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ScienceWeek
NEUROSCIENCE: ON WORKING MEMORY
The following points are made by Adrian M. Owen (Current Biology 2004 14:R573):
1) Working memory has been described in various ways: as a cognitive system for both the temporary storage and manipulation of remembered information [1,2]; as the type of memory that is active and only relevant for a short period of time [3,4]; and as the specific process by which a remembered stimulus is held "on-line" to guide behavior in the absence of external cues or prompts [5]. While definitions vary, few disagree that working memory capacity refers to the number of items of information that can be maintained over a short delay interval and that working memory contributes to performance on a wide variety of cognitive tasks, from complex decision making to selective attention and language [1].
2) In spite of this, a precise neurophysiological correlate of working memory storage capacity has not previously been identified. A common assumption in both electrophysiological recording studies in monkeys and functional neuroimaging studies in humans is that activity that is sustained during a delay period reflects maintenance or storage processes, and may therefore provide a key to the neural basis of known limits on storage capacity. For example, several early single-unit recording studies revealed sustained neuronal firing in the lateral frontal cortex of monkeys during the retention interval of delayed response tasks, suggesting the temporary storage of some aspect of the stimulus to be remembered. But visual-form-specific neurons with activity that is sustained during a delay have also been found in area TE and in perirhinal cortex in the anterior temporal lobe, while delay-dependent spatial working memory neurons have been identified in the parietal cortex.
3) In humans, several event-related functional magnetic resonance imaging (fMRI) studies have also found persistent frontal-lobe activity during retention intervals of delayed response tasks. But some of these studies have failed to show a relationship between frontal activity and memory load in such tasks, arguing against a role for this region in basic storage processes. Moreover, sustained activity during delays, broadly similar to that observed in the frontal-lobe, has also been reported in other cortical regions, such as the dorsal premotor and posterior parietal cortices.
4) The question that arises, therefore, is whether working memory storage can in fact be localised at all, or whether it is simply a general property of a distributed neural system. Recent studies have shed light on this issue by identifying, for the first time, neurophysiological correlates of working memory capacity in humans. These studies [Vogel and Machizawa (Nature 2004 428:748) and Todd and Marois (Nature 2004 428:751)] are entirely consistent with those working memory models that have assumed that storage of information occurs in posterior cortical association areas, including the parietal cortex, while the frontal-lobe contribution is via various "top-down" executive processes on those stored representations [2]. According to such models, sustained frontal-lobe activity does not reflect storage per se, but rather, operations such as active rehearsal and strategic encoding ("chunking", for example) that are essential for maintaining a stored representation across prolonged and/or interrupted delays and when storage demands exceed capacity.
References (abridged):
1. Baddeley, A.D. (1986). Working Memory. (New York: Oxford University Press)
2. Owen, A.M., Evans, A.C. and Petrides, M. (1996). Evidence for a two-stage model of spatial working memory processing within the lateral frontal cortex: a positron emission tomography study. Cereb. Cortex 6, 31-38
3. Fuster, J.M. (1995). Memory in the Cerebral Cortex: an empirical approach to neural networks in the human and nonhuman primate brain. (Cambridge, MA: MIT press)
4. Goldman-Rakic, P.S. (1995). Architecture of the prefrontal cortex and the central executive. Ann. N.Y. Acad. Sci. 769, 71-83
5. Goldman-Rakic, P.S. (1996). The prefrontal landscape: Implications of functional architecture for understanding human mentation and the central executive. Phil. Trans. R. Soc. Lond. B. Biol. Sci. 351, 1445-1453
Current Biology http://www.current-biology.com
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NEUROSCIENCE: ON COGNITIVE MEMORY
The following points are made by Yasushi Miyashita (Science 2004 306:435):
1) Since the pioneering observations on patient H.M., who developed a severe and selective deficit in the formation of explicit (or declarative) memory after a bilateral resection of the medial temporal lobe (i.e., the hippocampus and nearby regions), subsequent studies of patients have located the source of various types of impairment in explicit memory in many brain areas (1). Notably, although patients with localized frontal lobe lesions do not have an amnesia typically observed in patients with medial temporal lobe lesions, they do exhibit impairments in memory of temporal context or temporal order, memory of the source of facts or events, or metamemory (i.e., knowledge about one's memory capabilities and about strategies that can aid memory) (2-4).
2) The identified brain-wide distributed network, called here the "cognitive memory system", is composed of three major subsystems, namely, the medial temporal lobe, the temporal cortex, and the frontal cortex. Although the ultimate storage sites for explicit memories appear to be in the cortex [but see (5) for another strong position], the medial temporal lobe plays a critical enabling role necessary for storage to take place. Domain-specific cortical regions in the temporal lobes are reactivated during remembering and contribute to the contents of a memory. The reactivation process is mediated by various signals, such as the top-down signal from the prefrontal cortex or the backward signal from the limbic cortex. Frontal regions mediate the strategic attempts for retrieval and encoding and also monitor its outcome, with the dissociated frontal regions making functionally separate contributions.
3) This large-scale cognitive network was initially identified in humans by using neuropsychology and functional imaging. However, molecular, cellular, and network components of this cognitive system have been systematically dissected by recent technical advancements, particularly in animal studies. These include cell type-restricted gene manipulations in mice, a combination of molecular biology and single-unit recording in monkeys, and a sophisticated scan design of event-related functional magnetic resonance imaging (fMRI) in humans.
