Challenging the classical distinction between long-term and short-term memory: reconsidering the role of the hippocampus

Scale-invariant Characteristics of Forgetting: Toward a Unifying Account of Hippocampal Forgetting across Short and Long Timescales

After over 100 years of relative silence in the cognitive literature, recent advances in the study of the neural underpinnings of memory-specifically, the hippocampus-have led to a resurgence of interest in the topic of forgetting. This review draws a theoretically driven picture of the effects of time on forgetting of hippocampus-dependent memories. We review evidence indicating that time-dependent forgetting across short and long timescales is reflected in progressive degradation of hippocampal-dependent relational information. This evidence provides an important extension to a growing body of research accumulated in recent years, showing that-in contrast to the once prevailing view that the hippocampus is exclusively involved in memory and forgetting over long timescales-the role of the hippocampus also extends to memory and forgetting over short timescales. Thus, we maintain that similar rules govern not only remembering but also forgetting of hippocampus-dependent information over short and long timescales.

Working Memory in Alzheimer's Disease and Parkinson's Disease

Working memory impairments are frequently observed in patients with Alzheimer's disease (AD) and Parkinson's disease (PD). Recent research suggests that the mechanisms underlying these deficits might be dissociable using sensitive tasks, specifically those that rely on the reproduction of the exact quality of features held in memory.In patients with AD, working memory impairments are mainly due to an increase in misbinding errors. They arise when patients misremember which features (e.g., color, orientation, shape, and location) belong to different objects held in memory. Hence, they erroneously report features that belong to items in memory other than the one they are probed on. This misbinding of features that belong to different objects in memory can be considered a form of interference between stored items. Such binding errors are evident even in presymptomatic individuals with familial AD (due to gene mutations) who do not have AD yet. Overall, these findings are in line with the role of the medial temporal lobes, and specifically the hippocampus, in retention of feature bindings, regardless of retention duration, i.e., in both short- or long-term memory.Patients with PD, on the other hand, do not show increased misbinding. Their working memory deficits are associated with making more random errors or guesses. These random responses are not modulated by manipulations of their dopaminergic medication and hence may reflect involvement of non-dopaminergic neurotransmitters in this deficit. In addition, patients with PD demonstrate impairments in gating of information into relevant vs. irrelevant items in memory, a cognitive operation that is modulated by dopaminergic manipulation in line with a frontal executive effect of this neurotransmitter. Thus, although AD and PD are both associated with working memory impairments, these surface manifestations appear to be underpinned by very different mechanisms.

Executive dysfunction is associated with an altered executive control network in pediatric temporal lobe epilepsy

OBJECTIVES

Children with temporal lobe epilepsy (TLE) exhibit executive dysfunction on traditional neuropsychological tests. However, there is limited evidence of neural network alterations associated with this clinical executive dysfunction. The objective of this study was to characterize working memory deficits in children with TLE via activation of the executive control network on functional magnetic resonance imaging (fMRI) and determine the relationships to fMRI behavioral findings and traditional neuropsychological tests.

EXPERIMENTAL DESIGN

Functional magnetic resonance imaging was conducted on 17 children with TLE and 18 healthy control participants (age 8-16 years) while they performed the N-back task in order to assess activation of the executive control network. N-back accuracy, N-back reaction time, and traditional neuropsychological tests (Delis-Kaplan Executive Function System [D-KEFS] color-word interference and card-sort test) were also assessed.

PRINCIPAL OBSERVATIONS

Children with TLE exhibited executive dysfunction on D-KEFS testing, reduced N-back accuracy, and increased N-back reaction time compared with healthy controls; D-KEFS and N-back behavioral findings were significantly correlated. Children with TLE also exhibited significant reduction in activation of the frontal lobe within the executive control network compared to healthy controls. These alterations were significantly correlated with N-back behavioral findings and D-KEFS testing.

CONCLUSIONS

Children with TLE exhibit executive dysfunction, which correlates with executive control network alterations. This lends validity to the theory that the executive control network contributes to working memory function. The findings also indicate that children with TLE have network alterations in nontemporal brain regions.

