The Aging Navigational System

The contribution of visual attention and declining verbal memory abilities to age-related route learning deficits

Our ability to learn unfamiliar routes declines in typical and atypical ageing. The reasons for this decline, however, are not well understood. Here we used eye-tracking to investigate how ageing affects people's ability to attend to navigationally relevant information and to select unique objects as landmarks. We created short routes through a virtual environment, each comprised of four intersections with two objects each, and we systematically manipulated the saliency and uniqueness of these objects. While salient objects might be easier to memorise than non-salient objects, they cannot be used as reliable landmarks if they appear more than once along the route. As cognitive ageing affects executive functions and control of attention, we hypothesised that the process of selecting navigationally relevant objects as landmarks might be affected as well. The behavioural data showed that younger participants outperformed the older participants and the eye-movement data revealed some systematic differences between age groups. Specifically, older adults spent less time looking at the unique, and therefore navigationally relevant, landmark objects. Both young and older participants, however, effectively directed gaze towards the unique and away from the non-unique objects, even if these were more salient. These findings highlight specific age-related differences in the control of attention that could contribute to declining route learning abilities in older age. Interestingly, route-learning performance in the older age group was more variable than in the young age group with some older adults showing performance similar to the young group. These individual differences in route learning performance were strongly associated with verbal and episodic memory abilities.

The potential of virtual reality for spatial navigation research across the adult lifespan

Older adults often experience serious problems in spatial navigation, and alterations in underlying brain structures are among the first indicators for a progression to neurodegenerative diseases. Studies investigating the neural mechanisms of spatial navigation and its changes across the adult lifespan are increasingly using virtual reality (VR) paradigms. VR offers major benefits in terms of ecological validity, experimental control and options to track behavioral responses. However, navigation in the real world differs from navigation in VR in several aspects. In addition, the importance of body-based or visual cues for navigation varies between animal species. Incongruences between sensory and motor input in VR might consequently affect their performance to a different degree. After discussing the specifics of using VR in spatial navigation research across species, we outline several challenges when investigating age-related deficits in spatial navigation with the help of VR. In addition, we discuss ways to reduce their impact, together with the possibilities VR offers for improving navigational abilities in older adults.

Spatial updating deficits in human aging are associated with traces of former memory representations

The ability to update spatial memories is important for everyday situations, such as remembering where we left our keys or parked our car. Although rodent studies have suggested that old age might impair spatial updating, direct evidence for such a deficit in humans is missing. Here, we tested whether spatial updating deficits occur in human aging, whether the learning mode influences spatial updating, and what mnemonic mechanism underlies the presumed deficits. To address these questions, younger and older participants had to indicate the latest location of relocated items, following either incidental or intentional learning. Using eye tracking, we further quantified memory traces of the original and updated locations. We found that older participants were selectively impaired in recalling locations of relocated items. Furthermore, they depicted relatively stronger representations of the original locations, which were correlated with their spatial updating deficits. The findings demonstrate that stronger representations of former spatial contexts can impair spatial updating in aging, a mechanism that can help explain the commonly observed age-related decline in spatial memory.

Increased posterior default mode network activity and structural connectivity in young adult APOE-ε4 carriers: a multimodal imaging investigation

Young adult APOE-ε4 carriers show increased activity in posterior regions of the default mode network (pDMN), but how this is related to structural connectivity is unknown. Thirty young adults (one half of whom were APOE-ε4 carriers; mean age 20 years) were scanned using both diffusion and functional magnetic resonance imaging. The parahippocampal cingulum bundle (PHCB)-which links the pDMN and the medial temporal lobe-was manually delineated in individual participants using deterministic tractography. Measures of tract microstructure (mean diffusivity and fractional anisotropy) were then extracted from these tract delineations. APOE-ε4 carriers had lower mean diffusivity and higher fractional anisotropy relative to noncarriers in PHCB, but not in a control tract (the inferior longitudinal fasciculus). Furthermore, PHCB microstructure was selectively associated with pDMN (and medial temporal lobe) activity during a scene discrimination task known to be sensitive to Alzheimer's disease. These findings are consistent with a lifespan view of Alzheimer's disease risk, where early-life, connectivity-related changes in specific, vulnerable "hubs" (e.g., pDMN) lead to increased neural activity. Critically, such changes may reflect reduced network efficiency/flexibility in APOE-ε4 carriers, which in itself may portend a faster decline in connectivity over the lifespan and ultimately trigger early amyloid-β deposition in later life.

