[Paleopsych] SW: On Cognitive Memory

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Neuroscience: On Cognitive Memory
http://scienceweek.com/2004/sb041126-3.htm

    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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    Related Material:
    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
    --------------------------------
    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.
    --------------------------------
    Related Material:
    NEUROBIOLOGY: ON THE BIOLOGICAL BASIS OF MEMORY
    Notes by ScienceWeek
    Exactly 100 years ago, two psychologists, G.E. Mueller and A.
    Pilzecker, proposed what came to be called the
    perseveration-consolidation hypothesis of memory. In studies with
    human subjects, Mueller and Pilzecker found that memory of newly
    learned information was disrupted by the learning of other information
    shortly after the original learning, and they suggested that processes
    underlying new memories initially persist in a fragile state and then
    consolidate over time. This consolidation hypothesis still guides
    research, particularly research in neurobiology on the time-dependent
    involvement of neural systems and cellular processes enabling lasting
    memory.
    At the present time, the concept of "synaptic plasticity" underlies
    nearly all theories of memories, the term referring to changes in the
    behavior of the junction (synapse) between two nerve cells resulting
    from past history.
    Two prominent aspects of synaptic plasticity considered to be related
    to memory are "facilitation" and "potentiation". The term
    "facilitation" refers to a progressive increase in the amount of
    *neurotransmitter substance released at a synapse by successive nerve
    impulses (action potentials), the increase occurring during an input
    barrage consisting of repetitive stimulation (stimulus train). The
    term "potentiation" refers to an increase in neurotransmitter
    substance released by an action potential following repetitive
    stimulation of a synapse.
    Both facilitation and potentiation can be long-lasting, and "long-term
    potentiation" has been a focus of much research on the cellular basis
    of memory, particularly in the hippocampus, a brain cortex structure
    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 the rat indicates that the hippocampus of the rat is also
    involved in memory.
    The following points are made by James L. McGaugh (Science 2000
    287:248):
    1) The author points out that the idea that synaptic mechanisms of
    long-term potentiation and long-term facilitation underlie memory
    remains a hypothesis.
    2) The author points out that although studies of long-term
    potentiation and memory have focused on the involvement of the
    hippocampus, much evidence indicates that the hippocampus has only a
    time-limited role in the consolidation and/or stabilization of lasting
    memory.
    3) The author points out that there are forms of memory that
    apparently do not involve the hippocampus and that may not use any
    known mechanisms of synaptic plasticity.
    4) The author points out that despite theoretical conjectures, little
    is known about system and cellular processes mediating consolidation
    that continues for several hours or longer after learning,
    consolidation that creates lifelong memories.
    Concerning the above caveats, the author concludes: "These issues
    remain to be addressed in this new century of research on memory
    consolidation."
    Science http://www.sciencemag.org
    --------------------------------
    Notes by ScienceWeek:
    neurotransmitter substance: Neurotransmitters are chemical substances
    released at the terminals of nerve axons in response to the
    propagation of an impulse to the end of that axon. The
    neurotransmitter substance diffuses into the synapse, the junction
    between the presynaptic nerve ending and the postsynaptic neuron, and
    at the membrane of the postsynaptic neuron the transmitter substance
    interacts with a receptor. Depending on the type of receptor, the
    result may be an excitatory or an inhibitory effect on the
    postsynaptic nerve cell.