The hippocampus supports high-precision binding in visual working memory

Aberrant glutamatergic systems underlying impulsive behaviors: Insights from clinical and preclinical research

Impulsivity is a broad construct that often refers to one of several distinct behaviors and can be measured with self-report questionnaires and behavioral paradigms. Several psychiatric conditions are characterized by one or more forms of impulsive behavior, most notably the impulsive/hyperactive subtype of attention-deficit/hyperactivity disorder (ADHD), mood disorders, and substance use disorders. Monoaminergic neurotransmitters are known to mediate impulsive behaviors and are implicated in various psychiatric conditions. However, growing evidence suggests that glutamate, the major excitatory neurotransmitter of the mammalian brain, regulates important functions that become dysregulated in conditions like ADHD. The purpose of the current review is to discuss clinical and preclinical evidence linking glutamate to separate aspects of impulsivity, specifically motor impulsivity, impulsive choice, and affective impulsivity. Hyperactive glutamatergic activity in the corticostriatal and the cerebro-cerebellar pathways are major determinants of motor impulsivity. Conversely, hypoactive glutamatergic activity in frontal cortical areas and hippocampus and hyperactive glutamatergic activity in anterior cingulate cortex and nucleus accumbens mediate impulsive choice. Affective impulsivity is controlled by similar glutamatergic dysfunction observed for motor impulsivity, except a hyperactive limbic system is also involved. Loss of glutamate homeostasis in prefrontal and nucleus accumbens may contribute to motor impulsivity/affective impulsivity and impulsive choice, respectively. These results are important as they can lead to novel treatments for those with a condition characterized by increased impulsivity that are resistant to conventional treatments.

The contribution of short-term memory for sound features to speech-in-noise perception and cognition

Speech-in-noise (SIN) perception is a fundamental ability that declines with aging, as does general cognition. We assess whether auditory cognitive ability, in particular short-term memory for sound features, contributes to both. We examined how auditory memory for fundamental sound features, the carrier frequency and amplitude modulation rate of modulated white noise, contributes to SIN perception. We assessed SIN in 153 healthy participants with varying degrees of hearing loss using measures that require single-digit perception (the Digits-in-Noise, DIN) and sentence perception (Speech-in-Babble, SIB). Independent variables were auditory memory and a range of other factors including the Pure Tone Audiogram (PTA), a measure of dichotic pitch-in-noise perception (Huggins pitch), and demographic variables including age and sex. Multiple linear regression models were compared using Bayesian Model Comparison. The best predictor model for DIN included PTA and Huggins pitch (r2 = 0.32, p < 0.001), whereas the model for SIB included the addition of auditory memory for sound features (r2 = 0.24, p < 0.001). Further analysis demonstrated that auditory memory also explained a significant portion of the variance (28 %) in scores for a screening cognitive test for dementia. Auditory memory for non-speech sounds may therefore provide an important predictor of both SIN and cognitive ability.

Anatomo-functional changes in neural substrates of cognitive memory in developmental amnesia: Insights from automated and manual Magnetic Resonance Imaging examinations

Despite bilateral hippocampal damage dating to the perinatal or early childhood period and severely impaired episodic memory, patients with developmental amnesia continue to exhibit well-developed semantic memory across the developmental trajectory. Detailed information on the extent and focality of brain damage in these patients is needed to hypothesize about the neural substrate that supports their remarkable capacity for encoding and retrieval of semantic memory. In particular, we need to assess whether the residual hippocampal tissue is involved in this preservation, or whether the surrounding cortical areas reorganize to rescue aspects of these critical cognitive memory processes after early injury. We used voxel-based morphometry (VBM) analysis, automatic (FreeSurfer) and manual segmentation to characterize structural changes in the brain of an exceptionally large cohort of 23 patients with developmental amnesia in comparison with 32 control subjects. Both the VBM and the FreeSurfer analyses revealed severe structural alterations in the hippocampus and thalamus of patients with developmental amnesia. Milder damage was found in the amygdala, caudate, and parahippocampal gyrus. Manual segmentation demonstrated differences in the degree of atrophy of the hippocampal subregions in patients. The level of atrophy in CA-DG subregions and subicular complex was more than 40%, while the atrophy of the uncus was moderate (-24%). Anatomo-functional correlations were observed between the volumes of residual hippocampal subregions in patients and selective aspects of their cognitive performance, viz, intelligence, working memory, and verbal and visuospatial recall. Our findings suggest that in patients with developmental amnesia, cognitive processing is compromised as a function of the extent of atrophy in hippocampal subregions. More severe hippocampal damage may be more likely to promote structural and/or functional reorganization in areas connected to the hippocampus. In this hypothesis, different levels of hippocampal function may be rescued following this variable reorganization. Our findings document not only the extent, but also the limits of circuit reorganization occurring in the young brain after early bilateral hippocampal damage.