Visual short-term memory binding deficit in familial Alzheimer's disease

Long-term episodic memory deficits in Alzheimer's disease (AD) are well characterised but, until recently, short-term memory (STM) function has attracted far less attention. We employed a recently-developed, delayed reproduction task which requires participants to reproduce precisely the remembered location of items they had seen only seconds previously. This paradigm provides not only a continuous measure of localization error in memory, but also an index of relational binding by determining the frequency with which an object is misplaced to the location of one of the other items held in memory. Such binding errors in STM have previously been found on this task to be sensitive to medial temporal lobe (MTL) damage in focal lesion cases. Twenty individuals with pathological mutations in presenilin 1 or amyloid precursor protein genes for familial Alzheimer's disease (FAD) were tested together with 62 healthy controls. Participants were assessed using the delayed reproduction memory task, a standard neuropsychological battery and structural MRI. Overall, FAD mutation carriers were worse than controls for object identity as well as in gross localization memory performance. Moreover, they showed greater misbinding of object identity and location than healthy controls. Thus they would often mislocalize a correctly-identified item to the location of one of the other items held in memory. Significantly, asymptomatic gene carriers - who performed similarly to healthy controls on standard neuropsychological tests - had a specific impairment in object-location binding, despite intact memory for object identity and location. Consistent with the hypothesis that the hippocampus is critically involved in relational binding regardless of memory duration, decreased hippocampal volume across FAD participants was significantly associated with deficits in object-location binding but not with recall precision for object identity or localization. Object-location binding may therefore provide a sensitive cognitive biomarker for MTL dysfunction in a range of diseases including AD.

tES Stimulation as a Tool to Investigate Cognitive Processes in Healthy Individuals

Abstract. This paper is aimed at providing an introduction to up-to-date noninvasive brain stimulation tools that have been successful in modulating higher-level cognitive functions in healthy individuals. The current review focuses on transcranial electrical stimulation (tES) studies aiming to explore cognitive models from an experimental rather than clinical viewpoint. It focuses primarily on major advances in language, working memory, learning, response inhibition, and other executive functions in healthy individuals, and the use of different methods of electrical brain stimulation such as transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), and transcranial random noise stimulation (tRNS). The final section summarizes the scientific novelty of the reviewed papers and discusses the possible roles of brain stimulation in future experimental research and clinical applications.

Thalamo-Cortical Disruption Contributes to Short-Term Memory Deficits in Patients with Medial Temporal Lobe Damage

Short-term (STM) and long-term memory (LTM) have largely been considered as separate brain systems reflecting fronto-parietal and medial temporal lobe (MTL) functions, respectively. This functional dichotomy has been called into question by evidence of deficits on aspects of working memory in patients with MTL damage, suggesting a potentially direct hippocampal contribution to STM. As the hippocampus has direct anatomical connections with the thalamus, we tested the hypothesis that damage to thalamic nuclei regulating cortico-cortical interactions may contribute to STM deficits in patients with hippocampal dysfunction. We used diffusion-weighted magnetic resonance imaging-based tractography to identify anatomical subdivisions in patients with MTL epilepsy. From these, we measured resting-state functional connectivity with detailed cortical divisions of the frontal, temporal, and parietal lobes. Whereas thalamo-temporal functional connectivity reflected LTM performance, thalamo-prefrontal functional connectivity specifically predicted STM performance. Notably, patients with hippocampal volume loss showed thalamic volume loss, most prominent in the pulvinar region, not detected in patients with normal hippocampal volumes. Aberrant thalamo-cortical connectivity in the epileptic hemisphere was mirrored in a loss of behavioral association with STM performance specifically in patients with hippocampal atrophy. These findings identify thalamo-cortical disruption as a potential mechanism contributing to STM deficits in the context of MTL damage.

Battery powered thought: enhancement of attention, learning, and memory in healthy adults using transcranial direct current stimulation

This article reviews studies demonstrating enhancement with transcranial direct current stimulation (tDCS) of attention, learning, and memory processes in healthy adults. Given that these are fundamental cognitive functions, they may also mediate stimulation effects on other higher-order processes such as decision-making and problem solving. Although tDCS research is still young, there have been a variety of methods used and cognitive processes tested. While these different methods have resulted in seemingly contradictory results among studies, many consistent and noteworthy effects of tDCS on attention, learning, and memory have been reported. The literature suggests that although tDCS as typically applied may not be as useful for localization of function in the brain as some other methods of brain stimulation, tDCS may be particularly well-suited for practical applications involving the enhancement of attention, learning, and memory, in both healthy subjects and in clinical populations.

