### MindNet is no longer active.

#### Back to MindNet Index

    ================================================================
         MindNet Journal - Vol. 1, No. 62c * [Part 3 of 4 parts]
    ================================================================
         V E R I C O M M / MindNet         "Quid veritas est?"
    ================================================================

    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

    ================================================================
    [Continued from part 2]

    IV. ALPHA FREQUENCY AND SEMANTIC MEMORY PROCESSES

    34. Some researchers have suggested that the EEG frequency within
    the alpha band (of about 8-13 Hz) stems from the thalamus and
    induces synchronized neuronal activity in the cortex (Andersen &
    Andersson, 1968). In terms of EEG frequencies, the ideas
    described so far can be summarized as follows. A search processes
    in LTM starts if thalamo-cortical pathways are selectively
    activated, which means that a particular subset of these pathways
    shift their resting frequency within the alpha band. This shift
    in frequency serves as input frequency f to the cortical storage
    network where the search process spreads between different access
    points. In response to the search process, different cell
    assemblies start to oscillate with different frequencies f'
    within the alpha, beta and gamma bands. The status of the search
    process is constantly fed back by each activated cortical field
    (alpha field) that is served by a thalamo-cortical feedback loop
    oscillating within the range of alpha frequency. The higher
    cortical frequency f' is, the stronger is the concomitant
    increase in alpha frequency. The relevant information can be read
    out via those feedback loops responding with the highest
    frequency. Because we have assumed that alpha oscillations
    reflect the information processing in thalamocortical feedback
    loops, whereas gamma oscillations reflect pure cortical
    processing, a specific transition or interface between alpha and
    gamma oscillations must be postulated. The following two facts
    support this idea. First, Steriade et al. (1990, p. 147) report
    that the gamma rhythm (the 40 Hz rhythm in particular) is driven
    by neurons located in that cortical layer, which receives
    thalamic afferents. They conclude that thalamic input to the
    cortex serves as a trigger for rhythmic activation of specific
    cortical columns. Second, Pfurtscheller et al. (1994) observed a
    reciprocal relationship between alpha and the 40 Hz rhythm: If
    the 40 Hz rhythm synchronizes, alpha desynchronizes and vice
    versa.

    IV.1 EEG ALPHA FREQUENCY REFLECTS SEMANTIC AND THETA EPISODIC
    MEMORY PROCESSES

    35. If we proceed from the idea that memory codes are retrieved
    via longitudinal pathways linking thalamic nuclei with the
    cortex, and that alpha is the predominant rhythm reflecting the
    activity of these pathways, we arrive at the hypothesis that
    alpha frequency should be related to memory performance (see
    section I). We have tested this hypothesis (Klimesch, Schimke,
    Ladurner & Pfurtscheller, 1990; Klimesch, Schimke &
    Pfurtscheller, 1993) and found that - as compared to bad memory
    performers - good performers show a significantly higher alpha
    frequency. This result was found even in a resting state where
    subjects relaxed with eyes closed, but was most pronounced during
    actual retrieval attempts (Klimesch et al., 1990, Experiments 1
    and 2). Furthermore, it could be demonstrated that attentional
    demands or interindividually different responses to increasing
    memory load (as an indicator of STM-span) are not responsible for
    the higher alpha frequency of good memory performers (Klimesch et
    al., 1993).