Effects of Context Changes on Memory Reactivation

While the influence of context on long-term memory (LTM) is well documented, its effects on the interaction between working memory (WM) and LTM remain less understood. In this study, we explored these interactions using a delayed match-to-sample task, where participants (6 males, 16 females) encountered the same target object across six consecutive trials, facilitating the transition from WM to LTM. During half of these target repetitions, the background color changed. We measured the WM storage of the target using the contralateral delay activity in electroencephalography. Our results reveal that task-irrelevant context changes trigger the reactivation of long-term memories in WM. This reactivation may be attributed to content-context binding in WM and hippocampal pattern separation.

Is working memory domain-general or domain-specific?

Given the fundamental role of working memory (WM) in all domains of cognition, a central question has been whether WM is domain-general. However, the term 'domain-general' has been used in different, and sometimes misleading, ways. By reviewing recent evidence and biologically plausible models of WM, we show that the level of domain-generality varies substantially between three facets of WM: in terms of computations, WM is largely domain-general. In terms of neural correlates, it contains both domain-general and domain-specific elements. Finally, in terms of application, it is mostly domain-specific. This variance encourages a shift of focus towards uncovering domain-general computational principles and away from domain-general approaches to the analysis of individual differences and WM training, favoring newer perspectives, such as training-as-skill-learning.

The hippocampus supports precise memory for public events regardless of their remoteness

The necessity of the human hippocampus and surrounding medial temporal lobe structures to semantic memory remains contentious. Impaired semantic memory following hippocampal lesions could arise either due to partially intertwined episodic memories and/or retrograde/anterograde effects. In this study, we tested amnesic individuals with lesions in hippocampus and surrounding medial temporal lobe (n = 7) and age-matched controls (n = 14) on their ability to precisely recall the dates of famous public events that occurred either before (i.e., pre-lifetime) or after participants' birth date (lifetime). We show that deficits in dating precision are greatest for recent lifetime events, consistent with the notion that recent event memory may be particularly intertwined with episodic memory. At the same time, individuals with medial temporal lobe lesions showed more subtle impairments in their ability to date pre-birth and remote lifetime events precisely. Together, these findings suggest that the hippocampus and surrounding medial temporal lobe structures are important for representational precision of semantic memories regardless of their remoteness.

Neural correlates of retrieval success and precision: an fMRI study

Prior studies examining the neural mechanisms underlying retrieval success and precision have yielded inconsistent results. Here, their neural correlates were examined using a memory task that assessed precision for spatial location. A sample of healthy young adults underwent fMRI scanning during a single study-test cycle. At study, participants viewed a series of object images, each placed at a randomly selected location on an imaginary circle. At test, studied images were intermixed with new images and presented to the participants. The requirement was to move a cursor to the location of the studied image, guessing if necessary. Participants then signaled whether the presented image as having been studied. Memory precision was quantified as the angle between the studied location and the location selected by the participant. A precision effect was evident in the left angular gyrus, where BOLD activity covaried across trials with location accuracy. Multi-voxel pattern analysis also revealed a significant item-level reinstatement effect for high-precision trials. There was no evidence of a retrieval success effect in the angular gyrus. BOLD activity in the hippocampus was insensitive to both success and precision. These findings are partially consistent with prior evidence that success and precision are dissociable features of memory retrieval.

Repertoire of timescales in uni - and transmodal regions mediate working memory capacity

Working memory (WM) describes the dynamic process of maintenance and manipulation of information over a certain time delay. Neuronally, WM recruits a distributed network of cortical regions like the visual and dorsolateral prefrontal cortex as well as the subcortical hippocampus. How the input dynamics and subsequent neural dynamics impact WM remains unclear though. To answer this question, we combined the analysis of behavioral WM capacity with measuring neural dynamics through task-related power spectrum changes, e.g., median frequency (MF) in functional magnetic resonance imaging (fMRI). We show that the processing of the input dynamics, e.g., the task structure's specific timescale, leads to changes in the unimodal visual cortex's corresponding timescale which also relates to working memory capacity. While the more transmodal hippocampus relates to working memory capacity through its balance across multiple timescales or frequencies. In conclusion, we here show the relevance of both input dynamics and different neural timescales for WM capacity in uni - and transmodal regions like visual cortex and hippocampus for the subject's WM performance.

The cerebellum acts as the analog to the medial temporal lobe for sensorimotor memory

The cerebellum is critical for sensorimotor learning. The specific contribution that it makes, however, remains unclear. Inspired by the classic finding that, for declarative memories, medial temporal lobe structures provide a gateway to the formation of long-term memory but are not required for short-term memory, we hypothesized that, for sensorimotor memories, the cerebellum may play an analogous role. Here we studied the sensorimotor learning of individuals with severe ataxia from cerebellar degeneration. We dissected the memories they formed during sensorimotor learning into a short-term temporally-volatile component, that decays rapidly with a time constant of just 15-20sec and thus cannot lead to long-term retention, and a longer-term temporally-persistent component that is stable for 60 sec or more and leads to long-term retention. Remarkably, we find that these individuals display dramatically reduced levels of temporally-persistent sensorimotor memory, despite spared and even elevated levels of temporally-volatile sensorimotor memory. In particular, we find both impairment that systematically increases with memory window duration over shorter memory windows (<12 sec) and near-complete impairment of memory maintenance over longer memory windows (>25 sec). This dissociation uncovers a new role for the cerebellum as a gateway for the formation of long-term but not short-term sensorimotor memories, mirroring the role of the medial temporal lobe for declarative memories. It thus reveals the existence of distinct neural substrates for short-term and long-term sensorimotor memory, and it explains both newly-identified trial-to-trial differences and long-standing study-to-study differences in the effects of cerebellar damage on sensorimotor learning ability.

