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Courtney, S. M., Petit, L., Haxby, J. V., & Ungerleider, L. G. (1998). The role of prefrontal cortex in working memory: examining the contents of consciousness. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences, 353(1377), 1819-1828.

Working memory enables us to hold in our 'mind's eye' the contents of our conscious awareness, even in the absence of sensory input, by maintaining an active representation of information for a brief period of time. In this review we consider the functional organization of the prefrontal cortex and its role in this cognitive process. First, we present evidence from brain-imaging studies that prefrontal cortex shows sustained activity during the delay period of visual working memory tasks, indicating that this cortex maintains on- line representations of stimuli after they are removed from view. We then present evidence for domain specificity within frontal cortex based on the type of information, with object working memory mediated by more ventral frontal regions and spatial working memory mediated by more dorsal frontal regions. We also propose that a second dimension for domain specificity within prefrontal cortex might exist for object working memory on the basis of the type of representation, with analytic representations maintained preferentially in the left hemisphere and image-based representations maintained preferentially in the right hemisphere. Furthermore, we discuss the possibility that there are prefrontal areas brought into play during the monitoring and manipulation of information in working memory in addition to those engaged during the maintenance of this information. Finally we consider the relationship of prefrontal areas important for working memory, both to posterior visual processing areas and to prefrontal areas associated with long-term memory. gif

Haxby, J. V., Petit, L., Ungerleider, L. G., & Courtney, S. M. (2000). Distinguishing the functional roles of multiple regions in distributed neural systems for visual working memory. Neuroimage, 11(5), 380-391.

We have investigated the human neural systems for visual working memory using functional magnetic resonance imaging to distinguish sustained activity during memory delays from transient responses related to perceptual and motor operations. These studies have identified six distinct frontal regions that demonstrate sustained activity during memory delays. These regions could be distinguished from brain regions in extrastriate cortex that participate more in perception and from brain regions in medial and lateral frontal cortex that participate more in motor control. Moreover, the working memory regions could be distinguished from each other based on the relative strength of their participation in spatial and face working memory and on the relative strength of sustained activity during memory delays versus transient activity related to stimulus presentation. These results demonstrate that visual working memory performance involves the concerted activity of multiple regions in a widely distributed system. Distinctions between functions, such as perception versus memory maintenance, or spatial versus face working memory, are a matter of the degree of participation of different regions, not the discrete parcellation of different functions to different modules. (C) 2000 Academic Press.gif

Deco, G., Rolls, E. T., & Horwitz, B. (2004). "What'' and "where'' in visual working memory: A computational neurodynamical perspective for integrating fMRI and single- neuron data. Journal of Cognitive Neuroscience, 16(4), 683-701.

Single-neuron recordings, functional magnetic resonance imaging ( fMRI) data, and the effects of lesions indicate that the prefrontal cortex (PFC) is involved in some types of working memory and related cognitive processes. Based on these data, two different models of the topographical and functional organization of the PFC have been proposed: organization-by- stimulus-domain, and organization-by-process. In this article, we utilize an integrate-and-fire network to model both single- neuron and fMRI data on short-term memory in order to understand data obtained in topologically different parts of the PFC during working memory tasks. We explicitly model the mechanisms that underlie working-memory-related activity during the execution of delay tasks that have a "what''-then-"where'' design ( with both object and spatial delayed responses within the same trial). The model contains different populations of neurons ( as found experimentally) in attractor networks that respond in the delay period to the stimulus object, the stimulus position, and to combinations of both object and position information. The pools are arranged hierarchically and have global inhibition through inhibitory interneurons to implement competition. It is shown that a biasing attentional input to define the current relevant information ( object or location) enables the system to select the correct neuronal populations during the delay period in what is a biased competition model of attention. The processes occurring at the AMPA and NMDA synapses are dynamically modeled in the integrate-and-fire implementation to produce realistic spiking dynamics. It is shown that the fMRI data characteristic of the dorsal PFC and linked to spatial processing and manipulation of items can be reproduced in the model by a high level of inhibition, whereas the fMRI data characteristic of the ventral PFC and linked to object processing can be produced by a lower level of inhibition, even though the network is itself topographically homogeneous with no spatial topology of the neurons. This article, thus, not only presents a model for how spatial versus object short-term memory could be implemented in the PFC, but also shows that the fMRI BOLD signal measured during such tasks from different parts of the PFC could reflect a higher level of inhibition dorsally, without this dorsal region necessarily being primarily spatial and the ventral region object-related.