Spatial memory ability during middle age may depend on level of spatial similarity

Spatial memory impairment is well documented in old age; however, less is known about spatial memory during middle age. We examined the performance of healthy young, middle-aged, and older adults on a spatial memory task with varying levels of spatial similarity (distance). On low similarity trials, young adults significantly outperformed middle-aged adults, who significantly outperformed older adults (Ps < 0.05). On high similarity trials, young adults significantly outperformed middle-aged and older adults (Ps < 0.05); however, middle-aged and older adults did not differ. Subtle age-related changes in spatial memory may emerge during middle age, particularly when spatial similarity is high.

Time-of-Day and Age Impact on Memory in Elevated Plus-Maze Test in Rats

The purpose of the present study was to establish the effect of daytime and aging on memory in rats in the Elevated Plus-Maze (EPM) test. Young (2-months) and aged (18-months) male Wistar rats were exposed to the EPM test, at the beginning, mid-time or at the end of the light period. On the acquisition trial, the animals were placed individually at the end of one of the open arms of the EPM and the latency to enter in the enclosed arms was registered (cut-off time 60 s). The test was repeated 24 h later on. A longer latency period to reach the enclosed arm indicated poor retention compared to significantly shorter latencies. There were no significant differences between groups on the acquisition trial. In all tested periods, the latency time on the 24 h retention trial was significantly shorter in the young rats compared to the old ones. Furthermore, in the early and mid-time period of the light period, the young rats showed significantly decreased transfer latency (TL) time on the 24 h retention trial in comparison with the acquisition trial. In the aged rats, the TL time on the 24 h retention trial was significantly longer at the end of the light period, in comparison to the two other testing periods. In conclusion, aging significantly affects memory and the more critical period for memory process in both young and old animals, particularly at the end of the light period of the circadian cycle.

Social housing protects against age-related working memory decline independently of physical enrichment in rats

Longitudinal human studies suggest that as we age, sociality provides protective benefits against cognitive decline. However, little is known about the underlying neural mechanisms. Rodent studies, which are ideal for studying cognition, fail to examine the independent effects of social housing while controlling for physical enrichment in all groups. In this study, rats were socially housed or nonsocially housed throughout their lifespan and tested in the radial arm maze to measure working memory (WM) and reference memory longitudinally at 3 ages. In old age, exclusively, socially housed rats made significantly less WM errors than nonsocially housed rats, while reference memory errors did not differ between groups at any age. Anxiety, as assessed behaviorally and physiologically, could not account for the observed differences in WM. These data provide the first evidence that social enrichment alone can prevent age-related WM deficits in spite of the effects of practice seen in longitudinal designs. Importantly, our model will facilitate future investigations into the mechanisms underlying the neuroprotective benefits of sociability in old age.

Age-associated changes in waking hippocampal sharp-wave ripples

Hippocampal sharp-wave ripples are brief high-frequency (120-250 Hz) oscillatory events that support mnemonic processes during sleep and awake behavior. Although ripples occurring during sleep are believed to facilitate memory consolidation, waking ripples may also be involved in planning and memory retrieval. Recent work from our group determined that normal aging results in a significant reduction in the peak oscillatory frequency and rate-of-occurrence of ripples during sleep that may contribute to age-associated memory decline. It is unknown, however, how aging alters waking ripples. We investigated whether characteristics of waking ripples undergo age-dependent changes. Sharp-wave ripple events were recorded from the CA1 region of the hippocampus in old (n = 5) and young (n = 6) F344 male rats as they performed a place-dependent eyeblink conditioning task. Several novel observations emerged from this analysis. First, although aged rats expressed more waking ripples than young rats during track running and reward consumption, this effect was eliminated, and, in the case of track-running, reversed when time spent in each location was accounted for. Thus, aged rats emit more ripples, but young rats express a higher ripple rate. This likely results from reduced locomotor activity in aged animals. Furthermore, although ripple rates increased as young rats approached rewards, rates did not increase in aged rats, and rates in aged and young animals were not affected by eyeblink conditioning. Finally, although the oscillatory frequency of ripples was lower in aged animals during rest, frequencies in aged rats increased during behavior to levels indistinguishable from young rats. Given the involvement of waking ripples in memory retrieval, a possible consequence of slower movement speeds of aged animals is to provide more opportunity to replay task-relevant information and compensate for age-related declines in ripple rate during task performance.

