Exploring Information Flow From Posteromedial Cortex During Visuospatial Working Memory: A Magnetoencephalography Study Part 1

The posteromedial cortex (PMC) is a major hub of the brain's default mode network and is implicated in a broad range of internally driven cognitions, including visuospatial working memory. 

The brain's default mode network (DMN) refers to a specific neural circuit system in the human brain. The DMN is active in the absence of external stimulation, so it is also called the "self-awareness network" and is involved in complex actions such as thinking, remembering, planning, and self-evaluation. A growing body of research shows that the DMN is closely related to memory.

First, the DMN can play a key role in how we remember the past. The researchers found that the DMN was activated when subjects thought of and described previously experienced people, places, and events. This form, called "autobiographical memory," is based on self-awareness.

Secondly, the DMN is closely connected to the "seven-second rule" in the brain. The so-called "seven-second rule" means that people's brains will work automatically and continue for about seven seconds without stimulus input. This process can help the brain optimize its thinking and improve thinking flexibility and creativity. This has a positive effect on improving memory.

Finally, scientists have also found that with less background noise, the DMN can play an important role in how we hear and remember new information. This means that when we try to learn new content, we need to separate ourselves from distractions in our surroundings so that the DMN can process and store the information efficiently.

Taken together, the DMN is closely related to our memory in multiple ways. Encouraging self-awareness and training in deep thinking, introspection, and attention will have a positive impact on DMN development and improved memory abilities. Therefore, we should focus on exercising the brain's default mode network to improve our memory and cognitive abilities. It can be seen that we need to improve memory, and Cistanche deserticola can significantly improve memory because Cistanche deserticola is a traditional Chinese medicinal material that has many unique effects, one of which is to improve memory. The efficacy of minced meat comes from the various active ingredients it contains, including acid, polysaccharides, flavonoids, etc. These ingredients can promote brain health in various ways.

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However, its precise contribution to these cognitive processes remains unclear. Using MEG, we measured PMC activity in healthy human participants (young adults of both sexes) while they performed a visuospatial working memory task. 

Multivariate pattern classification analyses revealed stimulus-related information during encoding and retrieval in a set of a priori-defined cortical ROIs, including prefrontal, occipital, and centrotemporal cortices, in addition to PMC. 

We measured the extent to which this stimulus information was exchanged between areas in an information flow analysis, measuring Granger-causal relationships between areas over time. Consistent with the visual nature of the task, information from the occipital cortex shaped other regions across most epochs. However, the PMC shaped object representations in occipital and prefrontal cortices during visuospatial working memory, influencing the occipital cortex during retrieval and PFC across all task epochs. 

Our findings are consistent with a proposed role for the PMC in the representation of internal content, including remembered information, and in the comparison of external stimuli with remembered material.

Keywords: episodic memory; information flow analysis; MEG; posterior cingulate cortex; visuospatial memory.

Significance Statement

The human brain operates as a collection of highly interconnected regions. Mapping the function of this interconnectivity, as well as the specializations within different regions, is central to understanding the neural processes underlying cognition. 

The posteromedial cortex (PMC) is a highly connected cortical region, implicated in visuospatial working memory, although its precise contribution remains unclear. 

We measured the activity of PMC during a visuospatial working memory task, testing how different regions represented the stimuli, and whether these representations were driven by other cortical regions. 

We found that PMC influenced stimulus information in other regions across all task phases, suggesting that PMC plays a key role in shaping stimulus representations during visuospatial working memory.

Introduction

The posteromedial cortex (PMC) encompasses the posterior cingulate cortex and precuneus and is widely held to support a variety of internally driven forms of cognition (Andrews-Hanna, 2012). 

As a hub of the brain's default mode network, the PMC demonstrates extensive connections with the medial temporal lobe, as well as frontoparietal brain regions associated with cognitive control (Leech et al., 2012). 

fMRI studies delineating subdivisions within the PMC point to distinct patterns of connectivity with other areas during the resting state (Margulies et al., 2009; Bzdok et al., 2015; Kernbach et al., 2018; Khan et al., 2020) as well as Divergent task-related functional connectivity as task difficulty increases (Leech et al., 2011, 2012; Bzdok et al., 2015). 