Significance Statement

A key discovery about the neural underpinnings of memory, made more than half a century ago, is that long-term, but not short-term, memory formation depends on neural structures in the brain's medial temporal lobe (MTL). However, this dichotomy holds only for declarative memories - memories for explicit facts such as names and dates - as long-term procedural memories - memories for implicit knowledge such as sensorimotor skills - are largely unaffected even with substantial MTL damage. Here we demonstrate that the formation of long-term, but not short-term, sensorimotor memory depends on a neural structure known as the cerebellum, and we show that this finding explains the variability previously reported in the extent to which cerebellar damage affects sensorimotor learning.

Anatomo-functional changes in neural substrates of cognitive memory in developmental amnesia: Insights from automated and manual MRI examinations

Despite bilateral hippocampal damage dating to perinatal or early-childhood period, and severely-impaired episodic memory that unfolds in later childhood, patients with developmental amnesia continue to exhibit well-developed semantic memory across the developmental trajectory. Detailed information on the extent and focality of brain damage in these patients is needed to hypothesize about the neural substrate that supports their remarkable capacity for encoding and retrieval of semantic memory. In particular, we need to assess whether the residual hippocampal tissue is involved in this preservation, or whether the surrounding cortical areas reorganise to rescue aspects of these critical cognitive memory processes after early injury. We used voxel-based morphometry (VBM) analysis, automatic (FreeSurfer) and manual segmentation to characterize structural changes in the brain of an exceptionally large cohort of 23 patients with developmental amnesia in comparison with 32 control subjects. Both the VBM and the FreeSurfer analyses revealed severe structural alterations in the hippocampus and thalamus of patients with developmental amnesia. Milder damage was found in the amygdala, caudate and parahippocampal gyrus. Manual segmentation demonstrated differences in the degree of atrophy of the hippocampal subregions in patients. The level of atrophy in CA-DG subregions and subicular complex was more than 40% while the atrophy of the uncus was moderate (-23%). Anatomo-functional correlations were observed between the volumes of residual hippocampal subregions in patients and selective aspects of their cognitive performance viz, intelligence, working memory, and verbal and visuospatial recall. Our findings suggest that in patients with developmental amnesia, cognitive processing is compromised as a function of the extent of atrophy in hippocampal subregions, such that the greater the damage, the more likely it is that surrounding cortical areas will be recruited to rescue the putative functions of the damaged subregions. Our findings document for the first time not only the extent, but also the limits of circuit reorganization occurring in the young brain after early bilateral hippocampal damage.

Comparing episodic memory outcomes from walking augmented reality and stationary virtual reality encoding experiences

Episodic Memory (EM) is the neurocognitive capacity to consciously recollect personally experienced events in specific spatio-temporal contexts. Although the relevance of spatial and temporal information is widely acknowledged in the EM literature, it remains unclear whether and how EM performance and organisation is modulated by self-motion, and by motor- and visually- salient environmental features (EFs) of the encoding environment. This study examines whether and how EM is modulated by locomotion and the EFs encountered in a controlled lifelike learning route within a large-scale building. Twenty-eight healthy participants took part in a museum-tour encoding task implemented in walking Augmented Reality (AR) and stationary Virtual Reality (VR) conditions. EM performance and organisation were assessed immediately and 48-hours after trials using a Remember/Familiar recognition paradigm. Results showed a significant positive modulation effect of locomotion on distinctive EM aspects. Findings highlighted a significant performance enhancement effect of stairway-adjacent locations compared to dead-end and mid-route stimuli-presentation locations. The results of this study may serve as design criteria to facilitate neurocognitive rehabilitative interventions of EM. The underlying technological framework developed for this study represents a novel and ecologically sound method for evaluating EM processes in lifelike situations, allowing researchers a naturalistic perspective into the complex nature of EM.

Brain-wide human oscillatory local field potential activity during visual working memory

Oscillatory activity in the local field potential (LFP) is thought to be a marker of cognitive processes. To understand how it differentiates tasks and brain areas in humans, we recorded LFPs in 15 adults with intracranial depth electrodes, as they performed visual-spatial and shape working memory tasks. Stimulus appearance produced widespread, broad-band activation, including in occipital, parietal, temporal, insular, and prefrontal cortex, and the amygdala and hippocampus. Occipital cortex was characterized by most elevated power in the high-gamma (100-150 Hz) range during the visual stimulus presentation. The most consistent feature of the delay period was a systematic pattern of modulation in the beta frequency (16-40 Hz), which included a decrease in power of variable timing across areas, and rebound during the delay period. These results reveal the widespread nature of oscillatory activity across a broad brain network and region-specific signatures of oscillatory processes associated with visual working memory.

