I have been writing and fine-tuning this manuscript for more than ten years now and it is finally published. It outlines my model of working memory and my theory of how the brain and the mind are linked. They are linked by a very specific pattern of neural activity that creates the continuity of consciousness.
The open access article is titled:
Incremental Change in the Set of Coactive Cortical Assemblies Enables Mental Continuity
and can be read here for free:
Abstract
This opinion article explores how
sustained neural firing in association areas allows high-order mental
representations to be coactivated over multiple perception-action cycles, permitting
sequential mental states to share overlapping content and thus be recursively
interrelated. The term “state-spanning coactivity” (SSC) is introduced to refer
to neural nodes that remain coactive as a group over a given period of time.
SSC ensures that contextual groupings of goal or motor-relevant representations
will demonstrate continuous activity over a delay period. It also allows potentially
related representations to accumulate and coactivate despite delays between
their initial appearances. The nodes that demonstrate SSC are a subset of the
active representations from the previous state, and can act as referents to
which newly introduced representations of succeeding states relate. Coactive
nodes pool their spreading activity, converging on and activating new nodes,
adding these to the remaining nodes from the previous state. Thus, the overall
distribution of coactive nodes in cortical networks evolves gradually during
contextual updating. The term “incremental change in state-spanning coactivity”
(icSSC) is introduced to refer to this gradual evolution. Because a number of associated
representations can be sustained continuously, each brain state is embedded recursively
in the previous state, amounting to an iterative process that can implement
learned algorithms to progress toward a complex result. The longer
representations are sustained, the more successive mental states can share
related content, exhibit progressive qualities, implement complex algorithms,
and carry thematic or narrative continuity. Included is a discussion of the
implications that SSC and icSSC may have for understanding working memory,
defining consciousness, and constructing AI architectures.
1. Introduction
The present article will delineate a
simplistic but previously overlooked nonlinear dynamic pattern of brain
activity. Two hypothetical constructs are introduced to describe this pattern. The
first construct is state-spanning coactivity (SSC), which occurs when cortical nodes
exhibit sustained coactivity during the span of short-term memory. The gradual
evolution of SSC exhibits a distinctive spatiotemporal pattern of turnover as
it plays out in real time. The second construct introduced here, incremental
change in state-spanning coactivity (icSSC), refers to this pattern of turnover.
icSSC conveys that the set of nodes that are simultaneously coactive changes
incrementally as newly activated nodes are added and others are deactivated
while a distinct subset remains in SSC. Spreading activity from the nodes in
SSC select: 1) inactive neural nodes for activation, 2) active nodes for
deactivation, and 3) active nodes for sustained activation. Because a distinct
subset of nodes is always conserved from one brain state to the next, each
state is embedded recursively in the previous state, amounting to an iterative
process that has the potential to progress algorithmically toward a complex
result. The general intention of the present article is to propose a
qualitative model delineating the theoretical functions of SSC and icSSC from
the perspective of cognitive neuroscience.
The term SSC can be used either to denote a property or to designate a set
of neurons. icSSC denotes a property
or process. Both are related to the construct of working memory, which is
defined as a system responsible for the transient holding and processing of
attended information. The fundamental assumption made by this article is that
the content of working memory can be said to be in SSC; and as working memory
progresses over time, the content can be said to exhibit icSSC. This assumption
is applied not only to working memory as the same could be said of attention,
consciousness or short-term memory. icSSC can be taken to be the underlying neural
substrate of mental continuity. As proposed here, mental continuity is a
process where a gradually changing collection of mental representations held in
attention/working memory emerges from the icSSC of neural nodes. The thematic
and narrative quality created by this continuity during internally generated
thought may be largely congruent with key facets of conscious experience. In
the course of exploring how neural continuity creates mental continuity, this article
will attempt to integrate current theoretical approaches while remaining
consistent with prevailing knowledge.
Table 1: Definition
of Key Terms
|
Instantaneous
Coactivity
|
The coactivity of a
set of cortical nodes in a single instant or state.
|
|
State-spanning
Coactivity (SSC)
|
Sustained coactivity
exhibited by a set of two or more cortical nodes that spans two or more
consecutive brain states
|
|
Incremental Change
in State-spanning Coactivity (icSSC)
|
The process in
which a set of three or more neural nodes exhibiting SSC undergoes a shift in
group membership, where at least two nodes remain in SSC and at least one is
deactivated and replaced by a new node
|
|
Mental Continuity
|
The recursive
interrelatedness of consecutive mental states made possible by icSSC
|
Animals are information-processing agents.
They receive unprocessed data through sensory receptors, expose it to a
massively parallel network of nodes and channels, and allow the interaction
between the activity and the existing network to determine behavior. Even small
invertebrates with elementary nervous systems exhibit ongoing, internally
generated neural activity that temporarily biases the network weights. Because
it involves mechanisms that include sustained firing, this continuous
endogenous processing constitutes a fleeting form of SSC, even in animals like
the nematode and fruit fly. In vertebrates, however, SSC involves the
coactivation of high-level representations from long-term memory within a
single, massively interconnected representational network (telencephalon). Each
such representation is a record of the distribution of past neural activity
corresponding to a recognizable stimulus or motor pattern. An instantaneous
attentional state is composed of a novel combination of these template-like
representations which together create contextual, cognitive content. The
mammalian neocortex can hold a number of such mnemonic representations coactive
for hundreds of milliseconds, using them to make predictions by allowing them
to spread their activation energy together, throughout the thalamocortical
network. This activation energy converges on the inactive representations in
long-term memory that are the most closely connected with the current group of
active representations, making them active and pulling them into SSC. Thus, new
representations join the representations that recruited them, are incorporated
into the set of coactive parameters in SSC and used in subsequent searches.
