The Anatomical and Functional Evolution of the Default Mode Network
The default mode network (DMN) in humans is characterised as a closely-interconnected neural network that shows task-related de-activation. Whilst DMN abnormalities are associated with various neuropsychological disorders in humans, preclinical studies are generally limited. However, initial progress into the anatomy of proposed DMN homologues in rodents and non-human primates (NHPs) show the existence of a closely connected network functionally distant from primary sensory/motor regions in all these species, with a similar anterior-posterior subdivision. Examining the body of work in different species, the author proposes possible evolutionary developments in the anatomy of DMN homologues, starting with expansion of the frontotemporal subnetwork, an apomorphy of primates, and then transitioning from the ventral/frontotemporal pathway to the dorsal/frontoparietal pathway. Many symptoms of DMN-related disorders, including the shifting of attention (deficit in ADHD), the generation of social expectation by mirroring other individuals (deficit in autism spectrum disorders [ASD]), and object-recognition memory decline during ageing (probably dementia-specific), are not only conserved in common NHP species, but also provide further support for the frontotemporal-to-frontoparietal transition within primate species. Thus, the combined analysis suggests a very optimistic perspective that, with careful task design, behavioural studies with NHP will shed more light on the establishment of DMN as a clinical target.
Psychology: Is the default mode network the locus of imagination and morality, and what adaptive functions do these psychological faculties serve that distinguish humans from other primates? (Selwood-Metcalfe, E.)
Yuankai He Corpus Christi College, University of Cambridge Volume 1, Issue 2, pp. 89–98
Received: November 14, 2022 Revision recieved: December 10, 2022 Accepted: December 28, 2022 Published: January 20, 2023
Defying established beliefs that goal-directed behaviour is associated with regional brain activation, the default mode network (DMN) was identified as a widely distributed group of brain regions showing decreased activity during various types of attention-demanding, goal-directed behaviours. These areas include ventro- and dorsomedial prefrontal cortex (vmPFC and dmPFC), medial temporal cortex (MTC), posterior cingulate cortex (PCC), and the adjacent temporoparietal junction (TPJ; Andrews-Hanna et al., 2014; Raichle et al., 2001; Smallwood et al., 2021). Clinical correlates of DMN with depression (Yan et al., 2019), post-traumatic stress disorder (PTSD; Miller et al., 2017), Alzheimer’s disease (AD; Greicius et al., 2004), autism spectrum disorder (ASD; Padmanabhan et al., 2017), anxiety disorder (Zhao et al., 2007), and many other psychiatric conditions sparked wide-ranging interest in the functional role of the DMN in humans.
Many theories on the function of DMN have been proposed in light of its distinctive activation pattern, including maintenance of passive cognitive processes that are suppressed in active tasks (Shulman et al., 1997), interaction between self and non-self (Wen et al., 2020; Yeshurun et al., 2021), and various memory functions (Philippi et al., 2015; Yuan et al., 2021). Whilst these theories are supported by a wealth of neuroimaging data (including position electron tomography [PET] and functional magnetic resonance imaging [fMRI]) which establish robust brain activity-behaviour correlations, the establishment of DMN as a target of clinical interventions necessitates a causal link. With a limited range of strategies to study the anatomical and cellular mechanisms of DMN in humans, insights have to be drawn from animal-based studies. However, these are limited in at least two regards: anatomically, all component regions of the DMN are hotspots in cortical expansion during human evolution, limiting the anatomical translatability of animal studies (Fjell et al., 2015; Hill et al., 2010); functionally, complex behavioural correlates of the DMN in humans, like moral judgement (Chiong et al., 2013) and autobiographical memory (Wen et al., 2020), are difficult to observe with animals. Hence, an evolutionary perspective would facilitate the integration and clinical translation of the body of work on DMN. In particular, this review assesses evidence on the existence of DMN homologues in common model organisms, then compares between identified homologues to infer possible evolutionary developments in DMN connectivity, and finally attempts to relate the anatomical evolution of DMN to its functional evolution and thus to explore the possibility of establishing DMN as a clinical target.
IDENTIFYING ANIMAL HOMOLOGUES OF THE DMN
The human DMN was initially identified by task-related decreases in brain activity, with evidence showing higher resting-state blood flow in mPFC, PCC, and MTC, signalling higher resting-state activity (Raichle et al., 2001). However, only a few studies directly applied this criterion to animals to identify the DMN homologues. An initial exploratory study compared PET-based resting brain activities of human subjects and our closest living relative, the common chimpanzee (Pan troglodytes), showing highest resting brain activity in bilateral dmPFC, PCC, precuneus, and left TPJ in chimpanzees which largely overlapped with the regions of highest activity in humans, except for higher left-biased laterality and lower vmPFC activity in humans (Rilling et al., 2007). An extension to this study comparing chimpanzees’ brain activities at rest and during a working-memory image-matching task showed task-associated deactivation in bilateral vmPFC, PCC, precuneus, left TPJ, and more diffusely across the entire bilateral PFC (Barks et al., 2015). Since the lobular structure of the chimpanzee is highly similar to that of humans, this evidence overwhelmingly supports the existence of a DMN homologue in chimpanzees.