Exploring stress response's role in executive function impairments among adults with early adverse childhood experiences

Adverse childhood experiences (ACEs) are recognised as precursors to numerous physical and mental health challenges. However, research on their impact on inhibitory control and working memory, particularly among healthy young adults, remains limited. The role played by the stress response as a moderator in these effects is likewise underexplored. Our study addresses this gap by examining cognitive impairments in non-clinical adults with early childhood trauma, specifically trauma before the age of 13 years, and by assessing the influence of the stress response on these effects. A total of 15 participants with early ACEs were compared with a control group (n = 18) using the Corsi Block Tapping Test (CBTT) and Stroop Word Colour Test (SCWT). Results showed that participants with early ACEs exhibited lower scores on the SCWT but not the CBTT. The stress response emerged as a potential factor in the relationship between early ACEs and cognitive performance. The implications of these findings are then discussed in relation to the existing literature.

Analysis of telomere length and the relationship with neurocognitive functions in euthymic bipolar disorder: A cross-sectional pilot study

BACKGROUND

Telomere shortening has been considered a potential biological marker related to disease susceptibility and aging in psychiatric disorders. However, the relationship between telomere length and bipolar disorder (BD-I and BD-II) is uncertain. Moreover, whether telomere shortening is an independent factor of cognitive impairment in BD patients is still inconclusive.

METHODS

We explore telomere length and cognitive function in patients with bipolar disorder and the relationship between them. We enrolled three groups (35 patients with euthymic BD-I, 18 with euthymic BD-II, and 38 healthy controls). Telomere length was measured by fluorescent quantitative polymerase chain reaction (q-PCR), and cognitive function was evaluated by the MATRICS Consensus Cognitive Battery (MCCB). SPSS 24.0 was used for statistical analysis.

RESULTS

The telomere length of euthymic patients with BD-I and BD-II was shorter than that of healthy controls (F = 8.228, P = 0.001, η2 = 0.176). Telomere length was not significantly different between BD-I and BD-II. Compared to HCs, poor performance was detected in attention and vigilance in BD-I patients (F = 3.473, P = 0.036). Working memory was positively correlated with telomere length in BD-II patients (Beta = 0.5, P = 0.041, Adjusted R2 = 0.2).

CONCLUSIONS

The current study provided evidence of shortened telomere length in euthymic BD patients, indicating that telomere shortening might be a promising biomarker of susceptibility to bipolar disorder. The telomere length predicted the working memory in BD-II patients. Further studies are needed to clarify the role of accelerated aging on cognitive functioning in a young group of patients with BD.

Exploring the dynamic interplay between learning and working memory within various cognitive contexts

Introduction

The intertwined relationship between reinforcement learning and working memory in the brain is a complex subject, widely studied across various domains in neuroscience. Research efforts have focused on identifying the specific brain areas responsible for these functions, understanding their contributions in accomplishing the related tasks, and exploring their adaptability under conditions such as cognitive impairment or aging.

Methods

Numerous models have been introduced to formulate either these two subsystems of reinforcement learning and working memory separately or their combination and relationship in executing cognitive tasks. This study adopts the RLWM model as a computational framework to analyze the behavioral parameters of subjects with varying cognitive abilities due to age or cognitive status. A related RLWM task is employed to assess a group of subjects across different age groups and cognitive abilities, as measured by the Montreal Cognitive Assessment tool (MoCA).

Results

Analysis reveals a decline in overall performance accuracy and speed with differing age groups (young vs. middle-aged). Significant differences are observed in model parameters such as learning rate, WM decay, and decision noise. Furthermore, among the middle-aged group, distinctions emerge between subjects categorized as normal vs. MCI based on MoCA scores, notably in speed, performance accuracy, and decision noise.

Twelve weeks of physical exercise breaks with coordinative exercises at the workplace increase the sulcal depth and decrease gray matter volume in brain structures related to visuomotor processes

Physical exercise can evoke changes in the brain structure. Consequently, these can lead to positive impacts on brain health. However, physical exercise studies including coordinative exercises are rare. Therefore, in this study, we investigated how 12 weeks of physical exercise breaks (PEBs) with coordinative exercises, focusing mainly on juggling tasks, affected the brain structure. The participants were randomly allocated to an intervention group (IG, n = 16; 42.8 ± 10.2 years) and a control group (CG, n = 9; 44.2 ± 12.3 years). The IG performed the PEBs with coordinative exercises twice per week for 15-20 min per session. Before the intervention, after 6 weeks of the intervention, and after 12 weeks of the intervention, participants underwent a high-resolution 3T T1-weighted magnetic resonance imagining scan. Juggling performance was assessed by measuring the time taken to perform a three-ball cascade. A surface-based analysis revealed an increase in vertex-wise cortical depth in a cluster including the inferior parietal lobe after 6 and 12 weeks of training in the IG. After 12 weeks, the IG showed a decrease in gray matter (GM) volume in a cluster primarily involving the right insula and the right operculum. The changes in the GM volume were related to improvements in juggling performance. No significant changes were found for the CG. To conclude, the present study showed that regular engagement in PEBs with coordinative exercises led to changes in brain structures strongly implicated in visuomotor processes involving hand and arm movements.

