Differential impairment of remembering the past and imagining novel events after thalamic lesions

The future, before, and after: Bayesian and multivariate analyses reveal shared and unique neural mechanisms of imagining and remembering the same unique event

Research shows that the brain regions that subserve our ability to remember the past are also involved in imagining the future. Given this similarity in brain activity, it remains unclear how brain activity distinguishes imagination from memory. In the current work, we scanned participants using functional magnetic resonance imaging before and after they performed a highly unique and elaborate activity wherein they went skydiving for the first time in their lives. Multivariate pattern analysis, Bayesian inference, and a tightly controlled experimental design were used to identify the neural activity that differentiates between memory and imagination of the same events. The results showed that large swaths of the default mode network exhibited identical patterns of activity in recollection and imagination; several frontal areas were involved in imagination (but not in recollection). Representational similarity analysis revealed that the left ventral precuneus exhibited different patterns of memory and imagination. Further examination revealed that this subarea may be especially important for recollection of specific episodes. These results advance our understanding of how the critical distinction between the past and future might be manifested in the brain.

Elucidating the pivotal molecular mechanisms, therapeutic and neuroprotective effects of lithium in traumatic brain injury

INTRODUCTION

Traumatic brain injury (TBI) refers to damage to brain tissue by mechanical or blunt force via trauma. TBI is often associated with impaired cognitive abilities, like difficulties in memory, learning, attention, and other higher brain functions, that typically remain for years after the injury. Lithium is an elementary light metal that is only utilized in salt form due to its high intrinsic reactivity. This current review discusses the molecular mechanisms and therapeutic and neuroprotective effects of lithium in TBI.

METHOD

The "Boolean logic" was used to search for articles on the subject matter in PubMed and PubMed Central, as well as Google Scholar.

RESULTS

Lithium's therapeutic action is extremely complex, involving multiple effects on gene secretion, neurotransmitter or receptor-mediated signaling, signal transduction processes, circadian modulation, as well as ion transport. Lithium is able to normalize multiple short- as well as long-term modifications in neuronal circuits that ultimately result in disparity in cortical excitation and inhibition activated by TBI. Also, lithium levels are more distinct in the hippocampus, thalamus, neo-cortex, olfactory bulb, amygdala as well as the gray matter of the cerebellum following treatment of TBI.

CONCLUSION

Lithium attenuates neuroinflammation and neuronal toxicity as well as protects the brain from edema, hippocampal neurodegeneration, loss of hemispheric tissues, and enhanced memory as well as spatial learning after TBI.

The Role of Beta Oscillations in Mental Time Travel

The brain processes short-interval timing but also allows people to project themselves into the past and the future (i.e., mental time travel [MTT]). Beta oscillations index seconds-long-interval timing (i.e., higher beta power is associated with longer durations). Here, we used parietal transcranial alternating current stimulation (tACS) to investigate whether MTT is also supported by parietal beta oscillations and to test the link between MTT and short intervals. Thirty adults performed a novel MTT task while receiving beta and alpha tACS, in addition to no stimulation. Beta tACS corresponded to a temporal underestimation in past but not in future MTT. Furthermore, participants who overestimated seconds-long intervals also overestimated temporal distances in the past-projection MTT condition and showed a stronger effect of beta tACS. These data provide a unique window into temporal perception, showing how beta oscillations may be a common mechanism for short intervals and MTT.

An asymmetry in past and future mental time travel following vmPFC damage

The role of ventromedial prefrontal cortex (vmPFC) in mental time travel toward the past and the future is debated. Here, patients with focal lesions to the vmPFC and brain-damaged and healthy controls mentally projected themselves to a past, present or future moment of subjective time (self-projection) and classified a series of events as past or future relative to the adopted temporal self-location (self-reference). We found that vmPFC patients were selectively impaired in projecting themselves to the future and in recognizing relative-future events. These findings indicate that vmPFC damage hinders the mental processing of and movement toward future events, pointing to a prominent, multifaceted role of vmPFC in future-oriented mental time travel.