Infection Augments Expression of Mechanosensing Piezo1 Channels in Amyloid Plaque-Reactive Astrocytes

A defining pathophysiological hallmark of Alzheimer's disease (AD) is the amyloid plaque; an extracellular deposit of aggregated fibrillar Aβ1-42 peptides. Amyloid plaques are hard, brittle structures scattered throughout the hippocampus and cerebral cortex and are thought to cause hyperphosphorylation of tau, neurofibrillary tangles, and progressive neurodegeneration. Reactive astrocytes and microglia envelop the exterior of amyloid plaques and infiltrate their inner core. Glia are highly mechanosensitive cells and can almost certainly sense the mismatch between the normally soft mechanical environment of the brain and very stiff amyloid plaques via mechanosensing ion channels. Piezo1, a non-selective cation channel, can translate extracellular mechanical forces to intracellular molecular signaling cascades through a process known as mechanotransduction. Here, we utilized an aging transgenic rat model of AD (TgF344-AD) to study expression of mechanosensing Piezo1 ion channels in amyloid plaque-reactive astrocytes. We found that Piezo1 is upregulated with age in the hippocampus and cortex of 18-month old wild-type rats. However, more striking increases in Piezo1 were measured in the hippocampus of TgF344-AD rats compared to age-matched wild-type controls. Interestingly, repeated urinary tract infections with Escherichia coli bacteria, a common comorbidity in elderly people with dementia, caused further elevations in Piezo1 channel expression in the hippocampus and cortex of TgF344-AD rats. Taken together, we report that aging and peripheral infection augment amyloid plaque-induced upregulation of mechanoresponsive ion channels, such as Piezo1, in astrocytes. Further research is required to investigate the role of astrocytic Piezo1 in the Alzheimer's brain, whether modulating channel opening will protect or exacerbate the disease state, and most importantly, if Piezo1 could prove to be a novel drug target for age-related dementia.

Dexpramipexole enhances hippocampal synaptic plasticity and memory in the rat

Even though pharmacological approaches able to counteract age-dependent cognitive impairment have been highly investigated, drugs improving cognition and memory are still an unmet need. It has been hypothesized that sustaining energy dynamics within the aged hippocampus can boost memory storage by sustaining synaptic functioning and long term potentiation (LTP). Dexpramipexole (DEX) is the first-in-class compound able to sustain neuronal bioenergetics by interacting with mitochondrial F1Fo-ATP synthase. In the present study, for the first time we evaluated the effects of DEX on synaptic fatigue, LTP induction, learning and memory retention. We report that DEX improved LTP maintenance in CA1 neurons of acute hippocampal slices from aged but not young rats. However, we found no evidence that DEX counteracted two classic parameters of synaptic fatigue such as fEPSP reduction or the train area during the high frequency stimulation adopted to induce LTP. Interestingly, patch-clamp recordings in rat hippocampal neurons revealed that DEX dose-dependently inhibited (IC₅₀ 814 nM) the I_A current, a rapidly-inactivating K⁺ current that negatively regulates neuronal excitability as well as cognition and memory processes. In keeping with this, DEX counteracted both scopolamine-induced spatial memory loss in rats challenged in Morris Water Maze test and memory retention in rats undergoing Novel Object Recognition. Overall, the present study discloses the ability of DEX to boost hippocampal synaptic plasticity, learning and memory. In light of the good safety profile of DEX in humans, our findings may have a realistic translational potential to treatment of cognitive disorders.

Global Determinants of Navigation Ability

Human spatial ability is modulated by a number of factors, including age [1-3] and gender [4, 5]. Although a few studies showed that culture influences cognitive strategies [6-13], the interaction between these factors has never been globally assessed as this requires testing millions of people of all ages across many different countries in the world. Since countries vary in their geographical and cultural properties, we predicted that these variations give rise to an organized spatial distribution of cognition at a planetary-wide scale. To test this hypothesis, we developed a mobile-app-based cognitive task, measuring non-verbal spatial navigation ability in more than 2.5 million people and sampling populations in every nation state. We focused on spatial navigation due to its universal requirement across cultures. Using a clustering approach, we find that navigation ability is clustered into five distinct, yet geographically related, groups of countries. Specifically, the economic wealth of a nation was predictive of the average navigation ability of its inhabitants, and gender inequality was predictive of the size of performance difference between males and females. Thus, cognitive abilities, at least for spatial navigation, are clustered according to economic wealth and gender inequalities globally, which has significant implications for cross-cultural studies and multi-center clinical trials using cognitive testing.