Effects of Dietary Methionine Restriction on Cognition in Mice

Dietary restriction of the essential amino acid, methionine, has been shown to induce unique metabolic protection. The peripheral benefits of methionine restriction (MR) are well established and include improvements in metabolic, energy, inflammatory, and lifespan parameters in preclinical models. These benefits all occur despite MR increasing energy intake, making MR an attractive dietary intervention for the prevention or reversal of many metabolic and chronic conditions. New and emerging evidence suggests that MR also benefits the brain and promotes cognitive health. Despite widespread interest in MR over the past few decades, many findings are limited in scope, and gaps remain in our understanding of its comprehensive effects on the brain and cognition. This review details the current literature investigating the impact of MR on cognition in various mouse models, highlights some of the key mechanisms responsible for its cognitive benefits, and identifies gaps that should be addressed in MR research moving forward. Overall findings indicate that in animal models, MR is associated with protection against obesity-, age-, and Alzheimer's disease-induced impairments in learning and memory that depend on different brain regions, including the prefrontal cortex, amygdala, and hippocampus. These benefits are likely mediated by increases in fibroblast growth factor 21, alterations in methionine metabolism pathways, reductions in neuroinflammation and central oxidative stress, and potentially alterations in the gut microbiome, mitochondrial function, and synaptic plasticity.

ASSESSMENT AND CHARACTERIZATION OF COVID-19 RELATED COGNITIVE DECLINE: RESULTS FROM A NATURAL EXPERIMENT

Background

Cognitive impairment is the most common and disabling manifestation of post-acute sequelae of SARS-CoV-2. There is an urgent need for the application of more stringent methods for evaluating cognitive outcomes in research studies.

Objective

To determine whether cognitive decline emerges with the onset of COVID-19 and whether it is more pronounced in patients with Post-Acute Sequelae of SARS-CoV-2 or severe COVID-19.

Methods

This longitudinal cohort study compared the cognitive performance of 276 patients with COVID-19 to that of 217 controls across four neuroinflammation or vascular disease-sensitive domains of cognition using data collected both before and after the pandemic starting in 2015.

Results

The mean age of the COVID-19 group was 56.04±6.6 years, while that of the control group was 58.1±7.3 years. Longitudinal models indicated a significant decline in cognitive throughput ((β=-0.168, P=.001) following COVID-19, after adjustment for pre-COVID-19 functioning, demographics, and medical factors. The effect sizes were large; the observed changes in throughput were equivalent to 10.6 years of normal aging and a 59.8% increase in the burden of mild cognitive impairment. Cognitive decline worsened with coronavirus disease 2019 severity and was concentrated in participants reporting post-acute sequelae of SARS-CoV-2.

Conclusion

COVID-19 was most likely associated with the observed cognitive decline, which was worse among patients with PASC or severe COVID-19. Monitoring patients with post-acute sequelae of SARS-CoV-2 for declines in the domains of processing speed and visual working memory and determining the long-term prognosis of this decline are therefore warranted.

Structural plasticity in the entorhinal and perirhinal cortices following hippocampal lesions in rhesus monkeys

Immature neurons expressing the Bcl2 protein are present in various regions of the mammalian brain, including the amygdala and the entorhinal and perirhinal cortices. Their functional role is unknown but we have previously shown that neonatal and adult hippocampal lesions increase their differentiation in the monkey amygdala. Here, we assessed whether hippocampal lesions similarly affect immature neurons in the entorhinal and perirhinal cortices. Since Bcl2-positive cells were found mainly in areas Eo, Er, and Elr of the entorhinal cortex and in layer II of the perirhinal cortex, we also used Nissl-stained sections to determine the number and soma size of immature and mature neurons in layer III of area Er and layer II of area 36 of the perirhinal cortex. We found different structural changes in these regions following hippocampal lesions, which were influenced by the time of the lesion. In neonate-lesioned monkeys, the number of immature neurons in the entorhinal and perirhinal cortices was generally higher than in controls. The number of mature neurons was also higher in layer III of area Er of neonate-lesioned monkeys but no differences were found in layer II of area 36. In adult-lesioned monkeys, the number of immature neurons in the entorhinal cortex was lower than in controls but did not differ from controls in the perirhinal cortex. The number of mature neurons in layer III of area Er did not differ from controls, but the number of small, mature neurons in layer II of area 36 was lower than in controls. In sum, hippocampal lesions impacted populations of mature and immature neurons in discrete regions and layers of the entorhinal and perirhinal cortices, which are interconnected with the amygdala and provide major cortical inputs to the hippocampus. These structural changes may contribute to some functional recovery following hippocampal injury in an age-dependent manner.