Representation of viewed and recalled film clips in patterns of brain activity in a person with developmental amnesia

As clear memories transport us back into the past, the brain also revives prior patterns of neural activity, a phenomenon known as neural reactivation. While growing evidence indicates a link between neural reactivation and typical variations in memory performance in healthy individuals, it is unclear how and to what extent reactivation is disrupted by a memory disorder. The current study characterizes neural reactivation in a case of amnesia using Multivoxel Pattern Analysis (MVPA). We tested NC, an individual with developmental amnesia linked to a diencephalic stroke, and 19 young adult controls on a functional magnetic resonance imaging (fMRI) task during which participants viewed and recalled short videos multiple times. An encoding classifier trained and tested to identify videos based on brain activity patterns elicited at perception revealed superior classification in NC. The enhanced consistency in stimulus representation we observed in NC at encoding was accompanied by an absence of multivariate repetition suppression, which occurred over repeated viewing in the controls. Another recall classifier trained and tested to identify videos during mental replay indicated normal levels of classification in NC, despite his poor memory for stimulus content. However, a cross-condition classifier trained on perception trials and tested on mental replay trials-a strict test of reactivation-revealed significantly poorer classification in NC. Thus, while NC's brain activity was consistent and stimulus-specific during mental replay, this specificity did not reflect the reactivation of patterns elicited at perception to the same extent as controls. Fittingly, we identified brain regions for which activity supported stimulus representation during mental replay to a greater extent in NC than in controls. This activity was not modeled on perception, suggesting that compensatory patterns of representation based on generic knowledge can support consistent mental constructs when memory is faulty. Our results reveal several ways in which amnesia impacts distributed patterns of stimulus representation during encoding and retrieval.

Deficits in remembering the past and imagining the future in patients with prefrontal lesions

OBJECTIVE

Neuroimaging studies suggest that remembering the past and imagining the future engage a common brain network including several areas of the prefrontal cortex. Although patients with prefrontal damage often are described as blind to the future consequences of their decisions, and inclined to 'live in the here and now', little is known as to how the prefrontal cortex mediates past and future mental time travel.

METHOD

Nine patients with prefrontal lesions and nine healthy controls generated past and future events in response to different time periods. Event transcriptions were scored using the Autobiographical Interview protocol (Levine, Svoboda, Hay, Winocur, & Moscovitch, 2002), which provides a reliable system for categorizing internal (episodic) and external (semantic) information. For each event, participants answered a series of questions to assess self-reported phenomenal characteristics.

RESULTS

Patients with prefrontal lesions exhibited deficits in both remembering past events and imagining future events by generating fewer internal details than controls. This effect of group was larger in the past condition than in the future condition. In contrast, no group differences were seen in the number of external details, which were at the same level for patients and controls for both temporal conditions. There were no group differences in ratings of phenomenal characteristics.

CONCLUSION

Our findings suggest that damage to prefrontal structures adversely affects the retrieval of past and the construction of future events. In particular, prefrontal structures are critical for the production of episodic event-specific details when engaging in past and future mental time travel.

Neural correlates of episodic future thinking impairment in multiple sclerosis patients

BACKGROUND

Recent clinical investigations showed impaired episodic future thinking (EFT) abilities in multiple sclerosis (MS) patients. On these bases, the aim of the current study was to explore the structural and functional correlates of EFT impairment in nondepressed MS patients.

METHOD

Twenty-one nondepressed MS patients and 20 matched healthy controls were assessed with the adapted Autobiographical Interview (AI), and patients were selected on the bases of an EFT impaired score criterion. The 41 participants underwent a functional magnetic resonance imaging (fMRI) session, distinguishing the construction and elaboration phases of the experimental EFT, and the categorical control tests. Structural images were also acquired.

