Human beings are equipped with aremarkable profundity of ability, able to perceive external stimuli, generatecomplex novel ideas, and engage in profound self-reflection, as well as variousother sensory abilities.
Amidst this wealth of cognitive wherewithal, there areunderlying mechanisms that work in conjunction with one another to make such processingpossible. Without the neural proliferation and signal transferring that takesplace under the surface of everyday life, cognitive function ceases to exist.Cellular networking occurs on aggregate to actuate our behavior. Specificallyspeaking, there is structural connectivity, based on the “anatomical linkage ofthe brain’s neurons” (Bressler et al, 2010), which, as one would imagine,involves spatially adjacent brain structures.
Furthermore, there is functionalinterdependence (which can be, but is not limited to, structural association), orconnectivity, defined as the “statistical inter-relation of variablesrepresenting temporal changes in different network nodes” (Bressler et al,2010), or in more thoroughly defined terms, it “captures patterns of deviationsfrom statistical independence between distributed and often spatially remoteneuronal units, measuring their correlation/covariance, spectral coherence orphase-locking” (Sporns et al, 2004). For both functional and structuralconnectivity, these structures and their connecting “limbs,” so to speak, areformally known as “nodes” and “edges,” respectively, nodes being “excitatoryand inhibitory populations” whilst edges are defined as “long axon pathwaysthat connect from one neuronal population to another” (Bressler et al, 2010).Structural, or anatomical, connectivity is not contested, yet functionalconnectivity is, due to general uncertainty on what it means for two brainregions to operate similarly within the same time frame, as well as the methodsin which data is gathered. Of a more dubious nature is a particular instance offunctional connectivity – the default mode network (DMN), most generallycharacterized as a “large-scale network of brain areas that form an integratedsystem for self-related cognitive activity” (Bressler et al, 2010) or as a setof brain regions that show “greater activity during resting states than duringcognitive tasks” (Greicius et al, 2002). This paper serves to highlight thedebates concerning the question of which specific brain regions are associatedwith the network, under what circumstances the network is activated anddeactivated, and the veracity of the present claims.The DMN is a relatively nascentnetwork of functional connectivity, with its origins initially seen in 1997,where Schulman et al “first noted that a constellation of areas in the humancerebral cortex consistently reduced its activity while performing variousnovel, non-self referential, goal-directed tasks… when these tasks were comparedwith a control state of quiet repose (i.
e. a resting state of eyes closed orvisual fixation)” (Raichle, 2015). This led to an influx of intrigue in thesubject of activation during the brain’s resting, or “default,” mode, and as aresult more studies came about. The default mode network is defined by Bressleret al as a type of intrinsic connectivity network (ICN) – also known as a”resting-state network” (Hutchison et al, 2013) – which is a “large-scalenetwork of interdependent brain areas observed at rest.” Researchers haveattempted 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 islegitimately active in rest, while deactivated, or at least less active, inparticular states.
Now, what are these particular states? This is what the bulkof the debate hinges upon. One interpretation of the DMN’s activation is thatit “appears to persist during sensory tasks with low cognitive demand,” as wasseemingly the case in the analysis of the low-demand visual tasks (Greicius etal, 2002). In the study, a simple visual processing task was done in which 14healthy 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 “staticblack-and-white radial checkboard pattern,” and in the experimental conditioneverything remained the same, but “the same pattern was inverted (white sectionsbecome black, and black sections become white).” The subjects were told topassively view the checkboard under each condition, which in itself isreasonably difficult to control for obvious reasons. Naturally, it turned outthat there were task-related increases in activity in “extrastriate regionsbilaterally when flashing checkerboard epochs were contrasted with the staticcheckerboard epochs.” It is known that this region encompasses regions such asV3 and V4, visual areas of the brain, as well as V5, also known as MT, which isinvolved in motion perception. Therefore, it follows that there would be anincrease in cerebral function in the presence of dynamical visual stimulation.However, there was no task-related activity observed. The results of the visualprocessing task were much like the resting state task results; the participantswere instructed to “keep their eyes closed and to not think of anything inparticular.
” Despite this seeming lack of deactivation in the midst of sensoryprocessing tasks with limited cognitive demand, the paper made the claim thatthere was lowered activity in response to other external cues; in particular,there was a noted decrease (not complete cessation of activity) in activitywithin the working memory experimental task, in which there were sixalternating experimental and control epochs, each consisting of “16 stimulipresented for 500 ms each, with a 1,500-ms interstimulus interval.” Thepossibility of underlying emotional mechanisms was briefly suggested, but notdefinitively proven in the scope of the experiment – this assumption was laterconfirmed in a 2015 review by Raichle, where it was reported thatthe speculation of the DMN’sconnection to emotion was corroborated through the discovery of the ventromedialprefrontal cortex as a brain region implicated within the network (Barbas 2007). (Imagingstudies were done on normal individuals, showing that “the emotional state ofthe subject has a direct effect on the activity level in the VMPC component ofthe default mode network.” The extent to which the VMPC decreased in activitywith respect to the other elements of the network was “directly proportional tothe subject’s anxiety level” during the task, and it was also noted that ininstances of high anxiety, the change in VMPC activation was negligible.
