysio tical recordings and is 3 imnle spatial delaved r sponse tasks or only in more comple extension of the object-spatial (what'versus'where)visua situations,such as self-ordering tasks.Nonetheless,we will n posterior regions(Mishkin ompare the e two genera theories for their ability to accoun ore spec n and the imaging dat The data are i from the human psychological literature. on WM e dorsal to the principal sulcus code for r spatial information al. 1993 gene tial VLEC-DLEC distinction refects all com Perspectives from human cognitive psychology: WM:the'attentional,memorial and response control mechan multiple-component models isms'(Goldman-Rakic 1998).That is,there is no suggestio hich th 194 nerate develoned to account for a range of different wM functions The altemative,process-specific theory proposes that the from temporary maintenance of a single stimulus to the e the type of nipulation of multiple types of inform ratine on that material (Pe Shiffrin.1968).which acted simply as ag derives mainly from animal lesion data (Petrides,1994)and perception and LTM,to a multicomponent system in which has been extende to human lesion data (Petride a number of subsidiary 'slave'systems are coordinated by a wen et e ding to this th nmo centra ex The ave systen tion in WM.This infommation may have been perccived limited-capacity.material-specific stors.concemed with the recently or retrieved from LTM.DLFC.however.supports maintenance of verbal and visuospatial material respectively.852 P. C. Fletcher and R. N. A. Henson Frontal function in working memory tasks more complex processes operating on information that is The term ‘working memory’ is generally used to refer to the currently maintained in WM. These include processes such ability to maintain information on-line, often in the service as monitoring and higher-level planning. of a particular task or goal. However, the term has different The process-specific distinction can be illustrated by connotations in different fields. In the animal literature, it is comparing two types of WM task. ‘Delayed matching tasks’ often used to describe the ability of an animal to remember simply require that the subject decides whether a probe a stimulus for a short period after it is removed (in order stimulus matches one of a set of stimuli held in WM. This task requires maintenance only. In ‘self-ordered tasks’, to perform e.g. delayed matching-to-sample tasks). In the cognitive psychological literature, on the other hand, WM however, the subject must select one stimulus at a time from frequently refers to a mental workspace in which multiple the set presented previously, such that, over trials, every stimulus has been selected once (without repetition). This sources of information are manipulated in order to perform complex problem-solving tasks. We begin by introducing the requires that the subject not only selects stimuli from a set background to these two perspectives, before considering maintained in WM but also updates and monitors the set of recent imaging studies that have attempted to synthesize ideas previous responses. According to Petrides and colleagues, a delayed matching task would engage VLFC, whereas a self- from these traditionally quite distinct fields of investigation. ordering task would engage DLFC. Consistent with this view, DLFC lesions in primates produce deficits on self-ordering tasks but not typically on delayed-matching tasks (Petrides, Perspectives from animal studies: domain- 1995). Self-ordering deficits are also seen following frontal versus process-specific theories lesions in patients, which typically include DLFC (Petrides Two competing ideas concerning functional specialization and Milner, 1982; Owen et al., 1990). Furthermore, a review of FC in WM are ‘domain-specific’ and ‘process-specific’ by D’Esposito and Postle found no evidence that patients theories. These theories concentrate in particular on dis- with DLFC lesions were impaired on simple verbal or spatial sociations between ventral and dorsal regions of lateral FC. span tasks that require only maintenance of a stimulus on-line According to the domain-specific theory, FC is the primary (without any manipulation) (D’Esposito and Postle, 1999). site of WM processes and different regions within FC process Though often placed in opposition, the domain-specific different types of information (Goldman-Rakic, 1987, 1998). and process-specific theories are not necessarily incompatible. Specifically, VLFC is believed to be responsible for the FC may be functionally dissociable according to both the maintenance of stimulus form (object information), whereas type of material and the type of process. Moreover, the DLFC is believed to be responsible for the maintenance of precise site of lesions in the primate DLFC (e.g. Brodmann stimulus location (spatial information). This theory is based areas 9 or 46) can affect whether impairments are seen in largely on electrophysiological recordings and is an simple spatial delayed response tasks or only in more complex extension of the object–spatial (‘what’ versus ‘where’) visual situations, such as self-ordering tasks. Nonetheless, we will processing streams found in posterior regions (Mishkin et al., compare these two general theories for their ability to account 1983). More specifically, Wilson and colleagues found that for the human imaging data. The data are introduced later, FC cells ventral to the principal sulcus code for object after considering an alternative perspective on WM deriving information during a delay, whereas frontal cells within and from the human psychological literature. dorsal to the principal sulcus code for spatial information during a delay (Wilson et al., 1993). More generally, Goldman-Rakic and colleagues suggested that the object– Perspectives from human cognitive psychology: spatial VLFC–DLFC distinction reflects all components of WM: the ‘attentional, memorial and response control mechan- multiple-component models isms’ (Goldman-Rakic, 1998). That is, there is no suggestion Baddeley and Hitch’s theoretical model of WM function of specialization for different WM processes across FC, only (Baddeley and Hitch, 1974) has been highly influential in specialization for the domains over which these processes framing functional neuroimaging studies. This model was operate. developed to account for a range of different WM functions, The alternative, process-specific theory proposes that the from temporary maintenance of a single stimulus to the difference between VLFC and DLFC lies not in the type of manipulation of multiple types of information. It evolved from material being maintained but in the type of processes earlier conceptions of a single short-term buffer (Atkinson and operating on that material (Petrides, 1994, 1995). This theory Shiffrin, 1968), which acted simply as a gateway between derives mainly from animal lesion data (Petrides, 1994) and perception and LTM, to a multicomponent system in which has been extended to human lesion data (Petrides and Milner, a number of subsidiary ‘slave’ systems are coordinated by a 1982; Owen et al., 1990). According to this theory, VLFC common ‘central executive’. The slave systems, the supports processes that transfer, maintain and match informa- ‘phonological loop’ and ‘visuospatial scratch-pad’, are tion in WM. This information may have been perceived limited-capacity, material-specific stores, concerned with the recently or retrieved from LTM. DLFC, however, supports maintenance of verbal and visuospatial material respectively