Initial findings with the chimpanzees inspired studies exploring DMN homologues in other non-human primate (NHP) species. Compared to chimpanzee studies, which are limited by the slower life cycle of chimpanzees and various other practical or ethical reasons, rhesus macaques (Macaca mulatta) and, more recently, common marmosets (Callithrix jacchus) are much more established model species for neurobiological research. Whilst homologues of all DMN component regions (vmPFC/dmPFC, PCC, and the adjacent precuneus, MTC/TPJ) are found in the macaque (Reveley et al., 2017), and these areas generally show increased resting activity (Kojima et al., 2009), regions of task-associated deactivation do not fully coincide with the human/chimpanzee counterparts. Combined results from working-memory, attention, visual, and auditory tasks show that posterior DMN areas in PCC, precuneus, and TPJ coincide with the task-negative activation pattern in humans, whereas dmPFC shows task-related deactivation to a much lesser extent, and in vmPFC only the most dorsal part of BA32 is reported to show significant task-related deactivation (Hayden et al., 2009; Kojima et al., 2009; Mantini et al., 2011). It should, however, be noted that none of the putative DMN homologues showed task-induced activation. Hence, despite the interspecific difference in task-related activity levels, the existence of a default mode network in macaques over PFC, TPJ, and PCC/precuneus is generally supported. Although studies of DMN in marmosets have only recently started, a similar quantitative—but not qualitative—anatomical gap can be inferred. Only one study, to the author’s knowledge, compared the brain activity of marmosets at rest and during a simple visual attention task (i.e., fixate on stimulus when it appears), showing task-associated deactivation of PCC, precuneus, and TPJ. Whereas no task-induced inhibition is observed in the PFC, it displayed task-neutral behaviour (i.e., no significant change in activity during tasks) similar to macaques (Liu et al., 2019). Since these areas are also homologous to the human brain cytoarchitectonically (Paxinos et al., 2012), the existence of a DMN in marmosets can be confirmed. However, more information is needed to find a potential homologue of the frontal part of DMN in marmosets. Overall, evidence supports the conservation of the posterior parts of DMN in NHPs, whereas the functional cross-species continuity of the frontal DMN components, though anatomically homologous to humans, needs to be further examined.
Compared to NHPs, rodent studies offer a wider range of genetic/molecular intervention methods and a faster lifecycle, enabling larger-scale studies. However, though some studies have shown deactivation of certain rodent brain regions in response to sensory stimulation, including the retrosplenial cortex (RSP, homologous to human PCC), and their disconnection from other brain regions in both anaesthetised and awake animals (Ferrier et al., 2020; J. Li et al., 2015), whether stimulations compare to cognitive tasks in humans still needs to be established. Instead, the identification of DMN homologues in rodents is mainly based on the resting-state functional connectivity of candidate brain regions (see next section; Hsu et al., 2016; Lu et al., 2012; Stafford et al., 2014; Upadhyay et al., 2011; N. Zhang et al., 2010). Main candidate brain regions from cytoarchitectonic mapping include vmPFC homologues (prelimbic, infralimbic, medial orbital, and cingulate cortices), PCC homologue (RSP), TPJ homologue (temporal association [TEA] and the hippocampal formation), visual (V1/V2), and posterior parietal (PPC) cortices (Paxinos & Franklin, 2004).
Connectivity patterns of the DMN Although the cytoarchitectonic homologues of all DMN component regions exist in the model organisms, task-related activation patterns of these regions cast doubt on the extent to which the human DMN functions are represented in these model species. Hence, investigations into the connectivity patterns of the putative DMN homologues in different species could unravel the evolution of their functional roles, especially for the regions with discrepancies in task-related activity. One key piece of evidence is obtained from the raw resting-state fMRI signal, where co-activating regions of the brain show high temporal correlations of random resting-state fluctuations (Biswal et al., 1995). Hence, two key hypotheses can be formulated: (a) different DMN regions show high resting-state coherence with one another and low coherence with task-positive regions—as a result, all DMN regions should be included in a distinct cluster in resting-state correlations; (b) intrinsic organisation within the DMN can be revealed by identifying sub-clusters within the DMN. These hypotheses may be supported by task-based PET and fMRI studies (supporting functional connectivity), tractography, and anterograde/retrograde tracing studies (supporting anatomical connectivity).
Human DMN connectivity Findings from the human DMN are consistent with both hypotheses and have thus identified two main sub-networks within the DMN. Greicius and colleagues (2003) first studied the human DMN analysing the resting-state functional connectivity (FC) seeded in PCC and vmPFC, and found significant resting-state coherence within the DMN for both regions with significant anti-correlation with vlPFC/dlPFC, a known task-positive region (Desgranges et al., 1998). Similar coherence within the DMN is observed in studies using other data analysis approaches to resting-state fMRI data, including mixed-effect analysis, independent component analysis (Teipel et al., 2010; van Oort et al., 2014), and voxel-based clustering (Van Den Heuvel et al., 2009; Vatansever et al., 2015). A novel approach by Passow and colleagues (2015) utilising the resting fluctuations in metabolic activities measured by PET also produced a similar cluster covering DMN. Recent results also include the midline subcortical structures (caudate, central nuclei, and medial pulvinar nucleus of the thalamus) in the same cluster as DMN (J. Li et al., 2021), but further confirmation is needed to determine whether coherence of the resting-state PET signal implies functional co-activation. Overall, these results firmly establish the DMN as a closely interconnected network relatively functionally isolated from other cortical regions of the brain, confirming hypothesis (a).
