The same process that gives the octopus mobility and stability with its arms,
gives us mobility, stability and continuity in our thought.
Thinking about how thought changes
within the brain through time, I came to the conclusion that groups of neurons
interact in a very specific, repeating pattern over time. I tried to come up with an
analogy to explain this pattern. I
likened it to the pattern of footsteps taken by an octopus that is
“seafloor walking.” I feel the analogy works in a few different ways, but the
main point is that while much activation in the brain is often rapid and
transient, some activation is relatively more long lasting. I think that the
long lasting activations lend structure and continuity to thought and
contribute to the phenomenon of sentience.
Octopuses occasionally walk along
the seafloor and when they do, they often exhibit a particular stride. As they
move forward, they plant some of their arms on the sand below them and replant
arms that they have left behind. Imagine yourself climbing up a ladder. You
hold on to each rung until the rung is so low that you let go and reach for a
higher one. Now imagine climbing a ladder with many arms. They wouldn’t all
move at once, only the lowest ones would move. This is very similar to how an
octopus walks. The key idea is that, at any one point in time, some arms
retract from their positions, some arms are relocated to new positions but yet
most remain planted firmly where they are. This is very much like the pattern
of activation in the cerebral cortex.
Individual nodes (groups of cells
that are wired together) in the brain correspond to specific mental
representations. Each thought that we have necessitates large numbers of these
nodes to represent the various things going on in our mind. The octopus
analogy uniquely describes a system where certain nodes are conserved through
time as others come and go. The stride
of an octopus that plants the majority of its arms temporarily while actively
repositioning arms that have let go of their footholds, represents the
uninterrupted, nonlinear, spatio-temporal pattern of node activation,
deactivation and coactivation in the brain.
Some nodes can probably be retained even
after the transitions between a number of thoughts. This happens when your
thoughts cycle and change but hold a common element or theme constant. When we
attempt to solve a novel and complex problem we try to keep the majority of our
octopus arms firmly planted so that we can keep the problem set in mind. The
fact that some nodes (within association areas) remain active for prolonged
periods (i.e. the octopus arms remain planted), during reciprocal top-down to
bottom-up communications, accounts for the continuity found between
successive brain states. The longer nodes in association areas can be
continuously activated - over a series of states - the longer they can
influence sequences of bottom-up imagery in a sustained and consistent way
allowing modeling, planning and working memory in general. The result is a
stream of consciousness where each thought is quantitatively different from the
ones preceding, as newly relevant nodes are added and the least relevant ones
are removed.
The fact that the placement of some of
its arms are conserved, over sequential moments, gives the octopus balance and
stability and it also provides the physical basis for the continuity of thought. The nodes that are
conserved reside mostly in association areas whereas nodes in early sensory
areas are activated much more transiently. This is as if the arms in back of
the octopus (corresponding to posterior sensory cortices) move much more
quickly than the forelimbs (which find firm, reliable footholds in anterior
association cortices and the PFC). The octopus analogy has the advantage of
demonstrating how several interacting elements combine to allow thought. Also
because these elements remain active for different durations, thought does not
stop and go in discrete steps but is continually “carried along” by those
elements that endure through time. All elements, or neural nodes, will
deactivate within several seconds, but the intermingling of nodes of some
temporal stability with those of more fleeting persistence sustains the
associative bridges that allow the thematic and unifying consistency that is a
hallmark of cognition.
The routine of node activation and
deactivation is very similar to “polypedal locomotion” or movement in animals
with many legs. It is not much like the locomotion of an insect such as a
millipede or a centipede though because these animals move their legs in
stereotypical, repetitive ways where the placement of each leg is not actively
influenced by the placements of other legs or of the qualities of the
footholds. The pattern of activations in the brain is more like the polypedal
locomotion of an octopus that is seafloor walking because it is asymmetrical,
dynamic and the placement of the next legs is influenced by the octopus’
stance, posture and the characteristics of the footholds themselves. Most
importantly, this model can accommodate nonlinear aspects of neurodynamics. One
neural node does not activate the next in sequence. Several nodes are
coactivated together and they pool their activation energy to determine which nodes
will be activated next.
The total number of cortical nodes that
can be coactivated must be somewhat stable given known limitations on things
like neural excitability, cortical hemodynamics and working memory. In our
analogy the number of available octopus arms is very stable and this represents
our fixed, innate capacity for working memory. Even though the number of chunks
(psychologically perceptible units of perception and meaning) that can be held
in working memory, 7 plus or minus 2, coincidentally coincides with the number
of arms that a living octopus has (8), this is not a reliable indication of the
number of nodes that can be coactivated in association cortex. This is true
because even though chunks and nodes may be relatively congruent, the exact
relationship between them is currently unclear. It seems clear though that the
octopus has a relatively invariant number of arms and that perhaps, in order to
bring a new node into the train of thought it must first let go of some other node.