RESULTS

During the EFT fMRI task, increased cerebral activations were observed in patients (relative to healthy controls) within the EFT core network. These neural changes were particularly important during the construction phase of future events and involved mostly the prefrontal region. This was accompanied by an increased neural response mostly in anterior, and also posterior, cerebral regions, in association with the amount of detail produced by patients. In parallel, structural measures corroborated a main positive association between the prefrontal regions' volume and EFT performance. However, no association between the hippocampus and EFT performance was observed in patients, at both structural and functional levels.

CONCLUSION

We have documented significant overlaps between the structural and functional underpinnings of EFT impairment, with a main role of the prefrontal region in its clinical expression in MS patients.

Prospection and the Present Moment: The Role of Episodic Foresight in Intertemporal Choices between Immediate and Delayed Rewards

Humans are capable of imagining future rewards and the contexts in which they may be obtained. Functionally, intertemporal choices between smaller but immediate and larger but delayed rewards may be made without such episodic foresight. However, we propose that explicit simulations of this sort enable more flexible and adaptive intertemporal decision-making. Emotions triggered through the simulation of future situations can motivate people to forego immediate pleasures in the pursuit of long-term rewards. However, we stress that the most adaptive option need not always be a larger later reward. When the future is anticipated to be uncertain, for instance, it may make sense for preferences to shift toward more immediate rewards, instead. Imagining potential future scenarios and assessment of their likelihood and affective consequences allows humans to determine when it is more adaptive to delay gratification in pursuit of a larger later reward, and when the better strategy is to indulge in a present temptation. We discuss clinical studies that highlight when and how the effect of episodic foresight on intertemporal decision-making can be altered, and consider the relevance of this perspective to understanding the nature of self-control.

Into the future with little past: exploring mental time travel in a patient with damage to the mammillary bodies/fornix

OBJECTIVE

Remembering the past and imaging the future are both manifestations of 'mental time travel'. These processes have been found to be impaired in patients with bilateral hippocampal lesions. Here, we examined the question of whether future thinking is affected by other Papez circuit lesions, namely: damage to the mammillary bodies/fornix.

METHOD

Case (SL) was a 43-year-old woman who developed dense anterograde and retrograde amnesia suddenly, as a result of Wernicke-Korsakoff's syndrome. A region of interest volumetric Magnetic resonance imaging (MRI) analysis was performed. We assessed past and future thinking in SL and 11 control subjects of similar age and education with the adapted Autobiographical Interview (AI). Participants also completed a battery of neuropsychological tests.

RESULTS

Volumetric MRI analyses revealed severely reduced fornix and mammillary body volumes, but intact hippocampi. SL showed substantial, albeit temporally graded retrograde memory deficits on the adapted AI. Strikingly, whilst SL could not provide any specific details of events from the past two weeks or past two years and had impaired recall of events from her late 30s, her descriptions of potential future events were normal in number of event details and plausibility.

CONCLUSIONS

This dissociation of past and future events' performance after mammillary body and fornix damage is at odds with the findings of the majority of patients with adult onset hippocampal amnesia. It suggests that these non-hippocampal regions of the Papez circuit are only critical for past event retrieval and not for the generation of possible future events.

Distinct and common cerebral activation changes during mental time travel in relapsing-remitting multiple sclerosis patients

