The Major Memory Systems and Processing Routes of the
Human Cerebral Cortex
In
cognitive terms, the cortex of the human brain can be divided into 3
distinctive regions of processing. Firstly, from front-to-back (along the
rostral-caudal axis), regions at the front of the brain are associated with
internal management information processing, whilst regions at the back of the
brain are associated with external sensory information (e.g., from the eyes and
ears). Secondly, from the top-to-bottom (superior and inferior to the rostral-caudal
axis – dorsally and ventrally), regions at the top of the brain are associated
with contextual information processing (e.g. the spatial “what” processing
route), whilst regions at the bottom of the brain are associated with
qualifying information processing (e.g., the “where” processing route). Lastly,
from left-to-right (each hemisphere), regions in the left hemisphere are
associated with preparatory information processing (e.g., perceiving input
information), whilst regions in the right hemisphere are associated with formulation
information processing (e.g. formulating new perceptions).
These
divisions of the cortical surfaces are in general-level terms; that is they
indicate the overall trend in information processing type. In more accurate
terms most of these information processing types are distributed widely across
the brain’s cortical surface. It should also be noted that in regards to
hemispheric lateralisation of processing, the ‘preparatory’ and ‘formulation’
labels are somewhat deceptive. What is clearer is that the left hemisphere
tends to process information more in earlier stages of an overall process,
whilst the right hemisphere tends to process more information later in an
overall process (as observed in fMRI data during cognitive tasks).
The
brain itself is made up of 5 different major information processing systems. At
the most frontal part of the cortex is the quality-value system. This acts as a
motivator and is important in outcome evaluation and goal setting for cognitive
information processing. It has associations with social-cognition and
autobiographical memory. Next to this system is the principle-context system.
This system contextualises and links information by principles. It is largely
associated with episodic or time based processes and memory. In the middle is
the response-coordination system. This system is associated with bodily
movements and responses, and forms a bottleneck in cognitive processing. At the
back of the brain, at the top most and bottom most regions is the visual-spatial
system. This system processes sense information from the eyes into spatial and
object perceptions. Sandwiched between this, but reaching further into the
frontal regions than depicted in the above figure, is the audio-linguistic
system. This system processes information from the ears into language and other
auditory perceptions.
Overall, for the external information
processing systems of the brain, information ‘flows’ from the back of the brain
towards the central regions where the response coordination system is found.
Regions further to the back process and store finer details such as lines and
individual coordinates, whereas those closer to the centre of the brain process
and store more definitive objects and regions. The same overall information
flow pattern is also present for the internal information processing systems at
the front of the brain, except flowing from the front towards the middle. In
both cases regions further away from the response-coordination region of the
brain can be considered as more ‘abstract’ whilst those closer to the middle
can be considered more ‘concrete’.
Another
two major processing routes or pathways run along the lower and upper sides of
the brain, following the same inward flow pattern as described in the previous
paragraph. Information that flows along the lower pathway qualifies perceptions
and processes, whilst that which flows along the upper pathway contextualises
them. For example, a visual object perception (e.g., an apple) is given colour
and texture on the lower pathway, whilst the upper pathway defines its region
in space and relative position to other objects in the visual field. This same
general trend is also seen in all of the 5 major cognitive processing
modalities. It should be noted though, that although there are major trends in
information processing routes across the brains cortical regions and the 5
processing modalities, that these systems hold rich interconnections between
themselves as well. Every processing modality is connected to every other
modality via substantial fibre-tracts beneath the brain’s cortical surface.
Levels of Memory and Processing within the Major
Systems
The
manner in which information is processed and stored in memory within each of
the 5 major modalities also follows some overall trends. The physical networks
of the brain (i.e., neural networks) constitute what is called long-term
memory, whereas the activity of those networks constitutes what is called
working-memory. Long-term memory is information that the brain has stored
within itself, and may or may not be active during information processing.
Long-term memory is relatively static and unchanging, however is constantly
being ‘updated’ and ‘modified’ due to the ongoing information processing
activities of the cognitive system. On the other hand, long-term memory
networks that are actively processing information in the moment form immediate
memory and attention, known as working memory. Working memory is what a person
is thinking about, aware of, and attending to in any given moment. Information
that is actively processing may, or may not be, fully represented in long-term
memory, and it takes some time (approximately 90 minutes) before information is
transferred from immediate, working memory, into long-term memory storage.