Importantly, the same clustering approaches (Greicius et al., 2003; Passow et al., 2015) also showed that, whilst PCC has higher resting-state FC with the posterior sub-network of DMN (PCC/precuneus and TPJ), vmPFC is more strongly correlated with the anterior midline cortical regions (vmPFC, dmPFC, and ACC), outlining the two putative sub-networks which may be functionally different. The same two DMN sub-networks can be identified in the FC during fixation and a rapid-response visuomotor task, and this subdivision is functionally significant since the connectivity between these two subnetworks is reported to be associated with enhanced performance in the task (Vatansever et al., 2015). Thanks to recent advancements in high-resolution MRI, the structures of DMN subnetworks can be further confirmed by the tracking of axons with diffusion tensor imaging (DTI), which infers the axon direction from the higher diffusivity of protons along than across the axonal membrane (Basser, 1995). Main white matter tracts within the DMN, identified by DTI, include a major posterior network connecting bilateral PCC, precuneus, medial, and lateral temporal lobe, the genu of corpus callosum connecting bilateral mPFC, and two major tracts (cingulum and superior fronto-occipital fasciculus [SFOF]) connecting the anterior and posterior parts of DMN (Teipel et al., 2010; Van Den Heuvel et al., 2009; van Oort et al., 2014). These results clearly support the subdivision of DMN into two bilateral sub-networks (hypothesis (b)) whose function will be further analysed in the next section.
DMN connectivity in NHPs Since chimpanzees are not commonly used for preclinical research and have already shown highly similar task-induced activation patterns to humans, most studies of NHP DMN are devoted to the rhesus macaque. The first study to analyse the resting-state coherence of macaques on a network level used independent component analysis and identified four closely related clusters resembling the human DMN, including two overlapping clusters around the PCC, one around TPJ/MTC, and the other near medial parts of dlPFC (A8b/A46D, only slightly lateral to the human DMN; Hutchison et al., 2011). However, the PCC/precuneus cluster included the adjacent secondary somatosensory cortex (S2) and thus showed strong positive correlations with the task-positive primary somatosensory and primary motor cortices, a phenomenon incongruent with the findings with humans (Greicius et al., 2003; Passow et al., 2015). Conversely, though studies using the seed-based approach (Mantini et al., 2011; Margulies et al., 2009) and a novel diffusion embedding analysis strategy (Margulies et al., 2016) supported the general locations of DMN components, the clusters they identified are least connected to the primary areas. Compared to Hutchison and colleagues’ (2011) approach, the latter two studies excluded premotor and S2 cortices and included large parts of vmPFC, the resultant DMN homologue covering regions almost identical to that of humans. One factor for such divergent results might be the different pre-processing paradigms for raw fMRI data, which might affect the correlation between neighbouring brain regions. Despite these disparities, the agreement on the gross topography of the DMN, combined with its task-negative behaviour (Mantini et al., 2011), supports the anatomical homology between human and macaque DMNs. Findings from fluorescent neuron tracing studies further support the existence of a highly interconnected network amongst PCC, TPJ, precuneus, ACC, dlPFC (Parvizi et al., 2006), centrolateral and pulvinar nuclei of the thalamus (Gamberini et al., 2020) provide even stronger support for the existence of an independent DMN homologue (hypothesis (a)) in macaques with similar subcortical projections, although projections from the posterolateral thalamus nucleus to the DMN are not matched in humans.
The internal parcellation within the DMN is generally consistent with the two-component model in humans, with one centred around the medial dlPFC and the other centred around PCC, but surprisingly the TPJ region in macaques is more closely related to dlPFC than the posteromedial parts of the DMN despite being anatomically closer to the latter (Mantini et al., 2011). One potentially feasible explanation for this discrepancy is the different axon tract development in macaques: whereas the anterior-posterior connection within the human DMN mainly involves the cingulum and SFOF through the temporal lobe (Teipel et al., 2010), SFOF is not as well-developed in macaques and its function is instead likely performed by the inferior fronto-occipital fasciculus (IFOF) through the temporal lobe to TPJ; on the other hand, the forceps major tract system that connects most of the posteromedial and posterolateral cortices appears less extensive than in humans (Feng et al., 2017). Apart from that, the well-developed frontal decussating tract through the genu (ibid.) and dense connections within the posteromedial cortex (Parvizi et al., 2006) support the high degree of internal integrity within the frontal-temporal and posteromedial DMN sub-networks of macaques (hypothesis (b)), which agree with the overall DMN sub-division in humans.
