The functional dissociation of posterior parietal regions during multimodal memory formation

Distinguishable neural circuit mechanisms associated with the clinical efficacy of rTMS in aMCI patients

Repetitive transcranial magnetic stimulation is used in early-stage Alzheimer's disease to slow progression, but heterogeneity in response results in different treatment outcomes. The mechanisms underlying this heterogeneity are unclear. This study used resting-state neuroimaging to investigate the variability in episodic memory improvement from angular gyrus repetitive transcranial magnetic stimulation and tracked the neural circuits involved. Thirty-four amnestic mild cognitive impairment patients underwent angular gyrus repetitive transcranial magnetic stimulation (4 weeks, 20 Hz, 100% resting motor threshold) and were divided into high-response and low-response groups based on minimal clinically important differences in auditory verbal learning test scores. Baseline and pre/post-treatment neural circuit activities were compared. Results indicated that the orbital middle frontal gyrus in the orbitofrontal cortex network and the precuneus in the default mode network had higher local activity in the low-response group. After treatment, changes in local and remote connectivity within brain regions of the orbitofrontal cortex, default mode network, visual network, and sensorimotor network showed opposite trends and were related to treatment effects. This suggests that the activity states of brain regions within the orbitofrontal cortex and default mode network could serve as imaging markers for early cognitive compensation in amnestic mild cognitive impairment patients and predict the aftereffects of repetitive transcranial magnetic stimulation response.

Modulation of hippocampal theta oscillations via deep brain stimulation of the parietal cortex depends on cognitive state

The angular gyrus (AG) and posterior cingulate cortex (PCC) demonstrate extensive structural and functional connectivity with the hippocampus and other core recollection network regions. Consequently, recent studies have explored neuromodulation targeting these and other regions as a potential strategy for restoring function in memory disorders such as Alzheimer's Disease. However, determining the optimal approach for neuromodulatory devices requires understanding how parameters like selected stimulation site, cognitive state during modulation, and stimulation duration influence the effects of deep brain stimulation (DBS) on electrophysiological features relevant to episodic memory. We report experimental data examining the effects of high-frequency stimulation delivered to the AG or PCC on hippocampal theta oscillations during the memory encoding (study) or retrieval (test) phases of an episodic memory task. Results showed selective enhancement of anterior hippocampal slow theta oscillations with stimulation of the AG preferentially during memory retrieval. Conversely, stimulation of the PCC attenuated slow theta oscillations. We did not observe significant behavioral effects in this (open-loop) stimulation experiment, suggesting that neuromodulation strategies targeting episodic memory performance may require more temporally precise stimulation approaches.

Sense of own body shapes neural processes of memory encoding and reinstatement

How is the fundamental sense of one's body, a basic aspect of selfhood, incorporated into memories for events? Disrupting bodily self-awareness during encoding impairs functioning of the left posterior hippocampus during retrieval, which implies weakened encoding. However, how changes in bodily self-awareness influence neural encoding is unknown. We investigated how the sense of body ownership, a core aspect of the bodily self, impacts encoding in the left posterior hippocampus and additional core memory regions including the angular gyrus. Furthermore, we assessed the degree to which memories are reinstated according to body ownership during encoding and vividness during retrieval as a measure of memory strength. We immersed participants in naturalistic scenes where events unfolded while we manipulated feelings of body ownership with a full-body-illusion during functional magnetic resonance imaging scanning. One week later, participants retrieved memories for the videos during functional magnetic resonance imaging scanning. A whole brain analysis revealed that patterns of activity in regions including the right hippocampus and angular gyrus distinguished between events encoded with strong versus weak body ownership. A planned region-of-interest analysis showed that patterns of activity in the left posterior hippocampus specifically could predict body ownership during memory encoding. Using the wider network of regions sensitive to body ownership during encoding and the left posterior hippocampus as separate regions-of-interest, we observed that patterns of activity present at encoding were reinstated more during the retrieval of events encoded with strong body ownership and high memory vividness. Our results demonstrate how the sense of physical self is bound within an event during encoding, which facilitates reactivation of a memory trace during retrieval.

Lesion-behavior mapping indicates a strategic role for parietal substrates of associative memory

Numerous neuroimaging studies indicate that ventral parietal cortex (VPC), especially angular gyrus, plays an important role in episodic memory. However, the nature of the mnemonic processes supported by this region is far from clear. We previously found that stroke lesions in VPC and lateral temporal cortex caused deficits in cued recall of unimodal word pairs and picture pairs, and cross-modal picture-sound pairs, with larger deficits in the cross-modal task. However, those findings leave open the question whether those regions' integrity is necessary for maintenance of associative representations, or for strategic processes required for their recall. We addressed this question using associative recognition versions of those tasks. We additionally manipulated semantic relatedness of the associated memoranda, to assess VPC's involvement in semantic processing in the context of episodic memory. We analyzed performance of 62 first-event, sub-acute phase stroke patients (31 right- and 31 left-hemisphere damage) relative to 65 healthy participants, and employed voxel-based lesion-behavior mapping (VLBM) to identify task-relevant structures. Patients displayed greater false associative recognition of semantically related compared to unrelated recombined pairs. VLBM analysis implicated right lateral temporo-parietal regions in associative recognition deficits in the cross-modal pairs task, specifically for related recombined and new pairs, seemingly because of difficulty overcoming semantic relatedness bias effects on episodic discrimination. In contrast, damage to ventral parietal and lateral temporal cortex was not implicated in memory for unrelated memoranda. We interpret this pattern of lesion-behavior effects as indicating lateral temporo-parietal cortex involvement in strategic, rather than representational, roles in episodic associative memory.

Enriched environments enhance the development of explicit memory in an incidental learning task

Learning, rendered in an implicit (unconscious) or explicit (conscious) way, is a crucial part of our daily life. Different factors, like attention or motivation, influence the transformation from implicit to explicit memory. Via virtual reality a lively and engaging surrounding can be created, whereby motivational processes are assumed to be a vital part of the transition from implicit to explicit memory. In the present study, we tested the impact of an enriched virtual reality compared to two conventional, non-enriched 2D-computer-screen based tasks on implicit to explicit memory transformation, using an audio-visual sequential association task. We hypothesized, that the immersive nature of the VR surrounding enhances the transfer from implicit to explicit memory. Notably, the overall amount of learned sequence pairs were not significantly different between experimental groups, but the degree of awareness was affected by the different settings. However, we observed an increased level of explicitly remembered pairs within the VR group compared to two screen-based groups. This finding clearly demonstrates that a near-natural experimental setting affects the transformation process from implicit to explicit memory.