Mental time travel (MTT) entails the ability to mentally travel into autobiographical memory (AM) and episodic future thinking (EFT). While AM and EFT share common phenomenological and cerebral functional properties, distinctive characteristics have been documented in healthy and clinical populations. No report, to our knowledge, has informed on the functional underpinnings of MTT impairment in multiple sclerosis (MS) patients, hence the aim of this work. We studied 22 relapsing-remitting MS patients and 22 matched controls. Participants underwent an AM/EFT assessment using the Autobiographical Interview (Levine et al. 2002), followed by a functional MRI session. The latter consisted in AM and EFT tasks, distinguishing the construction and elaboration phases of events. The results showed impaired performance for AM and EFT in patients, accompanied by increased cerebral activations mostly located in the frontal regions, which extended to the parietal, lateral temporal and posterior regions during AM/EFT tasks, relative to healthy controls. Enhanced brain activations in MS patients were particularly evident during the EFT task and involved the hippocampus, frontal, external temporal, and cingulate regions. The construction phase required greater fronto-parieto-temporal activations in MS patients relative to both healthy controls, and the elaboration phase. Taking together, our results suggested the occurrence of cerebral activation changes in the context of MTT in MS patients, expressed by distinct and common mechanisms for AM and EFT. This study may provide new insights in terms of cerebral activation changes in brain lesion and their application to clinical settings, considering AM/EFT's central role in everyday life.

Cueing the personal future to reduce discounting in intertemporal choice: Is episodic prospection necessary?

How does the ability to imagine detailed future experiences (i.e., episodic prospection) contribute to choices between immediate and delayed rewards? Individuals with amnesia do not show abnormally steep discounting in intertemporal choice, suggesting that neither medial temporal lobe (MTL) integrity nor episodic prospection is required for the valuation of future rewards (Kwan et al. (), Hippocampus, 22:1215-1219; Kwan et al. (2013), J Exp Psychol, 142:1355-1369 2013). However, hippocampally mediated episodic prospection in healthy adults reduces the discounting of future rewards (Peters and Büchel (2010), Neuron, 66:138-148; Benoit et al. (2011), J Neurosci, 31:6771-6779), raising the possibility that MTL damage causes more subtle impairments to this form of decision-making than noted in previous patient studies. Intertemporal choice appears normal in amnesic populations, yet they may be unable to use episodic prospection to adaptively modulate the value assigned to future rewards. To investigate how the extended hippocampal system, including the hippocampus and related MTL structures, contributes to the valuation of future rewards, we compared the performance of six amnesic cases with impaired episodic prospection to that of 20 control participants on two versions of an intertemporal choice task: a standard discounting task, and a cued version in which cues prompted them to imagine specific personal future events temporally contiguous with the receipt of delayed rewards. Amnesic individuals' intertemporal choices in the standard condition were indistinguishable from those of controls, replicating previous findings. Surprisingly, performance of the amnesic cases in the cued condition indicates that amnesia does not preclude flexible modulation of choices in response to future event cues, even in the absence of episodic prospection. Cueing the personal future to modulate decisions appears to constitute a less demanding or a qualitatively different (e.g., personal semantic) form of prospection that is not as sensitive to MTL damage as prospective narrative generation.

Errorless learning of prospective memory tasks: An experimental investigation in people with memory disorders

The term prospective memory (PM) refers to memory for future intentions. PM problems are frequent in people with cognitive impairment and, because they are central to the realisation of many everyday goals, are important in rehabilitation. Event-based PM tasks (EBPM) are environmentally-cued and have primarily mnemonic demands, whereas time-based PM tasks (TBPM) require self-initiated retrieval, and have greater executive demands. Errorless learning (EL) is an encoding method that results in superior retrospective memory compared with “errorful” learning (EF). As this EL advantage (ELA) likely stems from its reduced explicit memory demands, and there is no such advantage for executive tasks, a greater ELA for EBPM than TBPM was predicted. Fourteen adults with neurological memory impairment completed PM tasks under four counterbalanced conditions: EL of EBPM, EL of TBPM, EF of EBPM, and EF of TBPM. A significant ELA was observed for EBPM (d = .63), but not TBPM (d = –.01). These results extend the evidence for EL within cognitive rehabilitation, by showing for the first time that the method can benefit future action in addition to retrospective memory. The clinical implications are also clear: errorless learning techniques may be usefully employed to support completion of day-to-day EBPM tasks.

