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     MindNet Journal - Vol. 1, No. 62b * [Part 2 of 4 parts]
     V E R I C O M M / MindNet         "Quid veritas est?"

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The following is reproduced here with the express permission of
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Permission is given to reproduce and redistribute, for
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Editor: Mike Coyle 

Assistant Editor: Rick Lawler

Research: Darrell Bross

[Continued from part 1]


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.


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.


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


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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