Nyberg, L., Forkstam, C., Petersson, K. M., Cabeza, R., & Ingvar, M. (2002). Brain imaging of human memory systems: between- systems similarities and within-system differences. Cognitive Brain Research, 13(2), 281-292.

There is much evidence for the existence of multiple memory systems. However, it has been argued that tasks assumed to reflect different memory systems share basic processing components and are mediated by overlapping neural systems. Here we used multivariate analysis of PET- data to analyze similarities and differences in brain activity for multiple tests of working memory, semantic memory, and episodic memory. The results from two experiments revealed between-systems differences, but also between-systems similarities and within- system differences. Specifically, support was obtained for a task-general working-memory network that may underlie active maintenance. Premotor and parietal regions were salient components of this network. A common network was also identified for two episodic tasks, cued recall and recognition, but not for a test of autobiographical memory. This network involved regions in right inferior and polar frontal cortex, and lateral and medial parietal cortex. Several of these regions were also engaged during the working-memory tasks, indicating shared processing for episodic and working memory. Fact retrieval and synonym generation were associated with increased activity in left inferior frontal and middle temporal regions and right cerebellum. This network was also associated with the autobiographical task, but not with living/non-living classification, and may reflect elaborate retrieval of semantic information. Implications of the present results for the classification of memory tasks with respect to systems and/or processes are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Passingham, D., & Sakai, K. (2004). The prefrontal cortex and working memory: physiology and brain imaging. Current Opinion in Neurobiology, 14(2), 163-168.

Sustained activity has been recorded in the prefrontal cortex during working memory tasks. First, we compare the anatomical distribution of this activity in humans and monkeys. Then, we show that it reflects many factors, maintenance of the items presented, preparation for the response, transformation of the items during the delay, task rules and task goals. Finally, we point out that sustained activity has also been recorded in other areas, such as the parietal cortex. We suggest that the key to prefrontal cortex lies not in the maintenance of sensory information but in the prospective use of that information for behaviour.gif

Ungerleider, L. G., Courtney, S. M., & Haxby, J. V. (1998). A neural system for human visual working memory. Proceedings of the National Academy of Sciences of the United States of America, 95(3), 883-890.

Working memory is the process of actively maintaining a representation of information for a brief period of time so that it is available for use, In monkeys, visual working memory involves the concerted activity of a distributed neural system, including posterior areas in visual cortex and anterior areas in prefrontal cortex, Within visual cortex, ventral stream areas are selectively involved in object vision, whereas dorsal stream areas are selectively involved in spatial vision, This domain specificity appears to extend forward into prefrontal cortex, with ventrolateral areas involved mainly in working memory for objects and dorsolateral areas involved mainly in working memory for spatial locations, The organization of this distributed neural system for working memory in monkeys appears to be conserved in humans, though some differences between the two species exist, In humans, as compared with monkeys, areas specialized for object vision in the ventral stream have a more inferior location in temporal cortex, whereas areas specialized for spatial vision in the dorsal stream have a more superior location in parietal cortex, Displacement of both sets of visual areas away from the posterior perisylvian cortex may be related to the emergence of language over the course of brain evolution, Whereas areas specialized for object working memory in humans and monkeys are similarly located in ventrolateral prefrontal cortex, those specialized for spatial working memory occupy a more superior and posterior location within dorsal prefrontal cortex in humans than in monkeys, As in posterior cortex, this displacement in frontal cortex also may be related to the emergence of new areas to serve distinctively human cognitive abilities.gif

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