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         MindNet Journal - Vol. 1, No. 62b * [Part 2 of 4 parts]
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         V E R I C O M M / MindNet         "Quid veritas est?"
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    The views and opinions expressed below are not necessarily the
    views and opinions of VERICOMM, MindNet, or the editors unless
    otherwise noted.

    The following is reproduced here with the express permission of
    the author.

    Permission is given to reproduce and redistribute, for
    non-commercial purposes only, provided this information and the
    copy remain intact and unedited.

    Editor: Mike Coyle

    Assistant Editor: Rick Lawler

    Research: Darrell Bross

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    [Continued from part 1]

    III. COMPONENTS OF MEMORY PROCESSES: THE WORKING MEMORY AND
    LONG-TERM MEMORY SYSTEM

    19. Two basic aspects of memory processes are emphasized. The one
    refers to a close interaction between the working memory system
    (WMS, see, e.g., Baddeley, 1992) with the long-term memory system
    (LTMS). This interaction plays an important role for encoding,
    searching, retrieving, and recognizing information. The other
    aspect refers to the meaning of monitoring processes which are
    considered a heterogeneous class of processes within the WMS that
    operate under voluntary control. The close interaction between
    the WMS and LTMS can be demonstrated by considering a fundamental
    cognitive process such as recognizing a familiar object. The
    crucial idea here is that after a sensory code is established,
    bottom up processes access semantic information in LTM that is
    used to identify the perceived object. If the process of
    identification which is considered a matching processes yields a
    positive result, the object is recognized which in turn leads to
    the creation of a STM code. The matching process activates
    pathways that are similar or identical to those that serve to
    retrieve information from LTM. Top down processes which are
    guided by expectancy and selective attention are capable of
    directing the matching process towards a certain outcome by
    preactivating or preselecting appropriate templates or prototypes
    in LTM. Recent models, such as Grossberg's adaptive resonance
    theory (ART), also proceed from the basic assumption that
    templates or prototypes stored in LTM are activated during a
    matching process which is characterized by a close interaction
    between STM and LTM (e.g., Grossberg, 1980; Grossberg & Stone,
    1986; Carpenter & Grossberg, 1993).

    20. Note that encoding has two different meanings. The encoding
    of sensory information (as a process of recognition) aims at the
    semantic understanding of perceived information. LTM holds that
    information which is essential for this encoding process. Within
    the framework of STM, encoding means the creation of a new code
    that primarily comprises episodic information. According to
    Tulving (e.g., Tulving, 1984), episodic information is that type
    of contextual information which keeps an individual
    autobiographically oriented within space and time.

    III.1 MONITORING PROCESSES, EPISODIC INFORMATION AND THE
    HIPPOCAMPUS

    21. The necessity for monitoring processes is related to the
    permanent need to update episodic information. An episodic code
    which is created through the action of monitoring processes
    reflects primarily subjective information, such as context,
    expectancy, emotion, and certain autobiographic aspects. Because
    time changes the context permanently, there is a permanent need
    to update and store episodic information. STM serves this vital
    need to store episodic information within certain capacity
    limits. Beyond these limits episodic information may be stored
    into a more permanent memory system. In contrast to episodic
    information, the encoding of new semantic information requires in
    most cases special mnemo-techniques ("learning"). Because
    "learning" is guided by complex monitoring processes of the WMS,
    it is assumed that new semantic and episodic information use
    identical encoding pathways into LTM.