    36. In a recently performed study, Klimesch, Schimke and
    Schwaiger (1994) were able to demonstrate that alpha power
    selectively responds to semantic LTM demands, whereas theta power
    selectively responds to episodic STM demands. In this study, an
    experimental design was used, that already proved useful in
    distinguishing semantic LTM from episodic STM (Kroll & Klimesch,
    1992). The experimental design consisted of two parts. Subjects
    first performed a semantic congruency task in which they had to
    judge whether or not the sequentially presented words of
    concept-feature pairs (such as "eagle-claws" or "pea-huge") were
    semantically congruent. Then, without prior warning, they were
    asked to perform an episodic recognition task. This was done in
    an attempt to prevent subjects from using semantic encoding
    strategies and thus to increase episodic memory demands. In the
    episodic task, the same concept-feature pairs were used as
    targets and were presented together with distractor pairs
    (generated by repairing known concept-feature pairs). Now
    subjects had to judge whether or not a particular concept-feature
    pair was already presented during the semantic task. The results
    of Kroll and Klimesch (1992) indicated that semantic features
    speeded up semantic, but slowed down episodic decision times (for
    an extensive review on this topic see also Klimesch, 1994). With
    respect to the purpose of the present study, this result
    indicates that semantic and episodic memory processes can
    effectively be differentiated by using the design underlying
    Experiment 4 in Kroll and Klimesch (1992). According to the
    proposed hypothesis, it is expected that only in the semantic
    task should the most pronounced desynchronization (decrease in
    alpha band power) be observed in the alpha band. Because pairs of
    items are presented and because a subject can only perform the
    episodic and semantic task after the second item of a pair (i.e.,
    the feature) is presented, a decrease in alpha band power as a
    response to increasing semantic task demands is expected only for
    the time period following the presentation of the feature. It is
    important to keep in mind that alpha power is well known to
    decrease (desynchronize) with increasing task difficulty and
    attentional demands. From the results found in Kroll and Klimesch
    (1992) we know that the episodic task is much more difficult than
    the semantic task. Thus, if task difficulty would be the only
    factor which is reflected by a decrease in alpha power, we would
    expect the most pronounced response to be observed during the
    presentation of the feature in the episodic task. This, however,
    was not the case. In support of our hypothesis it was found that
    alpha desynchronizes during the presentation of the feature in
    the semantic task. In the episodic task, on the other hand, theta
    power increased (synchronized) during the processing of the
    feature.

    V. THETA FREQUENCY AND EPISODIC MEMORY PROCESSES

    37. Since Scoville and Milner (1957) reported a severe
    anterograde amnesia for patient H.M. who had undergone a
    bilateral temporal lobectomy, including the hippocampal
    formation, and since Green and Arduini (1954) have found a
    dominant rhythmic electrical activity within the theta band in
    the hippocampus of rats, it has become obvious that theta
    activity of the hippocampus might be related to the encoding
    and/or retrieval of new information. Positive evidence came from
    studies which have documented that there is a preference for
    long-term potentiation (LTP) to occur in the hippocampal
    formation, and that theta activity induces or at least enhances
    LTP (e.g., Larson, Wong & Lynch, 1986; and Greenstein, Pavlides &
    Winson, 1988). The fact that LTP is considered the most important
    electrophysiological correlate for encoding new information,
    underlines the potential importance of hippocampal theta for
    memory processes in the WMS.

    38. In trying to explain the possible functional significance of
    the theta rhythm in the human EEG, we assume that synchronized
    bursts of a small set of hippocampal pyramidal cells induce theta
    activity in selected but distributed cortical regions which are
    relevant for performing a particular task. Empirical findings
    support this view and indicate that theta band power increases
    with increasing (episodic) task demands (Klimesch, Schimke &
    Schwaiger, 1994). Research in animals also indicates that during
    behavioral activity, theta power increases (e.g., the review in
    Lopes da Silva, 1992).

    39. One of the first questions that may arise when considering
    the proposed hypothesis is, why - in contrast to animals - theta
    is not a dominant rhythm in the human EEG. In an attempt to
    answer this question we first proceed from a theoretical
    consideration that is similar to the mechanisms that were
    proposed for accessing and retrieving LTM codes. It is assumed
    that hippocampo-cortical feedback loops induce synchronized
    rhythmic theta activity onto different regions of the neocortex
    where (e.g., by means of LTP) new information is encoded or
    freshly encoded information is retrieved. Given the basic
    assumption that new information always will be "added" or
    "attached" to related but already encoded information, only a
    small subset of the hippocampo-cortical feedback loops which are
    related to relevant cortical areas will be needed and thus will
    actually show synchronized theta activity. Because the human
    cortex is much larger than those in lower mammals and, as a
    consequence, holds much more LTM-information, the encoding of new
    information is a much more distributed process than in animals.
    Thus, if the percentage of synchronized hippocampo-cortical
    feedback loops is related to the size of the cortex (and to the
    hippocampus too, which is relatively much smaller in humans),
    this percentage will be orders of magnitudes smaller for humans
    as compared to animals.

    40. Evidence for the view that only a small percentage of
    hippocampo-cortical feedback loops is synchronized comes from a
    re-examination of the pacemaker role of the septum in the
    production of the hippocampal theta rhythm (Petsche, Stumpf &
    Gogolak, 1962; Stewart & Fox, 1990). In addition to cholinergic
    projections, a large fraction of the septo-hippocampal
    projections terminate on inhibitory (GABA-ergic) hippocampal
    interneurons (Freund & Antal, 1988; see also the reviews in Lopes
    da Silva, 1992; and Stewart & Fox, 1990). Based on these and
    related findings, Stewart and Fox (1990) assume that the septal
    input might organize the hippocampal theta activity via rhythmic
    inhibition of hippocampal interneurons. This view is in agreement
    with the fact that hippocampal interneurons are more likely to
    behave as theta cells (Fox & Ranck, 1981) than burst firing
    pyramidal neurons. In agreement with this fact, a simulation
    model (Traub, Miles & Wong, 1989) reveals that in contrast to
    interneurons, only a small percentage of the pyramidal cells
    display synchrony.