When the activity of certain nodes can be
sustained for several seconds at a time, as in primate association cortex, the
complexity of search in such a system increases. Highly sustained activity
allows prioritized representations to act as search parameters for multiple
perception-action cycles. This permits more dynamic icSSC, whereby goal-relevant
representations can be held constant as others are allowed to change. The icSSC
taking place in association areas allows task-pertinent representations to be
maintained over multiple cycles, in order to direct complex sequences of
interrelated mental states. The individual states in a sequence of such states
can be considered interrelated because they share representational content. The
associations linking these sequences are saved to memory, impacting future
searches and ultimately permitting semantic knowledge, planning, and
systemizing.
2. Sustained firing, attentional updating, and memory decay
Mammals regularly encounter scenarios
involving sets of stimuli that may remain present (or relevant) throughout the
experience. In order to systemize such a scenario, it may be necessary to
maintain mental representations of the pertinent contextual stimuli during the
experience, and even afterward. Mammalian brains are well-equipped to do
exactly this. The glutamatergic pyramidal neurons in the prefrontal cortex (PFC),
parietal cortex, and other association cortices, are specialized for sustained
firing, allowing them to generate action potentials at elevated rates for
several seconds at a time (Fuster, 2009). In contrast, neurons in other brain
areas, including cortical sensory areas, often remain persistently active for
periods of mere milliseconds unless sustained input from either the environment
or association areas makes their continued activity possible (Fuster, 2009). A
neuron may exhibit tonic sustained firing due to temporary changes in the strength
of certain synapses (short-term synaptic modification (Stokes, 2015)), its
intrinsic biophysical properties, extrinsic circuit properties (reverberatory
circuits), or dopaminergic innervation (Durstewitz & Seamans, 2002). Prolonged
activity of neurons in association areas is largely thought to allow the
maintenance of specific features, patterns and goals (Baddeley, 2007).
Goldman-Rakic (1987; 1990) first suggested
that the phenomenon of sustained firing in the PFC is responsible for the
information maintenance capabilities of the temporary storage buffers of
working memory. Goldman-Rakic (1995) also proposed that the PFC is parceled
into several specialized regions, each of which is responsible for detecting,
representing and sustaining a different extraction of multimodal information. Since
then, the PFC, along with a number of association areas, has been divided into
increasingly smaller modules, each with unique receptive/projective fields and
functional properties including faculties such as short-term spatial memory,
short-term semantic memory, response switching, error detection, reward
anticipation, impulse suppression, and many others. Working memory, executive
processing and cognitive control are now widely thought to stem from the active
maintenance of patterns of activity in the PFC, especially the dorsolateral PFC,
that correspond to goal-relevant features and patterns (Fuster, 2002). The temporary
persistence of these patterns ensures that they continue to transmit their
effects on network weights as long as they remain active, biasing ongoing
processing, and affecting the interpretation of stimuli that occur during their
episode of continual firing (Miller & Cohen, 2001). This persistence
ensures that context from the recent past is taken into account during action
selection.
During any experience, some neural nodes
exhibit more prolonged sustained firing than others. I will assume that in
general the most enduringly active nodes correspond to what attention is most
focused on, or the underlying theme that remains most constant as other
contextual features change. From subjective introspection we know that when we envision
a scenario in our mind’s eye, we often notice it transform into a related but
distinctly different scenario (James, 1978). These two scenarios are related because
our brain is capable of icSSC. In other words, the distribution of active
neurons in the brain transfigures incrementally from one configuration to
another, instead of changing all at once. If it were not for the phenomenon of icSSC,
instantaneous information processing states would be time-locked and isolated
(as in most serial and parallel computing architectures), rather than
continuous with the states before and after them.
These observations point to the notion that
every cortical state is composed of a subset of elements from the previous
state, and also composed of increasingly smaller subsets of elements of states
directly before that. In fact, when comparing successive cortical states, the
shorter the time difference between two states (on the order of seconds to fractions
of milliseconds), the more similar in composition the two states will be. For
instance, over the span of 10 milliseconds, a relatively large proportion of nodes
will exhibit uninterrupted coactivity; however, over 10 seconds, this
proportion will be much smaller. Here,
we will be concerned with neural nodes exhibiting SSC at two distinct levels: A)
short-term memory/priming, i.e., elements of long-term memory activated above
baseline (for seconds to minutes); and B) the focus of attention/immediate
memory, i.e., a small, perhaps more active subset of A (for milliseconds to a
few seconds). Items in SSC within the focus of attention likely demonstrate
active neural binding whereas items in SSC within short-term memory may not.
Mental
continuity and icSSC require a densely interconnected representational system
such as a neural network that is capable of holding two or more representations
(each
specifying discrete and separate informational content) active over the course of two or more points in time
(Fig. 1). The sustained activity of a single representation over time does not
provide any context or associative/relational content, and so should not be
taken to be sufficient for mental continuity. More than one representation is
needed. Although its limits are presently being debated, the human neocortex is
clearly capable of holding numerous representations active over numerous points
in time.
To keep reading, click here:
http://www.sciencedirect.com/science/article/pii/S0031938416308289
http://www.sciencedirect.com/science/article/pii/S0031938416308289