Brain-wide human oscillatory LFP activity during visual working memory

Oscillatory activity is thought to be a marker of cognitive processes, although its role and distribution across the brain during working memory has been a matter of debate. To understand how oscillatory activity differentiates tasks and brain areas in humans, we recorded local field potentials (LFPs) in 12 adults as they performed visual-spatial and shape-matching memory tasks. Tasks were designed to engage working memory processes at a range of delay intervals between stimulus delivery and response initiation. LFPs were recorded using intracranial depth electrodes implanted to localize seizures for management of intractable epilepsy. Task-related LFP power analyses revealed an extensive network of cortical regions that were activated during the presentation of visual stimuli and during their maintenance in working memory, including occipital, parietal, temporal, insular, and prefrontal cortical areas, and subcortical structures including the amygdala and hippocampus. Across most brain areas, the appearance of a stimulus produced broadband power increase, while gamma power was evident during the delay interval of the working memory task. Notable differences between areas included that occipital cortex was characterized by elevated power in the high gamma (100-150 Hz) range during the 500 ms of visual stimulus presentation, which was less pronounced or absent in other areas. A decrease in power centered in beta frequency (16-40 Hz) was also observed after the stimulus presentation, whose magnitude differed across areas. These results reveal the interplay of oscillatory activity across a broad network, and region-specific signatures of oscillatory processes associated with visual working memory.

No convincing evidence the hippocampus is associated with working memory

In a previous discussion paper , twenty-six working memory fMRI studies that reported activity in the hippocampus were systematically analyzed. None of these studies provided convincing evidence that the hippocampus was active during the late delay phase, the only period in which working memory can be isolated from long-term memory processes. Based on these results, it was concluded that working memory does not activate the hippocampus. Six commentaries on the discussion paper were received from Courtney (2022), Kessels & Bergmann (2022), Peters and Reithler (2022), Rose and Chao (2022), Stern & Hasselmo (2022), and Wood, Clark, and Nee (2022). Based on these commentaries, the present response paper considered whether there is evidence of sustained hippocampal activity during the working memory delay period based on depth-electrode recording, whether there are activity-silent working memory mechanisms in the hippocampus, and whether there is hippocampal lesion evidence indicating this region is important for working memory. There was no convincing electrophysiological or neuropsychological evidence that the hippocampus is associated with working memory maintenance, and activity-silent mechanisms were arguably speculative. Given that only a small fraction (approximately 5%) of fMRI studies have reported hippocampal activity in working memory tasks and lesion evidence indicates the hippocampus is not necessary for working memory, the burden of proof is on proponents of view that the hippocampus is important for working memory to provide compelling evidence to support their position. To date, in my view, there is no convincing evidence that the hippocampus is associated with working memory.

There Is no Theory-Free Measure of "Swaps" in Visual Working Memory Experiments

Visual working memory is highly limited, and its capacity is tied to many indices of cognitive function. For this reason, there is much interest in understanding its architecture and the sources of its limited capacity. As part of this research effort, researchers often attempt to decompose visual working memory errors into different kinds of errors, with different origins. One of the most common kinds of memory error is referred to as a "swap," where people report a value that closely resembles an item that was not probed (e.g., an incorrect, non-target item). This is typically assumed to reflect confusions, like location binding errors, which result in the wrong item being reported. Capturing swap rates reliably and validly is of great importance because it permits researchers to accurately decompose different sources of memory errors and elucidate the processes that give rise to them. Here, we ask whether different visual working memory models yield robust and consistent estimates of swap rates. This is a major gap in the literature because in both empirical and modeling work, researchers measure swaps without motivating their choice of swap model. Therefore, we use extensive parameter recovery simulations with three mainstream swap models to demonstrate how the choice of measurement model can result in very large differences in estimated swap rates. We find that these choices can have major implications for how swap rates are estimated to change across conditions. In particular, each of the three models we consider can lead to differential quantitative and qualitative interpretations of the data. Our work serves as a cautionary note to researchers as well as a guide for model-based measurement of visual working memory processes.

The medial temporal lobe supports the quality of visual short-term memory representation

The quality of short-term memory (STM) underlies our ability to recall the exact details of a recent event, yet how the human brain enables this core cognitive function remains poorly understood. Here we use multiple experimental approaches to test the hypothesis that the quality of STM, such as its precision or fidelity, relies on the medial temporal lobe (MTL), a region commonly associated with the ability to distinguish similar information remembered in long-term memory. First, with intracranial recordings, we find that delay-period MTL activity retains item-specific STM content that is predictive of subsequent recall precision. Second, STM recall precision is associated with an increase in the strength of intrinsic MTL-to-neocortical functional connections during a brief retention interval. Finally, perturbing the MTL through electrical stimulation or surgical removal can selectively reduce STM precision. Collectively, these findings provide converging evidence that the MTL is critically involved in the quality of STM representation.

