A model of late long-term potentiation simulates aspects of memory maintenance.

Late long-term potentiation (L-LTP) denotes long-lasting strengthening of synapses between neurons. L-LTP appears essential for the formation of long-term memory, with memories at least partly encoded by patterns of strengthened synapses. How memories are preserved for months or years, despite molecular turnover, is not well understood. Ongoing recurrent neuronal activity, during memory recall or during sleep, has been hypothesized to preferentially potentiate strong synapses, preserving memories. This hypothesis has not been evaluated in the context of a mathematical model representing ongoing activity and biochemical pathways important for L-LTP. In this study, ongoing activity was incorporated into two such models - a reduced model that represents some of the essential biochemical processes, and a more detailed published model. The reduced model represents synaptic tagging and gene induction simply and intuitively, and the detailed model adds activation of essential kinases by Ca(2+). Ongoing activity was modeled as continual brief elevations of Ca(2+). In each model, two stable states of synaptic strength/weight resulted. Positive feedback between synaptic weight and the amplitude of ongoing Ca(2+) transients underlies this bistability. A tetanic or theta-burst stimulus switches a model synapse from a low basal weight to a high weight that is stabilized by ongoing activity. Bistability was robust to parameter variations in both models. Simulations illustrated that prolonged periods of decreased activity reset synaptic strengths to low values, suggesting a plausible forgetting mechanism. However, episodic activity with shorter inactive intervals maintained strong synapses. Both models support experimental predictions. Tests of these predictions are expected to further understanding of how neuronal activity is coupled to maintenance of synaptic strength. Further investigations that examine the dynamics of activity and synaptic maintenance can be expected to help in understanding how memories are preserved for up to a lifetime in animals including humans.

The effects of resistance training on well-being and memory in eldery volunteers

Objective: to determine the short- and long-term effects of resistance training on muscle strength, psychological well-being, control-beliefs, cognitive speed and memory in normally active elderly people. Methods: 46 elderly people (mean age 73.2 years; 18 women and 28 men), were randomly assigned to training and control groups (n = 23 each). Pre- and post-tests were administered 1 week before and 1 week after the 8-week training intervention. The training sessions, performed once a week, consisted of a 10 min warm-up phase and eight resistance exercises on machines. Results: there was a significant increase in maximum dynamic strength in the training group. This training effect was associated with a significant decrease in self-attentiveness, which is known to enhance psychological well-being. No significant changes could be observed in control-beliefs. Modest effects on cognitive functioning occurred with the training procedure: although there were no changes in cognitive speed, significant pre/post-changes could be shown in free recall and recognition in the experimental group. A post-test comparison between the experimental group and control group showed a weak effect for recognition but no significant differences in free recall. Significant long-term effects were found in the training group for muscular strength and memory performance (free recall) 1 year later. Conclusion: an 8-week programme of resistance training lessens anxiety and self-attentiveness and improves muscle strength.

Memory and attention problems in children with brain tumors at diagnosis

Purpose: Results from previous studies indicate that children with brain tumors (BT) might present with cognitive problems at diagnosis and thus before the start of any medical treatment. The question remains whether these problems are due to the underlying tumor itself or due to the high level of emotional and physical stress which is involved at diagnosis of a malignant disorder. All children with a de novo oncological diagnosis not involving the central nervous systems (CNS) are usually exposed to a comparable level of distress. However, patients with cancer not involving the CNS are not expected to show disease-related cognitive problems. Thus they serve as a well-balanced control group (CG) to help distinguish between the probable causes of the effect. Method: In a pilot study we analyzed an array of cognitive functions in 16 children with BT and 17 control patients. In both groups, tests were administered in-patient at diagnosis before any therapeutic intervention such as surgery, chemotherapy od irradiation. Results: Performance of children with BT was comparable to that of CG patients in the areas of intelligence, perceptual reasoning, verbal comprehension, working memory, and processing speed. In contrast, however, BT patients performded significantly worse in verbal memory and attention. Conclusion: Memory and attention seem to be the most vulnerable funstions affected by BT, with other functions being preserved at the time of diagnosis. It ist to be expected that this vulnerability might exacerbate the cognitive decline after chemotherapy and radiation treatment - known to impair intellectual performance. The findings highlight the need of early cognitive assessments in children with BT in order to introduce cognitive training as early as possible to minimize or even prevent cognitive long-term sequelae. This might improve long-term academic and professional outcome of these children, but especially helps their return to school after hospitalization.

Prospective Memory, Retrospective Memory, Executive Functions, and Metacognition: How they are linked longitudinally in young elementary school children