Though there have been few studies investigating the marmoset DMN homologue, initial fMRI-based results show fairly similar results to macaques, with the DMN relatively isolated from other brain networks (hypothesis (a)), consisting of one main component around PCC, precuneus, and medial posterior parietal cortex, and another main component involving MTC, dmPFC, and medial parts of dlPFC (A46D, also slightly lateral to the human homologue; hypothesis (b)). However, one marked disparity with macaques and humans is the low FC between vmPFC and other putative DMN component regions, with A45 in lateral orbitofrontal cortex (lOFC) more involved in the frontotemporal DMN-homologue sub-network instead (Belcher et al., 2013; Hori et al., 2022; Liu et al., 2019). These results can be further confirmed by a battery of retrograde-tracing studies which confirmed extensive reciprocal connections amongst PCC, dmPFC, TPJ, precuneus, and showed little connection with vmPFC (Buckner & Margulies, 2019; Majka et al., 2020; Majka et al., 2016). Subcortical DMN connectivity as revealed by resting-state fMRI showed congruent FC with the midline thalamic nuclei but also strong connectivity with the superior colliculus which is not found in humans or macaques (Hori et al., 2020; Liu et al., 2019).
In conclusion, though the existence of a DMN homologue and its subdivision into anterior and posterior sub-networks is common to both model NHPs and humans, the following developments are likely apomorphic to the common ancestors of humans and chimpanzees: (a) medial shift of the frontal DMN hub; (b) enhanced frontoparietal connectivity, likely by means of increased development of the dorsal fronto-occipital pathway. The subcortical connectivity of DMN also diverged greatly among primate species which might limit clinical translation of relevant studies.
Default-like network in rodents Despite the difficulty in measuring task-related brain activity in rodents, the anatomical conservation of the DMN component regions suggests the possibility of a functional homologue of DMN in rodents, which has been extensively investigated in resting-state fMRI studies. Results generally agree upon a highly conserved network centred around the retrosplenial cortex (RSP), but its components may vary greatly with respect to the analysis methods, animal wakefulness, and the particular anaesthetic used. Upadhyay and colleagues (2011) also identified the habituation of the awake animal to the scanner environment, a key factor affecting the resting-state connectivity, with many default-like behaviours only emerging after habituation, including the ACC-RSP, ACC-thalamic, ACC-temporo-association (TEA, the homologue of human TPJ) connections. Despite these varying factors, the existence of a closely connected network extending through the cingulate and bilaterally from RSP is generally agreed (see Figure 1), which is consistent with findings from primates. However, unlike in primate species, the DMN identified in many such studies also include wide ranges of primary sensory and motor areas, mostly in the visual cortex but also the S1/M1 and auditory areas which are known to be activated by sensory stimulations (Bigelow et al., 2022; C. Lee et al., 2022; C. C. Y. Lee et al., 2020; J. Y. Li et al., 2021). One explanation for this anomaly is the enhancement of temporal coherence of blood-oxygen-level-dependent signals by adjacency or shared vasculature. Indeed, the most robust network hub as shown in these studies (RSP, ACC) show some reciprocal neuron projections and dense projections within the posteromedial cortex as demonstrated by retrograde tracing, whereas projection density between these areas and the primary areas are very low (Oh et al., 2014). However, several other connections implicated in the primate DMN, including TPJ-PCC (rodent TEA-RSP), and PFC-MTC, are also not found in retrograde tracing (ibid.). Hence, the functional coupling between rodent RSP and wide ranges of the cortex (including primary areas) is likely indirectly mediated through subcortical structures, most likely the thalamus, which relay neural signals to diffuse cortical areas. High connectivity between the DMN and midline thalamus, hippocampus, and superior colliculus (Hsu et al., 2016) might support this relay theory. The reduction in cortico-cortical connectivity might constitute a major cross-species gap between the rodent and primate DMN. Despite this gap, the existence of a DMN homologue along the midline cortical areas (hypothesis (a)) is highly likely given the highly conserved connectivity pattern.
Flattened Representation of the Rodent DMN From Different Study Methods
The quest into the internal organisation of the rodent DMN homologue further filled in the cross-species gap by discovering its anterior/posterior subdivision, highly similar to that of primate species (hypothesis (b)). Whilst posteromedial structures and secondary visual (cf. primate precuneus) areas and frontal medial cortices (ACC and prelimbic/infralimbic, cf. primate vmPFC; Vogt & Paxinos, 2014) are consensually assigned to distinct clusters, there are disputes regarding the cluster assignment of TEA (Cui et al., 2018; Hsu et al., 2016; Lu et al., 2012). Whilst both clusters are supported by dense regional projections, TEA does not show significant projection to either cluster anatomically (Oh et al., 2014), so large cross-study variation may result from indirect connections between the hubs and TEA. One of the stronger indirect pathways to TEA projects from ACC to lOFC (ibid.) and from lOFC to TEA (Hoover & Vertes, 2011). Since co-activation of ACC and TEA relayed via lOFC may lead to the development of a direct frontotemporal projection, this pathway in rodents is likely homologous to the frontotemporal DMN subnetwork in marmosets, which also includes lOFC and MTC.