A current model of neural circuitry active in forming mental images

My aim here is to formulate a compact, intuitively understandable model of neural circuits active in imagination that would be consistent with the current state of knowledge, but that would be simple enough to be able to use for teaching. I argue that such a model should be based on the recent idea of "concept neurons" and circuits of 2 separate loops necessary for recalling mental images and consolidation of memory traces of long-term memory. This paper discusses the role of the hippocampus and temporal lobe, emphasizing the essential importance of recurrent pathways and oscillations occurring in the upper layers of hierarchical neural structures, as well as oscillations in thalamo-cortical loops. The elaborated model helps explain specific processes such as imagining future situations, novel objects, and anticipated action, as well as imagination concerning oneself, which is indispensable for the sense of identity and self-awareness. I attempt to present this compact, simple model of neural circuitry active in imagination by using some intuitive, demonstrative figures.

Associative learning beyond the medial temporal lobe: many actors on the memory stage

Decades of research have established a model that includes the medial temporal lobe, and particularly the hippocampus, as a critical node for episodic memory. Neuroimaging and clinical studies have shown the involvement of additional cortical and subcortical regions. Among these areas, the thalamus, the retrosplenial cortex, and the prefrontal cortices have been consistently related to episodic memory performance. This article provides evidences that these areas are in different forms and degrees critical for human memory function rather than playing only an ancillary role. First we briefly summarize the functional architecture of the medial temporal lobe with respect to recognition memory and recall. We then focus on the clinical and neuroimaging evidence available on thalamo-prefrontal and thalamo-retrosplenial networks. The role of these networks in episodic memory has been considered secondary, partly because disruption of these areas does not always lead to severe impairments; to account for this evidence, we discuss methodological issues related to the investigation of these regions. We propose that these networks contribute differently to recognition memory and recall, and also that the memory stage of their contribution shows specificity to encoding or retrieval in recall tasks. We note that the same mechanisms may be in force when humans perform non-episodic tasks, e.g., semantic retrieval and mental time travel. Functional disturbance of these networks is related to cognitive impairments not only in neurological disorders, but also in psychiatric medical conditions, such as schizophrenia. Finally we discuss possible mechanisms for the contribution of these areas to memory, including regulation of oscillatory rhythms and long-term potentiation. We conclude that integrity of the thalamo-frontal and the thalamo-retrosplenial networks is necessary for the manifold features of episodic memory.

The development of mental scenario building and episodic foresight

Episodic foresight is the future-directed counterpart of episodic memory. It is a sophisticated, potentially uniquely human capacity, with tremendous adaptive consequences. Here we review what is currently known about its development through early childhood. We tackle this from two distinct perspectives. First, we present the first systematic evaluation of the development of purported components of mental scenario building as highlighted by a theater metaphor: the stage, the playwright, the set, the actors, the director, the executive producer, and the broadcaster. We find that, although there are diverse developmental trajectories, by 4 years of age children have acquired the basic cognitive components required to mentally construct specific future events. Second, we examine recent attempts to test children's episodic foresight more directly and find that results are in line with those examining the development of required components. This is not to say that children younger than four have no inkling of upcoming events or that older children have nothing left to learn about constructing the future. Episodic foresight, and its neurocognitive foundations, continues to develop throughout childhood.

Episodic future thinking: linking neuropsychological performance with episodic detail in young and old adults

Episodic future thinking (EFT) has been linked with our ability to remember past events. However, its specific neurocognitive subprocesses have remained elusive. In Experiment 1, a study of healthy older adults was conducted to investigate the candidate subprocesses of EFT. Participants completed a standard EFT cue word task, two memory measures (Verbal Paired Associates I, Source Memory), and two measures of executive function (Trail Making Test, Tower Test). In Experiment 2, healthy young adults also completed an EFT task and neuropsychological measures. The link between neurocognitive measures and five characteristics of EFT was investigated. Specifically, it was found that Source Memory and Trail Making Test performance predicted the episodic specificity of future events in older but not younger adults. Replicating previous findings, older adults produced future events with greater semantic but fewer episodic details than did young adults. These results extend the data and emphasize the importance of the multiple subprocesses underlying EFT.