    41. With respect to the question, whether theta activity can be
    observed in the EEG, these findings which were obtained from
    microelectrodes in the hippocampus are of outstanding importance.
    Biophysically, theta frequency in the hippocampus, deep inside
    the brain, would be difficult to detect from scalp electrodes.
    The crucial condition to detect theta as a dominant rhythm in the
    EEG would be that most of the burst firing hippocampal pyramidal
    cells that project to other parts of the cortex would fire in
    synchrony. However, as we have already noted, according to Leung
    (1980), Traub et al. (1989) and Lopes da Silva (1992), this is
    not the case. And indeed, as judged by visual inspection but in
    contrast to spectral analysis, theta activity usually is absent
    in the EEG of normal, wake adults.

    42. The fact that only a small percentage of the pyramidal
    neurons displays synchrony, agrees with the idea that
    hippocampo-cortical feedback loops induce synchronous theta
    activity into selected cortical areas where new information is
    encoded or fresh information is retrieved. This is type 2 or
    selective synchronization that means activation. Given the fact
    that theta frequency induces or at least enhances LTP (see also
    Lopes da Silva, 1992), it seems tempting to assume that theta
    activity, induced into selected cortical areas, reflects a
    process to encode or retrieve new information by keeping or
    putting selected cortical areas into a state of resonance. This
    assumption comes very close to a theory of resonant phase-locked
    hippocampo-cortical loops, proposed by Miller (1991).

    VI. THETA FREQUENCY AND EVENT RELATED POTENTIALS

    43. A possible objection against our central hypothesis that
    brain oscillations are the basic phenomenon of cortical
    information processing may come from researchers using event
    related potentials (ERPs) to study cognitive and memory processes
    in particular. They may argue that late components of ERPs
    reflect completely different types of cortical processes such as,
    for example, time locked threshold changes that regulate the
    degree of excitability in neuronal networks (e.g., Birbaumer &
    Elbert, 1988; Elbert, 1992). Given the fact that almost all of
    the memory studies in cognitive electrophysiology focus on late
    components of ERPs, this objection could seriously threaten the
    general validity of our hypothesis.

    44. However, it may also be argued that late components of ERPs
    are the result of synchronous oscillations that are transiently
    phase locked in response to a relevant event or stimulus. Summed
    up over several trials, a waveform (the ERP) would be generated
    that shows the typical succession of positive and negative
    "peaks" (or ERP components). If we proceed from this idea, it
    becomes evident that only (or at least primarily) those types of
    oscillations would be capable of generating late ERP components
    that indeed respond with coupled type 2 synchronization to an
    increase in respective task demands. Note that alpha and beta
    tend to desynchronize with increasing task demands.
    Hypothetically, there are only two possible candidates (see
    section II.3; paragraph 15): theta and gamma frequency. Because
    gamma frequency is much too high and its amplitudes much too
    small to generate a typical ERP, theta frequency remains the most
    plausible candidate.

    45. This proposal, of course, does not mean that theta is the
    only generator for ERPs. In addition, there may be other
    processes such as very slow oscillations in the delta band (below
    4 Hz) and/or threshold changes in large parts of the cortical
    network that also may have a strong influence on the waveform of
    ERPs. Weak influences may even be due to type 2 synchronizations
    within the alpha band.

    46. The most obvious ERP component that might reflect phase
    locked evoked theta activity is the P300 for the following two
    reasons: First, because of the (typical) latency and form of the
    P300, this ERP component shows the most significant power in the
    theta band as frequency decompositions indicate (Basar &
    Stampfer, 1985). Second, the (typical) functional meaning of the
    P300, particularly the process of "updating" (Donchin & Coles,
    1988), is well related to central functions of the WMS.