The entorhinal-DG/CA3 pathway in the medial temporal lobe retains visual working memory of a simple surface feature

Classic models consider working memory (WM) and long-term memory as distinct mental faculties that are supported by different neural mechanisms. Yet, there are significant parallels in the computation that both types of memory require. For instance, the representation of precise item-specific memory requires the separation of overlapping neural representations of similar information. This computation has been referred to as pattern separation, which can be mediated by the entorhinal-DG/CA3 pathway of the medial temporal lobe (MTL) in service of long-term episodic memory. However, although recent evidence has suggested that the MTL is involved in WM, the extent to which the entorhinal-DG/CA3 pathway supports precise item-specific WM has remained elusive. Here, we combine an established orientation WM task with high-resolution fMRI to test the hypothesis that the entorhinal-DG/CA3 pathway retains visual WM of a simple surface feature. Participants were retrospectively cued to retain one of the two studied orientation gratings during a brief delay period and then tried to reproduce the cued orientation as precisely as possible. By modeling the delay-period activity to reconstruct the retained WM content, we found that the anterior-lateral entorhinal cortex (aLEC) and the hippocampal DG/CA3 subfield both contain item-specific WM information that is associated with subsequent recall fidelity. Together, these results highlight the contribution of MTL circuitry to item-specific WM representation.

Olfaction and declarative memory in aging: a meta-analysis

Olfactory and declarative memory performances are associated, as both functions are processed by overlapping medial-temporal and prefrontal structures and decline in older adults. While a decline in olfactory identification may be related to a decline in declarative memory, the relationship between olfactory detection threshold and declarative memory remains unclear. In this meta-analysis, we assessed (i) the relationship between olfactory identification/detection threshold and verbal declarative memory in cognitively normal older adults, and (ii) the effect of age on these relationships. We included articles from PsychNet, PubMed, and Academic Search Complete according to the following criteria: (i) inclusion of cognitively normal older adults; (ii) assessment of episodic or semantic memory; and (iii) assessment of olfactory identification or detection threshold. Seventeen studies and 22 effect sizes were eligible and included in this meta-analysis. Olfactory identification was associated with episodic (small effect size: r = 0.19; k = 22) and semantic memory (small effect size: r = 0.16; k = 23). Similarly, the olfactory detection threshold was associated with both episodic (small to medium effect size: r = 0.25; k = 5) and semantic memory (small effect size: r = 0.17; k = 7). Age was found to moderate the relationship between olfactory detection threshold and memory performance. Both olfactory identification and detection threshold performances are associated with declarative memory in older adults, and age only moderates the relationship between olfactory detection threshold and declarative memory performances.

Prefrontal and Medial Temporal Lobe Cortical Contributions to Visual Short-Term Memory

A number of recent studies have indicated that the medial temporal lobe (MTL) plays a critical role in working memory (WM) and perception, but these results have been highly controversial given the traditional association of MTL with long-term memory. We review the research and highlight important factors that need to be considered in determining the role of MTL in WM including set-size of used stimuli and feature complexity and/or feature conjunctions/bindings embedded in those stimuli. These factors relate to hierarchical and, accordingly, domain-specific theories of functional organization within the temporal lobe. In addition, one must consider process-specific theories too, because two key processes commonly understood to contribute recognition memory, namely, recollection and familiarity, also have robust support from neurophysiological and neuroimaging research as to their functional dissociations within MTL. PFC has long been heavily implicated in WM; however, relatively less is known about how the PFC contributes to recollection and familiarity, although dynamic prefrontal coding models in WM may help to explain their neural mechanisms. The MTL and PFC are heavily interconnected and do not operate independently in underlying WM. We propose that investigation of the interactions between these two regions in WM, particularly their coordinated neural activities, and the modeling of such interactions, will be crucial for the advancing understanding of the neural mechanisms of WM.

Inverse forgetting in unconscious episodic memory

Forming memories of experienced episodes calls upon the episodic memory system. Episodic encoding may proceed with and without awareness of episodes. While up to 60% of consciously encoded episodes are forgotten after 10 h, the fate of unconsciously encoded episodes is unknown. Here we track over 10 h, which are filled with sleep or daytime activities, the retention of unconsciously and consciously experienced episodes. The episodes were displayed in cartoon clips that were presented weakly and strongly masked for conscious and unconscious encoding, respectively. Clip retention was tested for distinct clips directly after encoding, 3 min and 10 h after encoding using a forced-choice test that demands deliberate responses in both consciousness conditions. When encoding was conscious, retrieval accuracy decreased by 25% from 3 min to 10 h, irrespective of sleep or wakefulness. When encoding was unconscious, retrieval accuracy increased from 3 min to 10 h and depended on sleep. Hence, opposite to the classic forgetting curve, unconsciously acquired episodic memories strengthen over time and hinge on sleep on the day of learning to gain influence over human behavior.

Intact high-resolution working memory binding in a patient with developmental amnesia and selective hippocampal damage

Debate continues regarding the possible role of the hippocampus across short-term and working memory tasks. The current study examined the possibility of a hippocampal contribution to precise, high-resolution cognition and conjunctive memory. We administered visual working memory tasks featuring a continuous response component to a well-established developmental amnesic patient with relatively selective bilateral hippocampal damage (Jon) and healthy controls. The patient was able to produce highly accurate response judgments regarding conjunctions of color and orientation or color and location, using simultaneous or sequential presentation of stimuli, with no evidence of any impairment in working memory binding, categorical accuracy, or continuous precision. These findings indicate that hippocampal damage does not necessarily lead to deficits in high-resolution cognitive performance, even when the damage is severe and bilateral.