Losing sight of the future: Impaired semantic prospection following medial temporal lobe lesions

The ability to imagine the future (prospection) relies on many of the same brain regions that support memory for the past. To date, scientific research has primarily focused on the neural substrates of episodic forms of prospection (mental simulation of spatiotemporally specific future events); however, little is known about the neural substrates of semantic prospection (mental simulation of future nonpersonal facts). Of particular interest is the role of the medial temporal lobes (MTLs), and specifically the hippocampus. Although the hippocampus has been proposed to play a key role in episodic prospection, recent evidence suggests that it may not play a similar role in semantic prospection. To examine this possibility, amnesic patients with MTL lesions were asked to imagine future issues occurring in the public domain. The results showed that patients could list general semantic facts about the future, but when probed to elaborate, patients produced impoverished descriptions that lacked semantic detail. This impairment occurred despite intact performance on standard neuropsychological tests of semantic processing and did not simply reflect deficits in narrative construction. The performance of a patient with damage limited to the hippocampus was similar to that of the remaining patients with MTL lesions and amnesic patients' impaired elaboration of the semantic future correlated with their impaired elaboration of the semantic past. Together, these results provide novel evidence from MTL amnesia that memory and prospection are linked in the semantic domain and reveal that the MTLs play a critical role in the construction of detailed, multi-element semantic simulations.

The future of memory: remembering, imagining, and the brain

During the past few years, there has been a dramatic increase in research examining the role of memory in imagination and future thinking. This work has revealed striking similarities between remembering the past and imagining or simulating the future, including the finding that a common brain network underlies both memory and imagination. Here, we discuss a number of key points that have emerged during recent years, focusing in particular on the importance of distinguishing between temporal and nontemporal factors in analyses of memory and imagination, the nature of differences between remembering the past and imagining the future, the identification of component processes that comprise the default network supporting memory-based simulations, and the finding that this network can couple flexibly with other networks to support complex goal-directed simulations. This growing area of research has broadened our conception of memory by highlighting the many ways in which memory supports adaptive functioning.

Lithium reduces BACE1 overexpression, β amyloid accumulation, and spatial learning deficits in mice with traumatic brain injury

Traumatic brain injury (TBI) leads to both acute injury and long-term neurodegeneration, and is a major risk factor for developing Alzheimer's disease (AD). Beta amyloid (Aβ) peptide deposits in the brain are one of the pathological hallmarks of AD. Aβ levels increase after TBI in animal models and in patients with head trauma, and reducing Aβ levels after TBI has beneficial effects. Lithium is known to be neuroprotective in various models of neurodegenerative disease, and can reduce Aβ generation by modulating glycogen synthase kinase-3 (GSK-3) activity. In this study we explored whether lithium would reduce Aβ load after TBI, and improve learning and memory in a mouse TBI model. Lithium chloride (1.5 mEq/kg, IP) was administered 15 min after TBI, and once daily thereafter for up to 3 weeks. At 3 days after injury, lithium attenuated TBI-induced Aβ load increases, amyloid precursor protein (APP) accumulation, and β-APP-cleaving enzyme-1 (BACE1) overexpression in the corpus callosum and hippocampus. Increased Tau protein phosphorylation in the thalamus was also attenuated after lithium treatment following TBI at the same time point. Notably, lithium treatment significantly improved spatial learning and memory in the Y-maze test conducted 10 days after TBI, and in the Morris water maze test performed 17-20 days post-TBI, in association with increased hippocampal preservation. Thus post-insult treatment with lithium appears to alleviate the TBI-induced Aβ load and consequently improves spatial memory. Our findings suggest that lithium is a potentially useful agent for managing memory impairments after TBI or other head trauma.