    47. Now, let us consider the most speculative part of our
    proposal which links phase locked theta activity to the P300
    component of event related potentials (ERPs). Keeping in mind
    that a single cycle of the theta rhythm consists of an inhibitory
    and a disinhibited or excitatory phase and that only in the
    latter, bursts of action potentials are sent to selected cortical
    areas, the question arises: With which phase of the cycle does
    task related (episodic) processing start? Does it start with the
    inhibitory or the excitatory phase? In referring to the argument
    that theta is induced in selected parts of the cortical network,
    it seems plausible to assume that episodic processing starts with
    the inhibitory phase in order to maximize the impact of the
    distributed activation of the relevant parts of the network by
    reducing irrelevant background activity through the inhibitory
    phase. As a result of this assumption, and because only a small
    percentage of the burst firing pyramidal cells are synchronized
    through the excitatory phase, the outcome should be a positive
    going deflection in the EEG, time locked to the presentation of
    an adequate stimulus.

    48. If this is true, it should be possible to record evoked theta
    activity from the scalp in response to an appropriate stimulus or
    event. Based on theoretical considerations and experimental
    evidence (e.g., Leung, 1980), Lopes da Silva (1992, p. 93)
    concludes that appropriate stimuli or events induce evoked
    responses that depend on the phase of theta frequency. Therefore,
    evoked theta activity may be viewed as synchronized phase locked
    and thus amplified theta frequency which occurs in response to an
    appropriate event. The issue of interest is whether evoked or
    event-related theta activity can be detected as a response to
    increased episodic memory demands.

    49. We have emphasized that one of the most important functions
    of the WMS that we have related to hippocampal information
    processing is the encoding of contextual or episodic information.
    Thus, if the P300 really stems from phase-locked hippocampal
    theta activity, the (typical) functional meaning of the P300
    should be related to the encoding of contextual and the encoding
    of new information.

    50. There is some evidence for this view. Donchin's updating
    hypothesis (e.g., Donchin & Coles, 1988) is one of the best
    examples. It is well established that the P300 amplitude is
    related to the degree of contextual encoding (e.g., Donchin &
    Coles, 1988), expectancy (or subjective probability), and the
    amount of effort which is also reflected by the amount of
    information transmitted to a subject (see e.g., Johnson's
    triarchic model in Johnson, 1986; and the summary in Verleger,
    1988, p. 351). It is important to note that Verleger (1988), who
    is challenging Donchin's updating hypothesis, is not challenging
    the significance of the P300 with respect to contextual encoding.
    His argument basically is that the P300 does not reflect the
    "updating" but instead the "closure" of expectancies.

    [Continued to part 4]
    ----------------------------------------------------------------
    MindNet Journal Archive Filename: [mn162c.txt]
    ================================================================

    To receive the MindNet Journal via email:

    Send message: [subscribe mindnet] to: .

    To unsubscribe:

    Send message: [unsubscribe mindnet] to: .

    Back issues of the MindNet Journal are available at our
    FTP Archive site:

     [/pub/users/vericomm/mindnet/]

    MindNet Journal Publication Index: [mnindex.txt]

    Submission of articles for publication within the MindNet
    Journal on the subjects of mind control, directed-energy
    weapons, non-lethal weapons, ritual abuse, UFO abductions,
    bioelectromagnetics, hypnosis, and other related topics
    will be accepted with the author's statement of permission
    to publish. The editor reserves the right to accept or
    reject for publication. The publisher disclaims all
    responsibility to return unsolicited matter. Send articles
    for submission to:

    , or VERICOMM BBS 510.891.0303, or VERICOMM,
    POB 32314, Oakland, CA 94604-2314 USA.

    VERICOMM / MindNet and its agents disclaim any and all
    responsibility or liability for any and all claims and/or
    guarantees, express or implied, and delivery of products,
    merchandise, and/or services offered for sale by
    advertisers and/or authors within the MindNet Journal.

    The MindNet mailing list is owned and maintained by Mike Coyle,
    , VERICOMM / MindNet, POB 32314 Oakland, CA
    94604-2314 USA.

    The MindNet Journal is published by VERICOMM / MindNet in
    cooperation with the Freedom Of Thought Foundation, POB 35072,
    Tucson, AZ 85740-5072 USA.

    ================================================================

    ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
    ||||  ||||  |      |       |  |      |      |   |   |   |   ||||
    |||||  ||  ||  |||||  |||  |  |  |||||  ||  |  | |  |  | |  ||||
    ||||||    |||  ,,,,|      ||  |  |||||  ||  |  |||  |  |||  ||||
    |||||||  ||||      |  |||  |  |      |      |  |||  |  |||  sm||
    ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
    ||||||||||||  VERICOMM / MindNet : MindNet@c2.org  |||||||||||||
    ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||