    22. As the classical case of patient H.M. (Scoville & Milner,
    1957; and the reviews in, e.g., Markowitsch, 1983, 1984, and
    Markowitsch & Pritzel, 1985, for similar cases) as well as a
    variety of more recent evidence demonstrates, the hippocampus
    (and other parts of the limbic system) are responsible for
    encoding (or retrieving) any new declarative information beyond
    the capacity limits of STM (Squire, 1992; Squire, Knowlton &
    Musen, 1993). Because of the permanent need to update episodic
    (but not semantic) information, the loss of freshly encoded
    episodic memory is such a dramatic symptom for anterograde
    amnesia that the concurrent failure to encode new semantic
    information appeared to be of minor significance (c.f. Baddeley's
    commentary to Tulving's target article in BBS for a similar
    statement; Baddeley, 1984, p. 239). The importance of the
    hippocampus for contextual encoding was proposed by several
    researchers. As an example, Teyler and DiScenna (1986) assume
    that the hippocampus stores at least initially some sort of
    "index" pointing towards those neocortical modules or cell
    assemblies (i.e., those LTM structures) that have been activated.
    In an interesting theory, Miller (1991) proposes that the
    hippocampus is important for contextual representations and might
    be involved in forming global cell assemblies. Squire (1992)
    emphasizes the "binding" function of the hippocampus and
    emphasizes that it is needed to bind together distributed cell
    assemblies (representing features) that together form the
    information of a single code. If the hippocampus is involved in
    the process of consolidation, contextual encoding and binding, we
    have to expect that extensive and widespread projections exist to
    the association cortices. This is indeed the case (e.g., Lopes da
    Silva, Witter, Boeijinga & Lohman, 1990).

    23. These conceptions of hippocampal functions fit very well with
    the hypothesis (proposed in section V, paragraph 42, see also
    paragraph 17) that the selective type 2 synchronization of a
    small percentage of hippocampo-cortical feedback loops actually
    reflects the encoding of new (episodic) information. The
    synchronization of selected, distributed cortical cell assemblies
    might allow for establishing a binding process that links
    features in a new way to create a new context. Carpenter and
    Grossberg (1993) add another interesting aspect to this view of
    hippocampal functions. They assume that the hippocampus
    represents something like an orienting system that allows one to
    orient towards the encoding of a new stimulus. It is important to
    note that all of the functions ascribed to the hippocampus are
    central functions of the WMS.

    III.2 SEARCHING LTM CODES AS A CORTICAL PROCESS OF SPREADING
    ACTIVATION

    24. It is assumed that LTM codes are represented by a distributed
    structure of nodes that establish a complex network or cell
    assembly in the neocortex. Even a single node is considered a
    structure of features that may be widely distributed throughout
    different regions of the neocortex. Features may be represented
    by smaller cell assemblies such as cortical columns or modules
    which serve as feature detectors when activated by perceptual
    processes (c.f. the close interaction between LTM and perception
    emphasized in paragraph 19). The assumption of highly distributed
    codes may explain why any attempt to localize a particular engram
    resulted in a failure (c.f. Lashley, 1950). This structural
    encoding assumption leads to the crucial question of how the
    different features belonging to a single code can be activated
    together as a functional unity without activating features of
    other overlapping but irrelevant codes. According to the
    traditional view as first proposed by Hebb (e.g., Hebb, 1949),
    one may assume that the features of a code are represented by a
    cell assembly of interconnected cells that are functionally
    characterized by a concurrent elevation of their average firing
    rate. Unlike more recent models, Hebb's conception has the
    disadvantage that in a particular cortical region and within a
    given time span, only a single code or feature can be activated,
    because the enhanced firing rate is the only cue which allows it
    to distinguish the relevant code from irrelevant information.
    During a search process in LTM, a huge variety of codes will be
    activated at the same time and possibly in the same brain region.
    Thus, different and topographically overlapping cell assemblies
    will be activated at the same time. Consequently, it will be
    impossible to distinguish between different codes. In trying to
    avoid this problem, one may instead assume that assemblies may be
    functionally defined by a state of synchronous firing of cortical
    neurons, rather than by an enhanced average firing rate. This
    means that in a particular cortical region and within a given
    time span, all of the cells may be highly active, but only those
    cells firing synchronously represent the relevant information
    comprised by a single code.