In conclusion, there is likely a DMN homologue in rodents with similar activity, connectivity, and anterior-posterior subdivision to primates, but with reduced cortico-cortical connectivity. The anatomical evolution of DMN likely begins with the development of direct frontotemporal projections (as illustrated by marmosets) in primate species, followed by the development of fronto-parieto-occipital pathways (SFOF and cingulum), which likely happened when Old World monkeys diverged. The latter pathway in the process then gains in connection strength, resulting in the highly continuous DMN in humans today. The reduced need for relays via subcortical nuclei is likely a factor in the remodelling of subcortical connections of DMN. Cortical expansion and the accompanying increase in local connections is also a marked development in DMN evolution.
Attention Three studies so far have suggested that the NHP DMN is responsible for the shift of attention during cognitive tasks. Premereur and colleagues (2018) trained rhesus macaques to perform a visually guided eye movement task and a visually guided arm movement task, where the target cues differ only in colour (monkey focuses on the same red fixation cross, a green target directs eye movement, and a blue target instructs arm movement). Compared to “stay” situations, where monkeys do two tasks of the same kind successively, multiple areas of the frontotemporal sub-network of DMN are more activated when monkeys switch in either direction between eye movement and arm movement tasks, including medial dlPFC (including frontal eye field), lOFC, ACC, and MTC. Importantly, increased event-related activation of the same areas is observed for macaques returning from either task to fixation, compared to continuous fixation, indicating that these effects are likely task-nonspecific. Increased frontotemporal DMN activity was also implicated in covert attention shifts in Arsenault and colleagues’ study (2018), who derived a sensorimotor task where macaques are exposed to a pair of relevant/irrelevant stimuli on the left/right sides of the screen when fixated to the centre of the display. When the relevant (but not irrelevant) stimulus dims, the macaque can interrupt a light beam to get a reward. The random timing of dimming events requires macaques to maintain covert attention (attention without observable fixation) on the relevant target to maximise reward. Event-related fMRI recordings show an increased activation in the frontotemporal DMN subnetwork (ACC, lOFC, medial dlPFC, ACC, TPJ) and PPC in shifts of covert attention in either direction between L/R compared to “stays” of covert attention. These results are highly consistent with human cognitive control studies which also show task-switching-associated activations in human mPFC, medial dlPFC, ACC, MTC, TPJ, and PPC (Dibbets et al., 2010; Lemire-Rodger et al., 2019). In contrast, Hayden and colleagues (2010) observed decreased PCC activity in preparation for task switches, with the type of task (visual attention or working memory) indicated by an advance cue. However, it should be noted that the anticipated (proactive) task switching in this study differs from the reactive task switching in the above studies, which might account for the difference.
Importantly, deficits in task switching is a marked symptom of ADHD (Cepeda et al., 2000; King et al., 2007; Luna-Rodriguez et al., 2018), and aberrant activation patterns are observed in ADHD patients in both anticipated (Cubillo et al., 2010) and reactive (Dibbets et al., 2010) task-switching paradigms, including aberrant deactivation in OFC, striatum and thalamus, activation of precuneus, and reduced local homogeneity in cingulate activity. As these regions largely overlap with the DMN, especially the cingulate region, which agree with the frontoparietal transition of the NHP frontotemporal DMN subnetwork, the high degree of functional conservation of the DMN in macaque attention functions would open up wide possibilities for preclinical studies.
Social interactions Many DMN component regions have been found to be associated with social functions in NHPs, especially in monitoring the actions of others. One innovative study design by Ninomiya and colleagues (2020) pairs two macaques together who switch between actor and observer roles; each macaque needs to plan its own action of button-pressing based on whether the partner’s previous action results in a reward (both receive a reward for correct choices). By measuring single-neuron activities at different loci of the macaque brain during this task, dense populations of neurons showing selective response to partner action and mirror neurons (responding to both self and partner action) were identified in the mPFC (ibid.) and TPJ (Ninomiya et al., 2021), which, importantly, activate preferentially in interactions with real macaques instead of pre-recorded macaque videos and non-living control objects, highlighting potential complex social function underlying this activation pattern. Whilst the coupling between these areas in the role-switching task is not studied, the coincidence between these areas and the proposed frontotemporal DMN subnetwork in macaques indicates a significant role of the DMN in social cognition.
Particularly noteworthy is the close association of the mirror neurons (i.e., neurons activated by observing conspecific behaviour) in human social cognition, which consists of mPFC and ventral parietal cortex (Rizzolatti & Craighero, 2004), with dysfunctions in mirror neurons closely associated with social impairments in autism spectrum disorders (Enticott et al., 2012; Oberman et al., 2005). Indeed, inhibiting the ventral premotor (containing upstream mirror neurons)-mPFC pathway in macaques greatly impaired the macaques’ performance in the role-switch task, especially when the partner made an error (Ninomiya et al., 2020). One plausible explanation is that the DMN compares mirror inputs and existing social memory in tracking the motion of conspecifics (so inhibiting mirror input will also impair social functions), thus calibrating social expectation. This hypothesis seems to gain support from the observation that macaque MTC/TPJ selectively activate in response to unexpected social events (Roumazeilles et al., 2021). Hence, although the distribution of mirror neurons is likely different between macaques (frontotemporal DMN subnetwork) and humans (frontoparietal DMN subnetwork), the overall conserved role of DMN in social functions across primates still makes NHPs a good model for ASD-related studies.