Given the richness of these connections and the heterogeneity of PMC subregions (Margulies et al., 2009; Leech et al., 2011), the PMC is, perhaps unsurprisingly, implicated across a diverse range of cognitive functions.

Such functions include self-referential processing, visual imagery, translating egocentric to allocentric representations, and modulating internally versus externally directed forms of cognition (for review, see Bzdok et al., 2015).

Despite no direct connections to the sensory cortex, the PMC appears ideally positioned to receive converging sensory-perceptual input, largely visual, to support the integration of visuospatial information (Conti and Irish, 2021). 

These representations can be maintained online and operated on (i.e., working memory) (Kravitz et al., 2011; Hunsaker and Kesner, 2018), or integrated into contextually rich reconstructions of past experiences (i.e., episodic memory) (Lega et al., 2017; Natu et al., 2019). 

A consistent finding in the memory literature is that of significant PMC activation during tasks that require the reinstatement of contextual information (Bird et al., 2015). 

Moreover, activity within PMC subregions has been shown to parametrically scale with the vividness of retrieved information parametrically, suggesting an important role in memory phenomenology. In contrast, other subregions of PMC have been suggested to represent unique configurations of event features (Cooper and Ritchey, 2019). 

Collectively, these studies indicate a central role for the PMC in representing and integrating different types of information in the service of memory (for review, see Ritchey and Cooper, 2020).

Studies exploring the evolving time course of PMC activity across task phases have consistently demonstrated an "encoding/ retrieval flip," where successful remembering is associated with attenuated PMC activity during encoding but increased activity during retrieval (Daselaar et al., 2004, 2009; Huijbers et al., 2012, 2013). 

Importantly, by comparing the locations of voxels driving associations between performance and attenuation, then facilitation, Vannini et al. (2011) demonstrated that the same PMC subregions modulate the encoding/retrieval flip. Patterns of functional connectivity with PMC have also been shown to evolve across memory task phases (e.g., Piccoli et al., 2015). 

For example, theta phase medial temporal lobe-neocortical synchrony reliably predicts the degree of visual imagery recollected during autobiographical memory retrieval, correlating specifically with activity in the precuneus (Fuentemilla et al., 2014), resonating with the proposed role of the PMC in the reinstatement of visuospatial sensory-perceptual details. 

Thus, while there is evidence to suggest information transfer between the PMC and other brain regions during memory performance (Canolty et al., 2006; Fell and Axmacher, 2011; Sauseng et al., 2019), the direction of this information exchange remains poorly understood.

The objective of this study was to leverage the temporal precision of MEG to establish the patterns of information flow between the PMC and other brain regions during visuospatial working memory performance. 

We used a novel measure of Granger-causal information exchange to test for evidence that PMC encodes stimulus attributes within different task phases and to determine whether any such information is transferred from the PMC to other brain regions. 

We predicted that, if connectivity between PMC and other regions reflects information exchange that is crucial for memory performance, we should find evidence that the PMC encodes the remembered stimulus and drives the encoding of this information in other regions.

Materials and Methods

Participants. We collected psychophysical, MEG, and MRI data from 12 participants (10 female, 2 male, age 19-31 years, mean = 23.8 years). Each participant completed the psychophysical experiment in a 1 h session, followed a week later by the MEG experiment in a 2 h session. 

The anatomic MRI images were acquired in a half-hour session on a separate day. One participant withdrew from the MEG study early because of a headache, so their data were excluded. 

All participants were healthy with no history of neurologic and/or psychiatric disorders and provided informed consent. Each participant had normal or corrected-to-normal visual acuity. 

Participant recruitment and experiments were conducted with the approval of the Macquarie University Human Research Ethics Committee and by the Declaration of Helsinki.

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