    25. Gray and Singer (1987) together with other researchers at the
    Frankfurt MPI (e.g., Gray, Koenig, Engel & Singer, 1989; Engel,
    Koenig, Kreiter, Schillen & Singer, 1992) have provided
    convincing evidence that a visual code, established through a
    perceptual process, can be described as a cell assembly which
    responds with a synchronous oscillatory discharge pattern within
    a broad frequency range of about 30 to 70 Hz which is termed
    gamma band. They assume that the synchronous oscillatory firing
    pattern of distributed cortical cells reflects a stage of
    cortical integration in the sense that the information provided
    by different feature detectors is integrated into a single visual
    code. This assumption is substantiated by the important finding
    that even widely distributed but synchronously oscillating cell
    assemblies fire with zero phase lag. Feedback loops, connecting
    different cell groups of the cortex, obviously are the means
    which enable this surprising ability. Oscillations are considered
    carrier signals for the relevant information which might be
    encoded by the synchronous modulation of frequencies.

    26. When applying the encoding principle described above to
    search processes in LTM, the interesting conclusion is that the
    search process would have to find cell assemblies that are
    capable of establishing a synchronous oscillatory firing pattern
    in response to the initiation of a search process. According to
    this concept, the relevant sought-after information would be
    characterized by a synchronous oscillatory discharge pattern.
    However, unlike a visual encoding process, where all the relevant
    information is given at the same time, during the course of a
    search process thousands of different codes will be activated at
    different times. Each code may respond with a synchronous
    oscillatory discharge pattern, but what should be the criterion
    to distinguish the sought-after relevant from irrelevant
    information? Establishing a synchronous oscillatory firing
    pattern along the search pathway would not allow the search
    process to selectively retrieve the relevant information.

    27. This question of how a search process finds the relevant
    information is called the search problem. It will be explained in
    terms of a spreading activation process that was described within
    the framework of the connectivity model, outlined in detail
    elsewhere (Klimesch, 1994). This model describes spreading
    activation in abstract terms of different activation values
    moving from one node to another. In the context of cortical
    activation the term "activation value" is translated into
    "frequency of an oscillatory neuronal discharge pattern". As an
    example, let us consider a completely interconnected code (with n
    = 5 nodes), in which each node is connected to each of the other
    nodes. At the beginning of an activation process each node
    representing a cell assembly (or cortical module) has zero
    activity which means that it oscillates with some low (resting)
    frequency. Activating a node means to put it in a state of
    oscillation with a frequency that is higher than its resting
    frequency. Now, if activation starts at one of the n = 5 nodes
    with frequency f, this activation spreads to all of the other n -
    1 nodes of that code. Accordingly nodes 2, 3, 4 and 5 are also
    put in oscillation with frequency f. Now in a second activation
    stage, the n - 1 nodes activate each other. Thus, each node
    receives activation from the remaining n - 2 nodes. With each
    additional activation, the n - 1 nodes increase their
    responsiveness which means that they increase their frequency
    proportional to the number of times they were activated. Note
    that all of the n - 1 nodes are completely interconnected and are
    thus n - 2 times activated which results in an increase in
    frequency from f to f'. In a third step, the increased frequency
    f' is fed back to that node where the activation process was
    initiated. Note that the increase from f to f' reflects the
    complexity of a code. The more nodes there are, the higher
    frequency f' and consequently, the faster the spreading
    activation process will be. Furthermore and most important, due
    to the interconnections between the n - 1 nodes, which are
    considered the features of a code, a synchronous oscillatory
    discharge pattern is established within all of the components of
    a code. Thus, in accordance with the findings of Gray & Singer
    (1987) which are partly summarized in Engel et al. (1992), a
    memory code can be characterized by a pattern of features
    oscillating synchronously. However, during the spreading
    activation process each code is activated at different times and,
    even more important, each code will respond with a different
    frequency, because frequency f' depends on geometric properties
    which differ between codes.

    28. According to the connectivity model, a search process
    terminates with a positive result if activation (i.e., some
    frequency f' which must be higher then input frequency f) spreads
    back as "echo" to one of those nodes where the search was
    initiated. Monitoring processes of the WMS do not guide the
    spreading activation process which follows automatically by local
    mechanisms. Their task is to select access points, when
    initiating a search process, and to retrieve the relevant
    information if a search process terminates with a positive
    result. The result of a search process can be judged by the
    strength of activation equaling frequency f' of activated codes.
    That code responding with the highest frequency represents the
    relevant information to be retrieved.