4) In summary:
a) A brain-wide distributed network orchestrates cognitive memorizing and remembering of explicit memory (i.e., memory of facts and events). The network was initially identified in humans and is being systematically investigated in molecular/genetic, single-unit, lesion, and imaging studies in animals.
b) The types of memory identified in humans are extended into animals as episodic-like (event) memory or semantic-like (fact) memory. The unique configurational association between environmental stimuli and behavioral context, which is likely the basis of episodic-like memory, depends on neural circuits in the medial temporal lobe, whereas memory traces representing repeated associations, which is likely the basis of semantic-like memory, are consolidated in the domain-specific regions in the temporal cortex. These regions are reactivated during remembering and contribute to the contents of a memory.
c) Two types of retrieval signal reach the cortical representations. One runs from the frontal cortex for active (or effortful) retrieval (top-down signal), and the other spreads backward from the medial temporal lobe for automatic retrieval. By sending the top-down signal to the temporal cortex, frontal regions manipulate and organize to-be-remembered information, devise strategies for retrieval, and also monitor the outcome, with dissociated frontal regions making functionally separate contributions.
d) The challenge is to understand the hierarchical interactions between these multiple cortical areas, not only with a correlational analysis but also with an interventional study demonstrating the causal necessity and the direction of the causality.
References (abridged):
1. L. R. Squire, D. L. Schacter, Neuropsychology of Memory (Guilford, New York, ed. 3, 2002)
2. D. T. Stuss, D. F. Benson, The Frontal Lobes (Raven, New York, 1986)
3. J. M. Fuster, The Prefrontal Cortex: Anatomy, Physiology, and Neuropsychology of the Frontal Lobe (Lippincott-Raven, Philadelphia, 1997).
4. M. Petrides, in Handbook of Neuropsychology, F. Boller, J. Grafman, Eds. (Elsevier, Amsterdam, 2000).
5. J. O'Keefe, L. Nadel, The Hippocampus as a Cognitive Map (Oxford Univ. Press, Oxford, 1978).
Science http://www.sciencemag.org
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ON THE NEUROBIOLOGY OF LEARNING AND MEMORY
The following points are made by H. Okano et al (Proc. Nat. Acad. Sci. 2000 97:12403):
1) The authors state they define memory as a behavioral change caused by an experience, and they define learning as a process for acquiring memory. According to these definitions, there are different kinds of memory. Some memories, such as those concerning events and facts, are available to our consciousness; this type of memory is called "declarative memory". However, another type of memory, called "procedural memory", is not available to consciousness. This is the memory that is needed, for example, to use a previously learned skill. We can improve our skills through practice: with training, the ability to play tennis, for example, will improve. Declarative memory and procedural memory are independent: there are patients with impaired declarative memory whose procedural memory is completely normal. Because of this fact, current researchers believe there must be separate mechanisms for each type of memory, and that these separate mechanisms probably also require separate brain areas as well.
2) The *cerebrum and *hippocampus are considered important for declarative memory, and the *cerebellum is considered important for procedural memory. The current belief is that memory requires alterations in the brain. The most popular candidate site for memory storage is the *synapse, where nerve cells communicate with each other. A change in the transmission efficacy at the synapse (called "synaptic plasticity") has been considered to be the cause of memory, and a particular pattern of synaptic usage or stimulation (conditioning stimulation) is believed to induce synaptic plasticity. Many questions remain to be answered, such as how synaptic plasticity is induced and how synaptic plasticity is implicated in learning and memory.
3) One current frontier in the study of synaptic plasticity is the attempt to clarify the role of plasticity in learning and memory. The strategy has been to examine the correlation between synaptic plasticity and learning by inhibiting the plasticity in a living animal. To do this, investigators have used inhibitors for certain molecules that are apparently required for synaptic plasticity. Another set of useful tools involves genetically engineered mutant mice, such as "knockout" and transgenic mice. A "knockout" mouse is a mutant mouse that is deficient in a specific native molecule. By using mutant mice, the relationship between synaptic plasticity and learning ability has been examined in detail.
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes by ScienceWeek:
cerebrum: What is called the "cerebrum" is the bulk of brain as seen by the naked eye, the "great ravelled knot" that sits on top of the phylogenetically older parts (brainstem and midbrain) of the whole brain. The surface of the cerebrum, an enormously extended surface because of the many deep folds of the cerebrum, is a thin sheet called the "cerebral cortex" (cortex = rind or bark).
hippocampus: A region of the cerebral cortex in the *medial part of the temporal lobe. In humans, among other functions, the hippocampus is apparently involved in short-term memory, and analysis of the neurological correlates of learning behavior in animals indicates that the hippocampus is also involved in memory in other species.
cerebellum: The human cerebellum is about the size of a large apple, is placed at the lower back of the head under the optic lobes of the cerebrum, and is apparently involved in the input-output control of automatic sensorimotor functions. If you are sitting at your breakfast table, holding a newspaper in one hand, and using the other hand to routinely and repetitively dip a spoon into cold cereal and bring the cold cereal to your mouth while you read the newspaper, it is the cerebellum which is governing the automatic feeding movements while your cerebral cortex processes the information that you read.
synapse: In general, nerve cells have a single long extension (the "axon") that propagates the electrical output (the action potential) of the cell. The term "synapse" refers to the junction between the terminal of a neuron's axon and another neuron. When studying the synapse, the first neuron is called the "presynaptic" neuron, and the second neuron is called the "postsynaptic" neuron.
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