Other social correlates reported for the macaque DMN component regions include face recognition of conspecifics and predators by mPFC (Dinh et al., 2018), forming social preferences by ACC (Basile et al., 2020), and dealing with social status-related stress by mPFC (Howell et al., 2014). Whilst many social behaviours are difficult to translate to NHPs, carefully designed behavioural paradigms are revealing more social functions of the NHP DMN, thus specifying the boundary of cross-species translatability in social cognition.
Ageing Cognitive decline as a result of senescence has been studied both structurally and functionally amongst NHPs, with the posteromedial cortex identified as a hotspot in both functional and structural development in ageing, consistent with the close link between the parietal lobe and AD in humans (for review, see Bruner & Jacobs, 2013). One recent study identified decreased performance in object-recognition working memory and associative learning, but not short-term spatial memory, in aged macaques, and correlated this decline with the degeneration of perineural networks around parvalbumin-expressing neurons in the PCC (Gray et al., 2022), a phenomenon also observed in mice and associated with aberrant firing in RSP of aged mice (Ueno et al., 2019). Importantly, this abnormality in PCC/RSP is shown to be only related to cognitive impairments instead of healthy ageing, as shown by the wide-ranging reduction of PCC/RSP FC in rats with memory impairments compared to age-comparable healthy rats (Ash et al., 2016), and in AD patients compared to healthy humans (Wu et al., 2011; H.-Y. Zhang et al., 2009). Though structural MRI studies found an increase in the volume of the PCC of macaques with age (Alexander et al., 2008; Koo et al., 2012), the loss of perineural networks provides a potential explanation for the functional decline despite volume increase: the firing rate of RSP pyramidal neurons greatly increases in the absence of perineural networks (Ueno et al., 2019), which might both increase local volume and promote the formation of a local circuitry, decreasing FC with other brain regions. However, in contrast to cognitively impaired rodents, AD patients show increased FC between PCC and the left somatosensory/motor areas (H.-Y. Zhang et al., 2009). This evolutionary gap in ageing/cognitive impairment-related behaviour is yet to be further investigated in NHPs.
Whilst there are few studies characterising the behavioural correlates of DMN in aged NHP, structural studies have provided initial evidence for the evolutionary continuity between the ageing process of human and macaque DMN: the conservation of volume in ageing in several regions of the frontoparietal DMN, including ACC, PCC (Alexander et al., 2008; Koo et al., 2012), and PFC (O’Donnell et al., 1999) compared to primary areas are highly consistent with the observation with aged humans (60 years and above; Frangou et al., 2022). One evolutionary gap identified in structural ageing studies is the dramatic decrease in lOFC volume in macaques which is not observed in humans (Alexander et al., 2008). It should however be noticed that lOFC does not constitute the DMN in humans. Hence, the structural similarity in DMN ontogeny between humans and NHP calls for more functional characterisation in aged NHPs.
The brain’s default mode network is perhaps the least functionally interpretable network of the brain, being most distant from the primary sensor/motor areas and showing general deactivation in tasks. As a result, despite wide-ranging DMN-related psychiatric disorders, pre-clinical studies of the DMN are still limited. Deciphering the evolutionary developments of the DMN would provide valuable insights into the translation of pre-clinical studies but also inspire more causal approaches to higher cognitive processes. This article examines the functional and anatomical evolution of the DMN across common clinical model organisms and humans, thus identifying the existence of a DMN homologue across all these species. Milestones in the anatomical evolution are likely the development of the frontotemporal network in primates and the subsequent frontotemporal-to-frontoparietal transition in the ancestors of chimpanzees and humans. Whilst the functional correlates of DMN in NHP are still poorly characterised due to difficulty of task design, initial studies reflect functional patterns generally coincident with this anatomical transition. Hence, more detailed characterisation of animal DMN-related behaviour would be an exciting frontier in research, with the objective of establishing the DMN as a potent clinical target.
 Seed regions are regions whose resting state time series of blood-O2-level-dependent signals are correlated with all other ROIs or all voxels of the brain.
 Aged here refers to more than 22-year-old macaques, equivalent to 66-year-old humans (Tigges et al., 1988).
Is the default mode network the locus of imagination and morality, and what adaptive functions do these psychological faculties serve that distinguish humans from other primates? Eve Selwood-Metcalfe Corpus Christ College, University of Cambridge
The default mode network (DMN) is an important neural network that has been implicated in a range of activities within human, nonhuman primate, cat, and rodent brains. It has consequently been linked to various neuropsychological disorders in humans, such as autism spectrum disorders, ADHD, and dementia. However, it has more recently been suggested that the DMN may be involved in more abstract psychological functions such as imagination, empathy and morality, qualities that have previously been difficult to investigate empirically and which seem to be largely unique to humans. This commentary will therefore aim to examine the potential role of the DMN in imagination and morality, considering their adaptive functions within humans in allowing both internal reflection and aiding social functions which have been crucial in characterising Homo sapiens.