A novel real-space navigation paradigm reveals age- and gender-dependent changes of navigational strategies and hippocampal activation

OBJECTIVE

To establish a novel multimodal real-space navigation paradigm and define age- and gender-related normative values for navigation performance and visual exploration strategies in space.

METHODS

A group of 30 healthy subjects (mean age 45.9 ± 16.5 years, 16 men) performed a real-space navigation paradigm, requiring allo- and egocentric spatial orientation abilities. Visual exploration behaviour and navigation strategy were documented by a gaze-controlled, head-fixed camera. Allo- and egocentric spatial orientation performance were compared in younger and older subjects (age threshold 50 years) as well as men and women. Navigation-induced changes of regional cerebral glucose metabolism (rCGM) were measured by [¹⁸F]-fluorodeoxyglucose-positron emission tomography in a subgroup of 15 subjects (8 men) and compared across age and gender.

RESULTS

The majority of healthy subjects (73.3%) completed the navigation task without errors. There was no gender difference in navigation performance. Normalized total error rates increased slightly, but significantly with age (r = 0.36, p = 0.05). Analysis of navigation path indicated a significantly reduced use of short cuts in older age (r = 0.44, p = 0.015). Visual exploration analysis revealed that older subjects made significantly more total saccades (r = 0.49, p = 0.006) and search saccades (r = 0.54, p = 0.002) during navigation. All visual exploration parameters were similar in men and women. Navigation-induced rCGM decreased with age in the hippocampus and precuneus and increased in the frontal cortex, basal ganglia and cerebellum. Women showed an increase of rCGM in the left hippocampus and right middle temporal gyrus, men in the superior vermis.

CONCLUSION

Real-space navigation testing was a feasible and sensitive method to depict age-related changes in navigation performance and strategy. Normalized error rates, total mean durations per item and total number of saccades were the most sensitive and practical parameters to indicate deterioration of allocentric navigation strategies and right hippocampal function in age irrespective of gender.

Virtual environments as memory training devices in navigational tasks for older adults

Cognitive training approaches using virtual environments (VEs) might counter age-related visuospatial memory decline and associated difficulties in wayfinding. However, the effects of the visual design of a VE in route learning are not fully understood. Therefore, we created a custom-designed VE optimized for route learning, with adjusted levels of realism and highlighted landmark locations (MixedVE). Herein we tested participants’ route recall performance in identifying direction of turn at the intersection with this MixedVE against two baseline alternatives (AbstractVE, RealisticVE). An older vs. a younger group solved the tasks in two stages (immediate vs. delayed recall by one week). Our results demonstrate that the MixedVE facilitates better recall accuracy than the other two VEs for both age groups. Importantly, this pattern persists a week later. Additionally, our older participants were mostly overconfident in their route recall performance, but the MixedVE moderated this potentially detrimental overconfidence. Before the experiment, participants clearly preferred the RealisticVE, whereas after the experiment, most of the younger, and many of the older participants, preferred the MixedVE. Taken together, our findings provide insights into the importance of tailoring visualization design in route learning with VEs. Furthermore, we demonstrate the great potential of the MixedVE and by extension, of similar VEs as memory training devices for route learning, especially for older participants.

Extrahippocampal Contributions to Age-Related Changes in Spatial Navigation Ability

Age-related decline in spatial navigation is well-known and the extant literature emphasizes the important contributions of a hippocampus-dependent spatial navigation system in mediating this decline. However, navigation is a multifaceted cognitive domain and some aspects of age-related navigational decline may be mediated by extrahippocampal brain regions and/or systems. The current review presents an overview of some key cognitive domains that contribute to the age-related changes in spatial navigation ability, and elucidates such domains in the context of an increased engagement of navigationally relevant extrahippocampal brain regions with advancing age. Specifically, this review focuses on age-related declines in three main areas: (i) allocentric strategy use and switching between egocentric and allocentric strategies, (ii) associative learning of landmarks/locations and heading directions, and (iii) executive functioning and attention. Thus far, there is accumulating neuroimaging evidence supporting the functional relevance of the striatum for egocentric/response strategy use in older adults, and of the prefrontal cortex for mediating executive functions that contribute to successful navigational performance. Notably, the functional role of the prefrontal cortex was particularly emphasized via the proposed relevance of the fronto-locus coeruleus noradrenergic system for strategy switching and of the fronto-hippocampal circuit for landmark-direction associative learning. In view of these putative prefrontal contributions to navigation-related functions, we recommend future spatial navigation studies to adopt a systems-oriented approach that investigates age-related alterations in the interaction between the prefrontal cortex, the hippocampus, and extrahippocampal regions, as well as an individual differences approach that clarifies the differential engagement of prefrontal executive processes among older adults.

