Persistent activity in the prefrontal cortex during working memory

Abstract

The dorsolateral prefrontal cortex (DLPFC) plays a crucial role in working memory. Notably, persistent activity in the DLPFC is often observed during the retention interval of delayed response tasks. The code carried by the persistent activity remains unclear, however. We critically evaluate how well recent findings from functional magnetic resonance imaging studies are compatible with current models of the role of the DLFPC in working memory. These new findings suggest that the DLPFC aids in the maintenance of information by directing attention to internal representations of sensory stimuli and motor plans that are stored in more posterior regions.Working memory refers to the temporary representation of information that was just experienced or just retrieved from long-term memory. These active representations are short-lived, but can be maintained for longer periods of time through active rehearsal strategies, and can be subjected to various operations that manipulate the information in such a way that makes it useful for goaldirected behavior. Most definitions of working memory include both storage and (executive) control components [1]. Cognitive neuroscientists are searching for ways to disassociate the separate components of working memory in attempts to localize and clearly characterize their neural implementation. The prefrontal cortex (PFC) is thought to be the most important substrate for working memory (Fig. 1). Two key findings from studies of monkeys performing delayed response tasks suggest a crucial role for the PFC in working memory. First, experimental lesions of the principal sulcus in the dorsolateral prefrontal cortex (DLPFC) cause delay-dependent impairments [2][3][4]. That is, forgetting increases not only when a delay is imposed but increases with the length of the delay. Second, neurophysiological unit recordings from the DLPFC often show persistent, sustained levels of neuronal firing during the retention interval of delayed response Fig. 1. Lateral surface of (a) macaque and (b) human brain. The PFC is composed of lateral, medial, and orbital sectors that are believed to be functionally distinct given the selective effects of damage and distribution of afferent and efferent projections. The tinted areas correspond to those defined by Petrides and Pandya [71] based on cytoarchitecture and connectivity. Notably, the mid-DLPFC comprises areas 46 and 9/46 and the mid-VLPFC comprises areas 45 and 47/12. Note that much of area 46 lies in the depths of the principle sulcus of the monkey and the intermediate frontal sulcus of the human. Frontal premotor regions are also highlighted. The frontal eye field (F) in the macaque lies in the anterior bank of the arcuate sulcus in area 8A. In the human, F is found in the vicinity of the precentral sulcus and superior frontal sulcus junction (area 6 and maybe the caudal-most portion of 8A). The frontal eye field is a premotor region involved in the control of eye movements. Broca's area (B, area 44) is also a premotor area that is involved in the product...

Beyond common resources: the cortical basis for resolving task interference

“…For example, the common load-sensitive region in the left prefrontal cortex has been implicated in both the maintenance of task goals during executive function tasks (MacDonald et al, 2000;Ullsperger and von Cramon, 2001) and WM rehearsal (Curtis and D'Esposito, 2003;D'Esposito et al, 1999;Rypma et al, 2002). Similarly, the dorsal ACC/pre-SMA region found to be common to both tasks has been widely identified across a range of tasks (Duncan and Owen, 2000), including inhibitory control (Garavan et al, 1999;Rubia et al, 2003) and WM (Kondo et al, 2004;Nyberg et al, 2003;Osaka et al, 2003).…”

Prefrontal cortex and sensory cortices during working memory: quantity and quality

“…Persistent activity in both sensory cortices and association areas (especially the prefrontal cortex, PFC) throughout the delay interval of a WM task after sensory stimulus presentation (sample) are usually considered to be critical for WM maintenance, and to bridge the temporal gap between the sample and the subsequent contingent response (see reviews [4,5] ). However, with regard to WM the role of the delay activity in sensory cortices has been thought to differ from that of the PFC.…”

Learning to memorize: Shedding new light on prefrontal functions

“…Among these regions, the PFC has drawn most attention due to the striking finding that individual neurons show persistent activity during the memory-retention period [1][2][3] (termed the delay period, a hallmark of working memory tasks): elevated activity persists after the sensory stimuli have been removed until the holding period is over (from seconds to tens of seconds) and the behavioral choice has been made. This raises the immediate question of whether the persistent activity in the PFC during the delay period encodes the contents of working memory (memory storage).…”

“…Therefore, it is not yet clear that the PFC is the site where the representations are stored. The fact that the PFC has been found to play important roles in executive functions [4] implies that its role in working memory might be controlling attention, selecting strategies, and manipulating information, rather than information storage [2,5] .…”

“…Therefore, it is not yet clear that the PFC is the site where the representations are stored. The fact that the PFC has been found to play important roles in executive functions [4] implies that its role in working memory might be controlling attention, selecting strategies, and manipulating information, rather than information storage [2,5] .To resolve this debate, it is therefore necessary to achieve a deeper understanding of the causal role of the PFC in working memory tasks. This would require temporally precise perturbation of neuronal activity in specific regions of the PFC during the delay period of a working memory task and monitoring its effect on task performance.…”