Binding deficits in memory following medial temporal lobe damage in patients with voltage-gated potassium channel complex antibody-associated limbic encephalitis

Some prominent studies have claimed that the medial temporal lobe is not involved in retention of information over brief intervals of just a few seconds. However, in the last decade several investigations have reported that patients with medial temporal lobe damage exhibit an abnormally large number of errors when required to remember visual information over brief intervals. But the nature of the deficit and the type of error associated with medial temporal lobe lesions remains to be fully established. Voltage-gated potassium channel complex antibody-associated limbic encephalitis has recently been recognized as a form of treatable autoimmune encephalitis, frequently associated with imaging changes in the medial temporal lobe. Here, we tested a group of these patients using two newly developed visual short-term memory tasks with a sensitive, continuous measure of report. These tests enabled us to study the nature of reporting errors, rather than only their frequency. On both paradigms, voltage-gated potassium channel complex antibody patients exhibited larger errors specifically when several items had to be remembered, but not for a single item. Crucially, their errors were strongly associated with an increased tendency to report the property of the wrong item stored in memory, rather than simple degradation of memory precision. Thus, memory for isolated aspects of items was normal, but patients were impaired at binding together the different properties belonging to an item, e.g. spatial location and object identity, or colour and orientation. This occurred regardless of whether objects were shown simultaneously or sequentially. Binding errors support the view that the medial temporal lobe is involved in linking together different types of information, potentially represented in different parts of the brain, regardless of memory duration. Our novel behavioural measures also have the potential to assist in monitoring response to treatment in patients with memory disorders, such as those with voltage-gated potassium channel complex antibody limbic encephalitis.

Decoding oscillatory representations and mechanisms in memory

A fundamental goal in memory research is to understand how information is represented in distributed brain networks and what mechanisms enable its reactivation. It is evident that progress towards this goal will greatly benefit from multivariate pattern classification (MVPC) techniques that can decode representations in brain activity with high temporal resolution. Recently, progress along these lines has been achieved by applying MVPC to neural oscillations recorded with electroencephalography (EEG) and magnetoencephalography (MEG). We highlight two examples of methodological approaches for MVPC of EEG and MEG data that can be used to study memory function. The first example aims at understanding the dynamic neural mechanisms that enable reactivation of memory representations, i.e., memory replay; we discuss how MVPC can help uncover the physiological mechanisms underlying memory replay during working memory maintenance and episodic memory. The second example aims at understanding representational differences between various types of memory, such as perceptual priming and conscious recognition memory. We also highlight the conceptual and methodological differences between these two examples. Finally, we discuss potential future applications for MVPC of EEG/MEG data in studies of memory. We conclude that despite its infancy and existing methodological challenges, MVPC of EEG and MEG data is a powerful tool with which to assess mechanistic models of memory.

Neural correlates of working memory in Temporal Lobe Epilepsy--an fMRI study

It has traditionally been held that the hippocampus is not part of the neural substrate of working memory (WM), and that WM is preserved in Temporal Lobe Epilepsy (TLE). Recent imaging and neuropsychological data suggest this view may need revision. The aim of this study was to investigate the neural correlates of WM in TLE using functional MRI (fMRI). We used a visuo-spatial 'n-back' paradigm to compare WM network activity in 38 unilateral hippocampal sclerosis (HS) patients (19 left) and 15 healthy controls. WM performance was impaired in both left and right HS groups compared to controls. The TLE groups showed reduced right superior parietal lobe activity during single- and multiple-item WM. No significant hippocampal activation was found during the active task in any group, but the hippocampi progressively deactivated as the task demand increased. This effect was bilateral for controls, whereas the TLE patients showed progressive unilateral deactivation only contralateral to the side of the hippocampal sclerosis and seizure focus. Progressive deactivation of the posterior medial temporal lobe was associated with better performance in all groups. Our results suggest that WM is impaired in unilateral HS and the underlying neural correlates of WM are disrupted. Our findings suggest that hippocampal activity is progressively suppressed as the WM load increases, with maintenance of good performance. Implications for understanding the role of the hippocampus in WM are discussed.