Introduction When the brain is not engaged in perception of the external world and subsequent directed action, it is not resting; it ‘defaults’ to activity in a neural network known as the default mode network (DMN; Carroll, 2020). The DMN consists of a range of brain regions that span from cortical areas—including the medial prefrontal cortex, the posterior cingulate cortex, the lateral parietal and temporal association cortices, and the medial temporal lobes (Andrews-Hanna et al., 2010; Buckner & DiNicola, 2019; Molnar-Szakacs & Uddin, 2013)—to subcortical regions such as the amygdala and hippocampus. The numerous neural regions of the DMN are associated with a range of different activities and functions, from episodic memory to morality and theory of mind among many more. In this way, the DMN can allow the mind to flexibly switch between attending to the external world and focusing on the internal world of imagination and morality as unique psychological facilities of the modern human.
This review will firstly examine how the psychological function of imagination may be to some extent centred in the default mode network and associated areas, evaluating the functional evolutionand adaptive significance of this. It will then consider another proposed role of the DMN regarding morality and empathy, and how DMN abnormalities such as those associated with psychopathy and dementia can further elucidate the morality function of the DMN.
Imagination and the default mode network Within the past decade, imagination has increasingly become the subject of empirical study in regards to the evolution of human cognition and cognitive neuroscience, allowing us to pinpoint such a uniquely human function to distinct regions of the brain as an identifiable set of cognitive components that serve adaptive functions (Carroll, 2020). In this way, imagination proves itself as an important factor in making humans a distinct, dominant species. Imagination can be defined as “the capacity to mentally transcend time, place and/or circumstance to think about what might have been, plan and anticipate the future, create fictional worlds, and consider remote and close alternatives to actual experiences” (Taylor, 2013, p. 791), with the brain inwardly focused on mental representations which are dissociated from the external environment. Such a definition seems to be consistent with the DMN, which has aptly been described as the “imagination network” (Kaufman & Gregoire, 2015, p. xxvii), the neurological locus of imagination. Buckner and colleagues (2008) describe the DMN as critically allowing the brain to simulate alternative perspectives to the present, supporting both self-directed functions such as daydreaming, as well as a more social function regarding imagining the inner life of others (i.e., theory of mind; Pace-Schlott, 2013).
Imagination seems to be a defining feature of the human experience, and though there is not conclusive evidence that non-human primates (NHP) are incapable of some degree of imagination, it seems reasonable to assume human imagination is somewhat unparalleled in other animals (Suddendorf, 2013). Humans are able to reflect on their own experiences, creating a life narrative, and imagine the inner lives of others (McAdams, 2015; McLean, 2016), as well as qualifying their imaginative conceptions of the world with moral values and even supernatural forces. When looking at the evolution of brain regions which may have allowed this unique function to arise in humans, there is evidence that as modern Homo sapiens evolved, parietal bulging occurred (Neubauer et al., 2018), increasing the size of inner parietal regions. One such area is the praecuneus, a central node of the DMN which is implicated in mental imagery concerning the self, representing other peoples’ inner minds, and higher creativity (Benedek & Jauk, 2018). Therefore, as the modern brain (and consequently the DMN) was evolving, Homo sapiens were becoming behaviourally modern and thus more imaginative, and this modern human faculty of imagination allowed cognitive and behavioural flexibility unique to humans (Margulies et al., 2016; Schacter, 2018; Schacter et al., 2018). This flexibility is closely linked to the aforementioned creativity, and both are powerful prerequisites for the modernisation of human behaviour.
There are three main processes of imagination that seem to be supported by the DMN: simulation, mental time travel, and perspective taking (or theory of mind), which then provide a basis for more complex imaginative functions such as dreaming, mind-wandering, auto-biographical narratives, counterfactual thinking, fantasising, moral reflection, comprehending narratives, and formulating intentional fictional constructs. Simulation is broadly understood as “mental representation”, essential for all imaginative experience (Buckner et al., 2008; Molnar-Szakacs & Uddin, 2013; Oatley, 2016; Roese & Epstude, 2017; Schacter, 2018; Tamir et al., 2015). Mental time travel is conscious perception of “personal identity as a continuous stream of experience over time” (Carroll, 2020, p. 38), allowing humans to make use of past and present memories to construct imaginative conceptions of the future. This flexible future planning has been proposed as an adaptive force which drives the expansion of human imagination, allowing humans to plan and prepare effectively for future contingencies (Andrews-Hanna, 2012; Buckner, 2012; Schacter, 2018). Finally, perspective taking allows navigation of the human social environment through an imaginative awareness of other people as individual agents with their own stream of consciousness, and their own desires, fears, beliefs, and values. This therefore allows humans to anticipate the thoughts and actions of others, predict their motives, and respond accordingly. The adaptive value of these three processes of imagination are supported by the fact that there are such substantial costs to these functions, yet they still exist within human behaviour. The DMN requires huge amounts of metabolic energy to function, and there are also risks associated with guiding behaviour based on imaginative conceptions of reality, as they can often prove to be wrong. Therefore, imagination seems to be essentially valuable as a human faculty with great adaptive functional power, otherwise it would have been selected against throughout the process of human evolution.