    29. It is important to remember the two different types of
    synchronization, outlined in section II.3. Synchronous (type 1)
    oscillations of large cell populations are considered to reflect
    a state of inhibition. As an example, at the beginning of a
    search process, large cortical areas may oscillate synchronously
    with resting frequency f. Because information is encoded by the
    modulation of frequencies, large cell populations oscillating
    with the same (low resting) frequency (within a narrow band) are
    not capable of transmitting information. However, as a result of
    a search (or spreading activation) process, different cortical
    cell assemblies change their frequency and establish a type 2
    synchronous oscillatory discharge pattern over a broad frequency
    range which may comprise the entire frequency band (comprising
    the theta, alpha, beta and gamma bands). This modulation or
    change in frequency reflects the process of transmitting
    information but is restricted to those cortical areas that are
    relevant for processing a task.

    30. With respect to the most dominant rhythm in the EEG, a
    spreading activation process of the type explained above should
    be reflected by a desynchronization of the alpha rhythm. And,
    indeed, as a variety of studies have shown, alpha
    desynchronization indicates a state of cortical activation (for
    recent reviews see, e.g., Pfurtscheller, 1992; Pfurtscheller &
    Klimesch, 1992; Pfurtscheller & Klimesch, 1991), whereas alpha
    synchronization reflects a state of cortical inactivity or
    "idling" (Pfurtscheller, 1992). Thus, alpha desynchronization
    might very well reflect the actual encoding process which is
    based on a frequency modulation in the relevant cell assemblies.
    The fact that during desynchronization a complex pattern of
    changes in EEG coherences can be found, as the interesting work
    of Petsche and his group demonstrates (e.g., Petsche and
    Rappelsberger, 1992) is well in line with the proposed
    interpretation.

    III.3 RETRIEVING LTM CODES AND THE POSSIBLE ROLE OF
    THALAMO-CORTICAL FEEDBACK LOOPS

    31. In the identification of the relevant nodes or codes,
    feedback loops may play a decisive role. During the course of a
    search process, the activation status of the searched network is
    constantly transmitted back by means of feedback loops to a
    control system. The basic idea is that a control network
    converging in a particular control system is mapped onto the
    storage network. Consequently, the control system should be
    connected with the cortex by a dense network of axonal
    connections. Besides the basal ganglia, the thalamus with its
    thalamo-cortical projections to virtually all different cortical
    regions (e.g., Hoehl-Abrahao & Creutzfeldt, 1991) is one of those
    brain structures that fulfills this requirement. It is important
    to see that as compared to the thalamo-cortical network, the
    cortical network is orders of magnitude denser. Thus, each
    feedback loop serves a relatively large cortical field which also
    will be termed "alpha field".

    32. To initiate a LTM search requires some vague information of
    where in the storage network to look for the relevant
    information. Because it would be highly inefficient to search the
    entire network once a search process is initiated, it is
    necessary to delimit the search area. In a theoretical sense,
    retrieval cues that give a rough description or some details of
    the relevant information can serve this purpose. In an anatomical
    sense, the thalamus might be a good candidate for delimiting the
    search area in the neocortex because the thalamo-cortical network
    may allow direct access to certain parts of the neocortex. Based
    on the current context of the WMS, retrieval cues are provided
    that enable the thalamus to activate particular thalamo-cortical
    pathways which start the search process in the neocortex.
    Thereby, specific and unspecific thalamic projections might
    provide access to specific sensoric or more abstract information,
    respectively. However, it should also be noted that in contrast
    to the traditional belief, the thalamus shows a much more complex
    pattern of different types of projections (e.g., Steriade et al.,
    1990, p. 40).

    33. The thalamus and hippocampus probably are involved in quite
    different functions. Whereas the thalamus might serve as a relay
    station for searching and retrieving pure LTM information, the
    hippocampus (as well as other parts of the limbic system) might
    be important for the encoding and retrieval of concomitant
    episodic information. Note, however, that any search and
    retrieval process is embedded within a particular autobiographic
    context which defines the particular episodic meaning of that
    information which is retrieved from LTM. Thus, it must be
    expected that the functions of the hippocampus and the thalamus
    are closely interrelated.

    [Continued to part 3]
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