Compromised Grid-Cell-like Representations in Old Age as a Key Mechanism to Explain Age-Related Navigational Deficits

A progressive loss of navigational abilities in old age has been observed in numerous studies, but we have only limited understanding of the neural mechanisms underlying this decline [1]. A central component of the brain’s navigation circuit are grid cells in entorhinal cortex [2], largely thought to support intrinsic self-motion-related computations, such as path integration (i.e., keeping track of one’s position by integrating self-motion cues) [3, 4, 5, 6]. Given that entorhinal cortex is particularly vulnerable to neurodegenerative processes during aging and Alzheimer’s disease [7, 8, 9, 10, 11, 12, 13, 14], deficits in grid cell function could be a key mechanism to explain age-related navigational decline. To test this hypothesis, we conducted two experiments in healthy young and older adults. First, in an fMRI experiment, we found significantly reduced grid-cell-like representations in entorhinal cortex of older adults. Second, in a behavioral path integration experiment, older adults showed deficits in computations of self-position during path integration based on body-based or visual self-motion cues. Most strikingly, we found that these path integration deficits in older adults could be explained by their individual magnitudes of grid-cell-like representations, as reduced grid-cell-like representations were associated with larger path integration errors. Together, these results show that grid-cell-like representations in entorhinal cortex are compromised in healthy aging. Furthermore, the association between grid-cell-like representations and path integration performance in old age supports the notion that grid cells underlie path integration processes. We therefore conclude that impaired grid cell function may play a key role in age-related decline of specific higher-order cognitive functions, such as spatial navigation.

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Grid-cell-like representations in human entorhinal cortex are compromised in old age

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This effect is predominantly driven by a lack of representational stability over time

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Path integration ability in old age is associated with grid-cell-like representations

The Mechanism of Cortico-Striato-Thalamo-Cortical Neurocircuitry in Response Inhibition and Emotional Responding in Attention Deficit Hyperactivity Disorder with Comorbid Disruptive Behavior Disorder

The neurocircuitries that constitute the cortico-striato-thalamo-cortical (CSTC) circuit provide a framework for bridging gaps between neuroscience and executive function in attention deficit hyperactivity disorder (ADHD), but it has been difficult to identify the mechanisms for regulating emotional problems from the understanding of ADHD comorbidity with disruptive behavior disorders (DBD). Research based on "cool" and "hot" executive functional theory and the dual pathway models, which are thought of as applied response inhibition and delay aversion, respectively, within the neuropsychological view of ADHD, has shed light on emotional responding before and after decontextualized stimuli, while CSTC circuit-related domains have been suggested to explain the different emotional symptoms of ADHD with or without comorbid DBD. This review discusses the role of abnormal connections in each CSTC circuit, especially in the emotion circuit, which may be responsible for targeted executive dysfunction at the neuroscience level. Thus, the two major domains - abstract thinking (cool) and emotional trait (hot) - trigger the mechanism of onset of ADHD.

Social targets improve body-based and environment-based strategies during spatial navigation

Encoding the position of another person in space is vital for everyday life. Nevertheless, little is known about the specific navigational strategies associated with encoding the position of another person in the wider spatial environment. We asked two groups of participants to learn the location of a target (person or object) during active navigation, while optic flow information, a landmark, or both optic flow information and a landmark were available in a virtual environment. Whereas optic flow information is used for body-based encoding, such as the simulation of motor movements, landmarks are used to form an abstract, disembodied representation of the environment. During testing, we passively moved participants through virtual space, and compared their abilities to correctly decide whether the non-visible target was before or behind them. Using psychometric functions and the Bayes Theorem, we show that both groups assigned similar weights to body-based and environment-based cues in the condition, where both cue types were available. However, the group who was provided with a person as target showed generally reduced position errors compared to the group who was provided with an object as target. We replicated this effect in a second study with novel participants. This indicates a social advantage in spatial encoding, with facilitated processing of both body-based and environment-based cues during spatial navigation when the position of a person is encoded. This may underlie our critical ability to make accurate distance judgments during social interactions, for example, during fight or flight responses.