Therefore, imagination as produced in the DMN has adaptive functions which have driven the evolution of the human species and its unique behavioural faculties. Humans are able to utilise their capacity for dreaming, mind-wandering, autobiographical narratives, counterfactual thinking, and so on, to produce a conception of the self, understanding the inner lives of others to satisfy social goals (both aiding cooperative behaviour within communities, as well as competitive manipulation between individuals), creating links between imaginative ideas, and appreciating multiple possible courses of action depending on conceptions of the future. These functions allow the creation of an imaginative inner world that can guide behaviour in the external world.
Morality and the default mode network As with imagination, moral decision making is also an internally-directed function that has been associated with the DMN. We use internal representations of our personal norms and values to formulate beliefs of what is right and wrong, and this contributes to our actions within the external world. Reniers and colleagues (2012) used fMRI to compare moral and non-moral decision-making using scenarios that required deliberate contemplation, examining the relationship between blood-oxygen-level-dependent (BOLD) signal and moral judgement competence, psychopathy, and empathy. They found that there were greater levels of activity in the DMN during moral decision-making, compared to non-moral decision making. High scores on primary psychopathy also correlated with a decreased difference between moral versus non-moral decision-making BOLD activation, suggesting that psychopathy is associated with lower activation of the DMN for moral decisions—i.e., moral decisions are viewed similarly to non-moral decisions. The study concluded that moral decision making demands a greater degree of internally directed processing (through the functioning of the DMN), including processes such as self-referential mental processing and the representation of intentions and feelings. Furthermore, Han (2017) meta-analysed studies investigating the neural mechanism of morality, by similarly comparing neural activity in moral task conditions to that in non-moral task conditions. Focusing on the neural correlates of both moral sensitivity and judgement (two functional components in the Neo-Kohlbergian model of moral functioning), the study showed that brain regions associated with the DMN were significantly more active for morality-related task conditions, compared to non-morality task conditions. These brain regions were also shown to be activated in both moral sensitivity and judgement task conditions, suggesting that the DMN is associated with multiple dimensions of morality. In conclusion, the study suggests that moral functioning is associated with self-related psychological processes occurring in the DMN.
We can understand the association between the DMN and morality by examining the consequences for moral understanding as a result of a breakdown of the DMN, as shown in some cases of psychopathy and dementia.
Pujol and colleagues (2012) used fMRI data to investigate alterations in the neural network supporting moral judgement (assumed to be the DMN) in criminal psychopaths, hypothesising that these alterations are not just confined to inadequate use of networks during moral judgement, but that a wider network breakdown would exist within these psychopaths, with dysfunctional alterations beyond moral dilemma situations. To do this, they measured (1) brain response to moral dilemmas; (2) task-induced deactivation of the network during a cognitive task; and (3) the strength of functional connectivity within the network. Results showed that the network dominating moral judgements (the DMN) was underactive in psychopathic subjects during moral dilemma situations. However, the study also demonstrated that there is a baseline network alteration beyond moral contexts in psychopaths, with a functional disconnection between emotional and cognitive elements that together form moral judgement. Therefore, psychopathy and the associated reduced moral judgement may, at least partly, be a result of a breakdown of the DMN, further elucidating the network’s significance in the psychological capacity of morality.
There is also a breakdown of the DMN and morality in dementia, as extrapolated by Chiong and colleagues (2013), who looked at decreased DMN functional activation during personal moral judgements in frontotemporal dementia patients. Previous research has shown that behavioural variant frontotemporal dementia (bvFTD) is characterised by decreased resting-state functional connectivity within the DMN (Zhou et al., 2010). However, the link between network changes and changes in patient’s behaviour is complex. For example, decision-making in personal moral dilemmas typically activates DMN nodes, yet choice behaviour in such dilemmas is unusually utilitarian in bvFTD (Mendez & Shapira, 2009), with patients being more likely to endorse inflicting harm on an innocent person to save a greater number of people, compared to healthy controls. Using fMRI data, the study showed that healthy control participants exhibited greater activation in nodes of the DMN (praecuneus, medial prefrontal cortex, and angular gyri) in moral personal dilemmas, compared to in nonmoral dilemmas or moral-impersonal dilemmas. bvFTD patients were, however, more likely to endorse utilitarian infringements of personal rights and had decreased praecuneus activation in moral-personal dilemmas, compared to healthy controls. As with psychopathy, the dysfunction of the DMN associated with dementia can be used to demonstrate the role of the DMN in personal moral reasoning.
Conclusion Despite the difficulty researching abstract psychological functions such as imagination and morality, it does seem that at least to some degree both faculties are related to the functioning of the DMN, allowing internal reflective function such as that involved in imagination, relating the external world to internal life, linking past, present and future, and allowing complex creative thought that is largely unique to humans. The DMN also appears important in a wider social context, further demonstrating its functional adaptiveness, as it has a role in understanding the internal minds of others and effectively functioning in a social context.
However, much more work needs to be done to firstly create more empirical and reliable definitions of concepts such as imagination and morality, as understandings of this vary across literature. It is also important to understand that such complex human psychology cannot be limited to one region of the brain, or even one network, and likely involves a range of interconnected webs of activity across the brain. Overall, the DMN does seem to be a significant brain network for a range of human functions.
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