Human make such processing possible. Without the neural proliferation

Human beings are equipped with a
remarkable profundity of ability, able to perceive external stimuli, generate
complex novel ideas, and engage in profound self-reflection, as well as various
other sensory abilities. Amidst this wealth of cognitive wherewithal, there are
underlying mechanisms that work in conjunction with one another to make such processing
possible. Without the neural proliferation and signal transferring that takes
place under the surface of everyday life, cognitive function ceases to exist.
Cellular networking occurs on aggregate to actuate our behavior. Specifically
speaking, there is structural connectivity, based on the “anatomical linkage of
the brain’s neurons” (Bressler et al, 2010), which, as one would imagine,
involves spatially adjacent brain structures. Furthermore, there is functional
interdependence (which can be, but is not limited to, structural association), or
connectivity, defined as the “statistical inter-relation of variables
representing temporal changes in different network nodes” (Bressler et al,
2010), or in more thoroughly defined terms, it “captures patterns of deviations
from statistical independence between distributed and often spatially remote
neuronal units, measuring their correlation/covariance, spectral coherence or
phase-locking” (Sporns et al, 2004). For both functional and structural
connectivity, these structures and their connecting “limbs,” so to speak, are
formally known as “nodes” and “edges,” respectively, nodes being “excitatory
and inhibitory populations” whilst edges are defined as “long axon pathways
that connect from one neuronal population to another” (Bressler et al, 2010).
Structural, or anatomical, connectivity is not contested, yet functional
connectivity is, due to general uncertainty on what it means for two brain
regions to operate similarly within the same time frame, as well as the methods
in which data is gathered. Of a more dubious nature is a particular instance of
functional connectivity – the default mode network (DMN), most generally
characterized as a “large-scale network of brain areas that form an integrated
system for self-related cognitive activity” (Bressler et al, 2010) or as a set
of brain regions that show “greater activity during resting states than during
cognitive tasks” (Greicius et al, 2002). This paper serves to highlight the
debates concerning the question of which specific brain regions are associated
with the network, under what circumstances the network is activated and
deactivated, and the veracity of the present claims.

The DMN is a relatively nascent
network of functional connectivity, with its origins initially seen in 1997,
where Schulman et al “first noted that a constellation of areas in the human
cerebral cortex consistently reduced its activity while performing various
novel, non-self referential, goal-directed tasks… when these tasks were compared
with a control state of quiet repose (i.e. a resting state of eyes closed or
visual fixation)” (Raichle, 2015). This led to an influx of intrigue in the
subject of activation during the brain’s resting, or “default,” mode, and as a
result more studies came about. The default mode network is defined by Bressler
et al as a type of intrinsic connectivity network (ICN) – also known as a
“resting-state network” (Hutchison et al, 2013) – which is a “large-scale
network of interdependent brain areas observed at rest.” Researchers have
attempted to tackle the exact function of this default mode network – that is,
come to a definite conclusion on what the DMN actually does and whether it is
legitimately active in rest, while deactivated, or at least less active, in
particular states. Now, what are these particular states? This is what the bulk
of the debate hinges upon. One interpretation of the DMN’s activation is that
it “appears to persist during sensory tasks with low cognitive demand,” as was
seemingly the case in the analysis of the low-demand visual tasks (Greicius et
al, 2002). In the study, a simple visual processing task was done in which 14
healthy right-handed individuals (seven males, seven females) were made to view
“experimental and control epochs that alternated every 20s for 6 cycles”
(Greicius et al, 2002). For the control state, there was a “static
black-and-white radial checkboard pattern,” and in the experimental condition
everything remained the same, but “the same pattern was inverted (white sections
become black, and black sections become white).” The subjects were told to
passively view the checkboard under each condition, which in itself is
reasonably difficult to control for obvious reasons. Naturally, it turned out
that there were task-related increases in activity in “extrastriate regions
bilaterally when flashing checkerboard epochs were contrasted with the static
checkerboard epochs.” It is known that this region encompasses regions such as
V3 and V4, visual areas of the brain, as well as V5, also known as MT, which is
involved in motion perception. Therefore, it follows that there would be an
increase in cerebral function in the presence of dynamical visual stimulation.
However, there was no task-related activity observed. The results of the visual
processing task were much like the resting state task results; the participants
were instructed to “keep their eyes closed and to not think of anything in
particular.” Despite this seeming lack of deactivation in the midst of sensory
processing tasks with limited cognitive demand, the paper made the claim that
there was lowered activity in response to other external cues; in particular,
there was a noted decrease (not complete cessation of activity) in activity
within the working memory experimental task, in which there were six
alternating experimental and control epochs, each consisting of “16 stimuli
presented for 500 ms each, with a 1,500-ms interstimulus interval.” The
possibility of underlying emotional mechanisms was briefly suggested, but not
definitively proven in the scope of the experiment – this assumption was later
confirmed in a 2015 review by Raichle, where it was reported that
the speculation of the DMN’s
connection to emotion was corroborated through the discovery of the ventromedial
prefrontal cortex as a brain region implicated within the network (Barbas 2007). (Imaging
studies were done on normal individuals, showing that “the emotional state of
the subject has a direct effect on the activity level in the VMPC component of
the default mode network.” The extent to which the VMPC decreased in activity
with respect to the other elements of the network was “directly proportional to
the subject’s anxiety level” during the task, and it was also noted that in
instances of high anxiety, the change in VMPC activation was negligible.) As
for the brain regions implicated in the aforementioned
activations/deactivations, Greicius and his colleagues posited that both the
posterior cingulate cortex (PCC) and the ventral anterior cingulate cortex (vACC)
“consistently show greater activity during resting states than during cognitive
tasks.” It was hypothesized that “if the network is suspended during
performance of cognitively demanding externally cued tasks,” then the activity
in the network during rest would likely exhibit an inverse correlation with the
brain regions that were activated during tasks, which in this case would be
three lateral prefrontal cortex regions that tend to show increased activity
during working memory tasks: the left ventrolateral prefrontal cortex (VLPFC),
right VLPFC, and the right dorsolateral prefrontal cortex (DLPFC). Subsequent
to testing, it was found that there were “significant inverse correlations
between all three ‘activated’ lateral prefrontal ROIs regions of interest and
the PCC during rest,” but the same relationship did not exist between the PFC
regions and the vACC. 

