The roles of prefrontal brain regions in components of working memory: Effects of memory load and individual differences

Abstract

Using an event-related functional MRI design, we explored the relative roles of dorsal and ventral prefrontal cortex (PFC) regions during specific components (Encoding, Delay, Response) of a working memory task under different memory-load conditions. In a group analysis, effects of increased memory load were observed only in dorsal PFC in the encoding period. Activity was lateralized to the right hemisphere in the high but not the low memory-load condition. Individual analyses revealed variability in activation patterns across subjects. Regression analyses indicated that one source of variability was subjects' memory retrieval rate. It was observed that dorsal PFC plays a differentially greater role in information retrieval for slower subjects, possibly because of inefficient retrieval processes or a reduced quality of mnemonic representations. This study supports the idea that dorsal and ventral PFC play different roles in component processes of working memory.Working memory (WM), the cognitive system that allows individuals to maintain and manipulate information held temporarily in mind, may be divided into separate processes such as those required for the retention of information and those required for allocating attention and coordinating information that is being temporarily maintained (1, 2). Evidence has accumulated to support the notion that PFC may be organized to support different WM processes. Imaging studies have observed that ventral PFC is engaged by simple rehearsal (or maintenance) processes (3, 4). Studies that use more complex WM tasks have shown PFC activations that often occur bilaterally and in regions dorsal to those found in studies of simple retention (Brodmann's areas 9 and 46) (5). One theory has proposed that PFC is functionally divided in a dorsal/ventral fashion according to the type of cognitive operation that must be performed on information held in WM (6). Empirical studies have supported the notion that ventral PFC mediates maintenance processes whereas dorsal PFC is recruited when additional processing of information, such as manipulation or monitoring, is necessary (7,8).Studies that use more complex tasks, such as n-back and dual-tasks (e.g., refs. 9-11, 5), have not clearly distinguished the role of PFC in these different WM processes because information processing demands increase concurrently with maintenance demands. In fact, one study that used a task with no overt requirements to manipulate information held in WM suggested a role for dorsal PFC in WM maintenance. Rypma et al. (12) observed dorsal PFC activation in a WM task in which subjects were required to maintain one, three, or six letters for 5 seconds. When subjects were required to maintain three letters in WM, relative to one letter, activation in frontal regions was limited to left ventral PFC (Brodmann's area 44). When subjects were required to maintain six letters, relative to one letter, additional activation of dorsal PFC was observed, similar to that observed during tasks in which successful perfo...

Age-related qualitative differences in auditory cortical responses during short-term memory

“…In the current study reductions in N100 amplitude with increasing memory load in young subjects may be attributable to memory load-dependent increases in inhibition from prefrontal cortex over auditory cortex. Consistent with this notion, studies in young subjects have shown increased activity in prefrontal cortex with increases in memory load (Braver et al, 1997;Jonides et al, 1997;Rypma and D'Esposito, 1999).…”

Are Individual Differences in Fatigue Vulnerability Related to Baseline Differences in Cortical Activation?

“…The Sternliquid crystal display screen positioned in front of their eyes. Participants berg task (Steinberg, 1966) used in the cur'ent study is a verbal working were instructed to hold two hand pads with their hands and to respond with memory task (Rypma & D'Esposito, 1999;Iypnia et al. 1999: Veltman et their thumbs (left or right) to "yes" or "no" (this order was randomized al.…”

“…Participants viewed the set provided evidence that the left hemisphere i6 dominant for verbal processes of letters for 3 s (recognition). They then maintained this set in mind during Rypma & D'Esposito, 1999;Rypma ct al., 1999; E. F. a 7-s delay (retention). Subsequently, a probe letter was presented on the ; E.E Smith & Jonides, 1498, Veltman et al-, 2()03).…”

Persistent activity in the prefrontal cortex during working memory

“…Three event-related fMRI studies that manipulated memory load have failed to find that the delay period activity was affected by load [37,40,41]. For example, Jha and McCarthy [37] reported that remembering three faces did not evoke greater delay period activity in the DLPFC than remembering one face at any point during 15 or 24 s memory delays.…”

“…Increasing storage demands beyond capacity limits might invoke strategic changes in the way information is represented [44]. Rypma et al have argued [41,43] that the increased signal changes with increased load in the DLPFC are the consequence of the strategic process of data compression (e.g. chunking) because these effects are most prominent during the cue period when encoding takes place.…”