Memory,
for both long-term storage and information processing is structured in a
hierarchical and categorical fashion. Categorical information that is ‘higher
up’ the memory network links to related sub-category or instance information
below that, which then links in the same kind of manner to memory below that,
and so on. For example, the idea of ‘animals’ may link to ‘mammals’, ‘fish’,
‘birds’, and ‘reptiles’. Each of those sub-categories then links to further
sub-categories or instances depending on how sophisticated and developed the
memory structure is for the person. Overall however, the links between levels
in the memory structure appears to be limited to about 4 bits of information
per level. This limit affects information processing and storage at the working
memory level, and probably represents limitations in the physical
interconnectivity between neurons and neural networks within the brain’s
cortex. Whilst it is easiest to understand such memory structures in terms of
definitive ‘bits’ of information like ‘animals’ and the colour ‘red’, in
practicality such ‘bits’ of information may be far more idiosyncratic and
abstract in terms of the true memory network structure (e.g. categories and
sub-categories may have many more than 4 instances associated with them; in
such cases non-descriptive linking sub-categories, or dummy-categories, may
intersperse levels within the structure).
When information is being actively
processed through a memory network, at the working memory level, activation may
flow either up or down the structure. Activated nodes within the network
increase the likelihood of all of the other connected nodes, above and below
it, to also become active. Active nodes may also decrease the likelihood of
other, unrelated memory network structures becoming active. Activation or
inhibition between networks is based on different types of neurotransmitter
systems, which themselves likely perform different types of connective function
within the brain.
The flow of
inhibition and activation through a memory network governs the manner in which
attention and information processing is focused and changed. Whilst there are
limitations on how much of a long-term memory network within one of the 5
modalities can be active at one time (approximately 4 bits of information),
active processing in working memory can occur concurrently within each of the 5
modalities as well as across the various divisions of that modality (i.e., left
and right hemisphere, and along the qualifying-contextualising axis). Because
the 5 modalities are interconnected, changes in one network can lead to changes
in another. As such information is parsed from one modality to another,
allowing for complex cognitive processing and thought when the brain is
considered as a whole.
Overall,
information processing in the internal management modalities at the front of
the brain are characterised by sustained activation, whilst those at the back
of the brain in the external sense modalities show more fluctuation. In both
cases, neural activity is biased towards processing changes and differences in
the cognitive processing environment, whilst that which stays the same is
likely accounted for by memory itself. Information about value-qualities and
principle-context information tend to be held consistent in order to focus
attention in a goal-direct and evaluative manner onto specific features and
elements of a more dynamic influx of sensory information that is processed by visual-spatial
and audio-linguistic modalities. The response-coordination system sits
somewhere in-between, having both internal-management and external-sense
related functions. Intended behavioural responses are most likely held
consistently online, whilst more sensory driven and automatic behaviours help
keep the sensory organs aligned and focused on specific aspects of the
environment.
Rhythms and Timing Intervals in Attention and Memory
There
are various timing intervals that are associated with active information
processing within a long-term memory network, as well as how long it takes for
information to be encoded into memory. These timing intervals create
limitations on the attentional and information processing capacities of the
human cognitive processing system and are based on biological processes in the
brain. Each timing interval follows a base-3 system; that is they are made up
of one unit of time length that occurs in processing streams of three units of
time in total length. The first unit is an active attention phase to
information being processed. The second unit is a linking phase, connecting the
previously attended to information to other information. The third and final
unit is an inactive, encoding/storage phase. Whilst individual processing
threads follow this pattern, multiple overlapping threads occur concurrently
with each other in a staggered manner. Half way through one processing threads
first unit marks the beginning of a second concurrent thread’s first unit, such
that active and linking phases link the threads together.
At
the level of working memory, the timing intervals have a base unit of around 3
seconds and a total interval length of around 9 seconds. These units, as with
all other levels of timing, are rough population averages, and vary from
individual to individual and are also influenced by other neurochemical factors
such as arousal level. Information that first enters into working memory then
takes approximately 90 minutes before it leads to structural changes in the
long-term memory networks of the brain, in a process known as
Long-Term-Potentiation. This 90 minute interval also follows the base-3 code,
with a 30 minute active phase, and 30 minute linking phase, and a 30 minute
encoding/storage phase. Between the 9 second interval level and the 90 minute
interval level, and also beyond that level there are also other levels of
timing interval. These levels of timing interval affect when, and for how long,
information becomes available and/or active in cognitive processing. For
example, the 90 minute interval level determines a person’s capabilities to
sustain attention on a subject and retain optimal memory performance; the
empirical data showing this to be about 45 minutes (2 attentional threads of 30
minutes that overlap). The following table indicates some of these timing
intervals.
For further information on Information Processing Modalities as well as discussion on Cognitive Control, read my PhD Critical Review;
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