Processing of embedded response plans is modulated by an interplay of fronto-parietal theta and beta activity

Even simple actions like opening a door require integration/binding and flexible re-activation of different motor elements. Yet, the neural mechanisms underlying the processing of such 'embedded response plans' are largely elusive, despite theoretical frameworks, such as the Theory of Event Coding, describing the involved cognitive processes. In a sample of N = 40 healthy participants we combine time-frequency decomposition and various beamforming methods to examine neurophysiological dynamics of such action plans - with special emphasis on the interplay of theta and beta frequency activity during the processing of these plans. We show that the integration and rule-guided reactivation of embedded response plans is modulated by a complex interplay of theta and beta activity. Pre-trial BBA is related to different functional neuroanatomical structures which are activated in a consecutive fashion. Enhanced preparatory activity is positively associated with higher binding-related BBA in the precuneus/parietal areas, indicating that activity in the precuneus/parietal cortex facilitates the execution of an embedded action sequence. Increased preparation subsequently leads to reduced working memory retrieval demands. A cascading pattern of interactions between pre-trial and within-trial activity indicates the importance of preparatory brain activity. The study shows that there are multiple roles of beta and theta oscillations associated with different functional neuroanatomical structures during the integration and reactivation of motor elements during actions.

Impaired allocentric spatial memory in patients with affective disorders

BACKGROUND

Memory disturbances are frequent in unipolar depression (UD) and bipolar disorder (BD) and may comprise important predisposing and maintaining factors. Previous studies have demonstrated hippocampal abnormalities in UD and BD but there is a lack of studies specifically assessing hippocampus-dependent memory.

METHODS

We used a virtual task to assess hippocampus-dependent (allocentric) vs non-hipppocampal (egocentric) spatial memory in remitted and partially remitted patients with UD or BD (N = 22) and a healthy control group (N = 32). Participants also completed a range of standard neuropsychological and functional assessments.

RESULTS

Participants in the UD/BD group showed selective impairments on high-load hippocampal (allocentric) memory compared to egocentric memory and this effect was independent of residual mood symptoms. Across both samples, both allocentric and egocentric spatial memory correlated with more general measures of memory and other aspects of cognition measured on standard neuropsychological tests but only high-load allocentric memory showed a significant relationship with functional capacity.

CONCLUSION

Results show a selective impairment in high-load allocentric spatial memory compared to egocentric memory in the patient group, suggesting impaired hippocampal functioning in patients with remitted UD/BD.

Examining the relationship between working memory consolidation and long-term consolidation

An emerging area of research is focused on the relationship between working memory and long-term memory and the likely overlap between these processes. Of particular interest is how some information first maintained in working memory is retained for longer periods and eventually preserved in long-term memory. The process of stabilizing transient memory representations for lasting retention is referred to as consolidation in both the working memory and long-term memory literature, although these have historically been viewed as independent constructs. The present review aims to investigate the relationship between working memory consolidation and long-term memory consolidation, which both have rich, but distinct, histories. This review provides an overview of the proposed models and neural mechanisms of both types of consolidation, as well as clinical findings related to consolidation and potential approaches for the manipulation of consolidation. Finally, two hypotheses are proposed to explain the relationship between working memory consolidation and long-term memory consolidation.

Blue light exposure increases functional connectivity between dorsolateral prefrontal cortex and multiple cortical regions

OBJECTIVE

Blue light is a powerful environmental stimulus that can produce significant phase shifts in the circadian rhythm of melatonin and sleep propensity as well as acute effects on alertness of neurobehavioral performance. Here, we undertook an expansion and reanalysis of our previously published findings to examine the effect of acute blue light exposure on the strength of resting-state functional connectivity (rsFC) between a previously identified region of the left dorsolateral prefrontal cortex (DLPFC) and 106 cortical and subcortical regions.

METHODS

Twenty-nine healthy adults (16 men and 13 women; age 18-32 years) completed a psychomotor vigilance test (PVT) before and after a single 30-min exposure to either blue (λ = 469 nm; n = 17) or amber wavelength (λ = 578 nm; n = 12) light, immediately followed by an rsFC scan.

RESULTS

Compared with amber light, blue light exposure produced significantly greater functional connectivity between the left DLPFC seed region and 30 cortical and subcortical regions (P < 0.05; false discovery rate-corrected). Although neurobehavioral performance did not differ between light conditions, only those exposed to blue light showed a significant association between rsFC and sustained PVT performance. Better sustained PVT performance was associated with greater connectivity between the left DLPFC and regions associated with visuospatial awareness/motion detection (right temporal-occipital middle temporal gyrus) and memory (left hippocampus), as well as reduced connectivity in a circuit associated with cognitive rumination and distraction (left parahippocampal gyrus).

CONCLUSION

Findings suggest that blue-wavelength light may facilitate acute alertness and improved cognitive performance through enhanced rsFC between the left DLPFC and cortical regions associated with visuospatial awareness.