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Another interpretation of the
default mode network argues that social cognition is marked by an increase in
default mode activity, which not only disagrees with the notion that the only
external cues that do not see a decrease in DMN activation are simple visual
processing tasks, but also includes additional brain regions that were not
mentioned within the network in the first argument. Research by Mars et al
disputes the notion that the DMN is by nature more active only in resting
states, save for low-demand visual processing, with research that posits the
existence of an overlap between the DMN and the social brain network. In a
study by Schilbach et al in 2008, “they performed a conjunction analysis on the
data from 12 studies from their lab, defining the DMN by looking for areas that
correlated negatively with the task-related regressors” (Mars et al, 2012). The
analysis showed activation in the left angular gyrus, the precuneus, and the
ventral anterior cingulate cortex. Strikingly, it was found that some of the
activations coincided with social cognition, namely “involvement of the
precuneus in social interactions, the left angular gyrus/TPJ temporoparietal
junction in differentiating between self and others, and anterior cingulate in
action monitoring in self and others.” Additionally, the researchers conducted
a meta-analysis to gauge the tasks that tend to activate the networks. As
expected, “a network highly reminiscent of DMN, showing bilateral inferior
parietal/TPJ, precuneus/posterior cingulate, and medial frontal activation…
loaded strongly and exclusively on only one behavioral domain, that of social
cognition.” The subsistence of social cognition in accordance with the DMN was
conjectured in Raichle’s review, where it was highlighted that the VMPC’s
anatomical circuitry reveals its potential involvement in the DMN as a
“sensory-visceromotor link” dealing with components important to personality,
such as mood control, social behavior, and motivational drive. These studies
bring forth strong evidence in support of the existence of a social cognition
function within the default mode network, bolstering the often unpopular belief
that the network has bearing in tasks that not only are external, but also
functionally challenging. 

In light of these differing
understandings of the role of the default mode network, it is necessary to note
many shortcomings in data collection and technique that could account for
confusion or lack of clarity regarding the complete function of the DMN and the
specific areas in which it holds domain. For instance, one critique of the
techniques used to assess functionality is that some activation patterns may be
due to “low signal-to-noise ratio (SNR), changing levels of non-neural noise
(e.g. from cardiac and respiratory processes and hardware instability), as well
as variations in the BOLD signal mean and variance over time,” all of which can
cause a change in functional connectivity metrics, resulting in data that seems
convincing but is not true to form (Hutchision et al, 2013). For areas that not
only functionally, but spatially connect (“i.e. the time series of a single
node may have partial correlations with that of multiple networks”), one should
be wary of activation patterns there as well, since the involvement of a region
could appear to change if the temporal windows of overlapping networks are not
properly separated from one another (Smith et al, 2012). It is also worth
questioning the validity of a claim that seems to somehow force the fact that a
network exists that is inversely correlated to higher-level cognitive
functions, whatever they may be. As was seen in the aforementioned studies,
some high-demand processes were coupled with an increase of activation, while
others were not. Also, the vACC was not shown to inversely correlate with any
of the three prefrontal ROIs in the initial study addressed, which partially
blurs the argument that there is any inverse correlation at all. It is quite
possible that the DMN simply functions in the resting state – even the
definition of resting state is not clear; how can one be sure that by telling
subjects to clear their minds that they are actually doing it? It is also
unclear what constitutes visual attention, and whether attention or intention
within the word-processing task, for instance, is what triggered a change in
network functionality. The environmental constraints, or the lack thereof, as
well as the usage of nonvisual sensory cues during a supposed resting state are
also concerns to consider when defining the function of the DMN. It seems as
though the DMN is not yet completely specified, but what seems most concretely
proven is the existence of emotional and social underpinnings.