) Asfor the brain regions implicated in the aforementionedactivations/deactivations, Greicius and his colleagues posited that both theposterior cingulate cortex (PCC) and the ventral anterior cingulate cortex (vACC)”consistently show greater activity during resting states than during cognitivetasks.” It was hypothesized that “if the network is suspended duringperformance of cognitively demanding externally cued tasks,” then the activityin the network during rest would likely exhibit an inverse correlation with thebrain regions that were activated during tasks, which in this case would bethree lateral prefrontal cortex regions that tend to show increased activityduring working memory tasks: the left ventrolateral prefrontal cortex (VLPFC),right VLPFC, and the right dorsolateral prefrontal cortex (DLPFC). Subsequentto testing, it was found that there were “significant inverse correlationsbetween all three ‘activated’ lateral prefrontal ROIs regions of interest andthe PCC during rest,” but the same relationship did not exist between the PFCregions and the vACC.
Another interpretation of thedefault mode network argues that social cognition is marked by an increase indefault mode activity, which not only disagrees with the notion that the onlyexternal cues that do not see a decrease in DMN activation are simple visualprocessing tasks, but also includes additional brain regions that were notmentioned within the network in the first argument. Research by Mars et aldisputes the notion that the DMN is by nature more active only in restingstates, save for low-demand visual processing, with research that posits theexistence of an overlap between the DMN and the social brain network. In astudy by Schilbach et al in 2008, “they performed a conjunction analysis on thedata from 12 studies from their lab, defining the DMN by looking for areas thatcorrelated negatively with the task-related regressors” (Mars et al, 2012). Theanalysis showed activation in the left angular gyrus, the precuneus, and theventral anterior cingulate cortex. Strikingly, it was found that some of theactivations coincided with social cognition, namely “involvement of theprecuneus in social interactions, the left angular gyrus/TPJ temporoparietaljunction in differentiating between self and others, and anterior cingulate inaction monitoring in self and others.
” Additionally, the researchers conducteda meta-analysis to gauge the tasks that tend to activate the networks. Asexpected, “a network highly reminiscent of DMN, showing bilateral inferiorparietal/TPJ, precuneus/posterior cingulate, and medial frontal activation…loaded strongly and exclusively on only one behavioral domain, that of socialcognition.” The subsistence of social cognition in accordance with the DMN wasconjectured in Raichle’s review, where it was highlighted that the VMPC’sanatomical 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 studiesbring forth strong evidence in support of the existence of a social cognitionfunction within the default mode network, bolstering the often unpopular beliefthat the network has bearing in tasks that not only are external, but alsofunctionally challenging. In light of these differingunderstandings of the role of the default mode network, it is necessary to notemany shortcomings in data collection and technique that could account forconfusion or lack of clarity regarding the complete function of the DMN and thespecific areas in which it holds domain. For instance, one critique of thetechniques used to assess functionality is that some activation patterns may bedue to “low signal-to-noise ratio (SNR), changing levels of non-neural noise(e.g. from cardiac and respiratory processes and hardware instability), as wellas variations in the BOLD signal mean and variance over time,” all of which cancause a change in functional connectivity metrics, resulting in data that seemsconvincing but is not true to form (Hutchision et al, 2013). For areas that notonly functionally, but spatially connect (“i.
e. the time series of a singlenode may have partial correlations with that of multiple networks”), one shouldbe wary of activation patterns there as well, since the involvement of a regioncould appear to change if the temporal windows of overlapping networks are notproperly separated from one another (Smith et al, 2012). It is also worthquestioning the validity of a claim that seems to somehow force the fact that anetwork exists that is inversely correlated to higher-level cognitivefunctions, whatever they may be. As was seen in the aforementioned studies,some high-demand processes were coupled with an increase of activation, whileothers were not.
Also, the vACC was not shown to inversely correlate with anyof the three prefrontal ROIs in the initial study addressed, which partiallyblurs the argument that there is any inverse correlation at all. It is quitepossible that the DMN simply functions in the resting state – even thedefinition of resting state is not clear; how can one be sure that by tellingsubjects to clear their minds that they are actually doing it? It is alsounclear what constitutes visual attention, and whether attention or intentionwithin the word-processing task, for instance, is what triggered a change innetwork functionality. The environmental constraints, or the lack thereof, aswell as the usage of nonvisual sensory cues during a supposed resting state arealso concerns to consider when defining the function of the DMN. It seems asthough the DMN is not yet completely specified, but what seems most concretelyproven is the existence of emotional and social underpinnings.