Surely the number of activatable nodes differs from area to area and from task
to task, but it probably remains relatively constant within tasks.
It is not always the case that the
majority of nodes are conserved from one thought to another. Most nodes can be
dropped or abandoned at the same time, i.e. when they become a lower priority.
This readily happens when we are exposed to a new, salient, perhaps emotionally
laden, stimulus. When this occurs, the octopus “jumps,” taking all of its arms
with it, and reorients to the new stimulus and its accompanying set of
features. Such a jump would constitute a disruption of mental continuity. So
clearly mental continuity can be viewed on a continuum where a high proportion
of nodes are conserved between brain states on one end of the continuum and a
low proportion are conserved on the other end. Disruptions in continuity might
occur due to a distracting stimulus in the environment, or from an internally
generated stimulus. Evolution has probably programmed the octopus to jump and
reposition its arms quickly in order to respond to important sensory stimuli,
so that mammals react to them with all of their cognitive resources. Mental
continuity is less easily disrupted in humans than it is in other mammals,
although perhaps more easily disrupted in people with habituation deficits.
Attention and distraction must be intimately related to the temporal
conservation of nodes. In fact, the extent of attention deficit and
distractibility should be inversely related to the neurological capacity to
conserve nodes in association areas from second to second.
Another analog of this analogy is the
idea that the octopus will “topple” if it loses its grip on a sufficient number
of nodes. This makes the body of the octopus analogous to consciousness because
brains become unconscious once coactivation (especially in the frontal and
parietal fields) is sufficiently diminished. Nodes in early sensory areas are
often active during unconscious states, but nodes in association areas are less
active and out of sync with those in sensory areas. Thus anterior-posterior
balance and coordination are important for our allegorical octopus.
Unlike subcortical areas, strictly
one-to-one, linear activation is probably rare in the cortex. Also, unlike
subcortical areas, information processing in the cortex is not
compartmentalized into individual nuclei that are relatively isolated from
processing occuring elsewhere. Rather, cortical nodes coactivate together to spread the activation energy
necessary to recruit, or converge upon, the next set of nodes that will be
coactivated with the remaining nodes
from the previous cycle. An node is released from coactivation when it no
longer receives sufficient activation energy from its inputs i.e. its relevance
to the processing demands diminishes. When a node is deactivated, the
perceptual or conceptual element corresponding to it becomes deactivated and
can no longer impact present experience. Whatever new node is introduced will
inform the present sum of coactivates in a unique and informative way.
The nodes that are coactivated, at any one time, sum their component
features together to create mental imagery and this occurs in both sensory and association
areas. Mental imagery changes plastically as nodes
that continue to be useful are maintained, nodes
that are rendered less useful are released from activation, and nodes that are newly recognized as useful are activated and
incorporated into the remaining amalgamation of useful coactivations.
Read the full article that I wrote on this topic here:
http://www.sciencedirect.com/science/article/pii/S0031938416308289
Table 1: Definitions of Terms
Psychological Aspects
|
Neurological Aspects
|
Octopus Analogy Analog
|
|
Neuron
|
Variable if not negligible
|
A single cell
|
A grain of sand on the cortical
seafloor that the octopus stands on
|
Neural Assembly
|
Element, feature, or fragment of a
construct in long-term memory
|
A cortical minicolumn or a
collection of cells with very similar receptive fields
|
A patch of sand that is currently
in contact with a suction cup on an octopus arm
|
Neural Ensemble
|
A psychologically perceptible
construct of long-term memory that can serve as a feature of a current
thought
|
A collection of coactivated
assemblies that are bound in a Hebbian manner
|
A region of sand that is currently
in contact with a single octopus arm and its suction cups
|
A Thought
|
A composite of several representations
that combine to create mental imagery
|
A set of coactivated ensembles
|
The set of all octopus arms that
are currently in contact with the cortical seafloor
|
Thinking / Consciousness
|
A progression of related imagery
formed through reciprocating transformations between association and sensory
cortex
|
A sequence of related sets of
coactivated ensembles where some remain active over the duration
|
The locomotive behavior or past
and present footsteps of the octopus
|
Unconscious Processes
|
Implicit mental behavior unavailable
to psychological introspection
|
The connectivity responsible for
the selection of assemblies and ensembles
|
The automatic processes
corresponding to the selection of arm placements
|
Read the full article that I wrote on this topic here:
http://www.sciencedirect.com/science/article/pii/S0031938416308289
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