Succeeding in everyday activities often requires executive functioning (EF), metacognitive abilities (MC) and memory skills such as prospective memory (PM) and retrospective memory (RM). These cognitive abilities seem to gradually develop in childhood, possibly influencing each other during development. From a theoretical point of view, it is likely that they are closely interrelated, especially in children. Their empirical relation, however, is less clear. A model that links these cognitive abilities can help to better understand the relation between PM and RM and other cognitive processes. In this project we studied the longitudinal development of PM, RM, EF, and MC in 7-8 year old elementary school children across half a year. 119 second graders (MT1 = 95 months, SDT1, = 4.8 months) completed the same PM, RM, EF and MC tasks twice with a time-lag of 7 months. The developmental progression was analysed using paired t-tests, the longitudinal relationships were analysed using confirmatory factor analysis and all fit indices are in accordance with Hu and Bentler (1998). In general, performance improved significantly (ps < .001) and effect sizes ranged from .45 to .62 (Cohen’s d). CFA revealed a good model fit, c2(227, 119) = 242.56, p = .23, TLI = .973, CFI = .979, RMSEA = .024. At T1, significant cross-sectional links were found between PM T1 and RM T1, between PM T1 and EF T1, and between EF T1 and MC T1. Moreover, significant longitudinal links were found between EFT1 and PMT2 and between EFT1 and MCT2; EF T1 and RM T2 were marginally linked. Results underline previous findings showing that PM, RM, EF, and MC develop significantly during childhood, even within this short time period. Results also indicate that these cognitive abilities are linked not only cross-sectionally, but longitudinally. Most relevant, however, is the predictive role of EF for both metacognition and memory.

Longitudinal relationships between prospective memory, executive functions, and metacognition in young elementary school children

Prospective Memory (PM), executive functions (EF) and metacognition (MC) are relevant cognitive abilities for everyday functioning. They all seem to develop gradually in childhood and appear to be theoretically closely related; however, their empirical links remain unclear, especially in children. As a recent study revealed significant cross-sectional links between PM and EF, and a weaker but close link between PM and MC in 2nd graders (Spiess, Meier, & Roebers, submitted), this study focused on their short-term relationships and on their development. 119 children (MT1 =95 months, SDT1, = 4.8 months) completed the same tasks (one PM, three EF, one MC task) twice with a time-lag of 7 months. T-tests showed significant improvements in all tasks, except in the updating task. Different structural equation models were contrasted (AMOS); the best fitting model revealed that PMT2 was similarly predicted by PMT1 (r = .33) and EFT1 (r = .34). Additionally, EFT1 predicted MCT2 (r = .44), chi2(118, 119) = 128.91, p = .23, TLI = .968, CFI = .978, RMSEA = .028. Results show that PM, EF, and MC develop during childhood and also demonstrate that they are linked not only cross-sectionally, but longitudinally. Findings are discussed in a broader developmental framework.

Implicit memory for the content but not the speaker of sleep-played messages

We presented 28 sentences uttered by 28 unfamiliar speakers to sleeping participants to investigate whether humans can encode new verbal messages, learn voices of unfamiliar speakers, and form associations between speakers and messages during EEG-defined deep sleep. After waking, participants performed three tests which assessed the unconscious recognition of sleep-played speakers, messages, and speaker-message associations. Recognition performance in all tests was at chance level. However, response latencies revealed implicit memory for sleep-played messages but neither for speakers nor for speaker-message combinations. Only participants with excellent implicit memory for sleep-played messages also displayed implicit memory for speakers but not speaker-message associations. Hence, deep sleep allows for the semantic encoding of novel verbal messages.

The effect of thyroxine and triiodothyronine on fatigue, depression, and memory in a hypothyroid population

Background. This study was planned at a time when important questions were being raised about the adequacy of using one hormone to treat hypothyroidism instead of two. Specifically, this trial aimed to replicate prior findings which suggested that substituting 12.5 μg of liothyronine for 50 μg of levothyroxine might improve mood, cognition, and physical symptoms. Additionally, this trial aimed to extend findings to fatigue. ^ Methods. A randomized, double-blind, two-period, crossover design was used. Hypothyroid patients stabilized on levothyroxine were invited to participate. Thirty subjects were recruited and randomized. Sequence one received their standard levothyroxine dose in one capsule and placebo in another during the first six weeks. Sequence two received their usual levothyroxine dose minus 50 μg in one capsule and 10 μg of liothyronine in another. At the end of the first six week period, subjects were crossed over. T tests were used to assess carry-over and treatment effects. ^ Results. Twenty-seven subjects completed the trial. The majority of completers had an autoimmune etiology. Mean baseline levothyroxine dose was 121 μg/d (±26.0). Subjects reported small increases in fatigue as measured by the Piper Fatigue Scale (0.9, p = 0.09) and in symptoms of depression measured by the Beck Depression Inventory-II (2.3, p = 0.16) as well as the General Health Questionnaire-30 (4.7, p = 0.14) while treated with substitution treatment. However, none of these differences was statistically significant. Measures of working memory were essentially unchanged between treatments. Thyroid stimulating hormone was about twice as high during substitution treatment (p = 0.16). Free thyroxine index was reduced by 0.7 (p < 0.001), and total serum thyroxine was reduced by 3.0 (p < 0.001) while serum triiodothyronine was increased by 20.5 (p < 0.001) on substitution treatment. ^ Conclusions. Substituting an equivalent amount of liothyronine for a portion of levothyroxine in patients with hypothyroidism does not decrease fatigue, symptoms of depression, or improve working memory. However, due to changes in serum hormone levels and small increments in fatigue and depression symptoms on substitution treatment, a question was raised about the role of T3 in the serum. ^

Non-Parametric Tests of Real Exchange rates in the Post-Bretton Woods Era

This paper examines the mean-reverting property of real exchange rates. Earlier studies have generally not been able to reject the null hypothesis of a unit-root in real exchange rates, especially for the post-Bretton Woods floating period. The results imply that long-run purchasing power parity does not hold. More recent studies, especially those using panel unit-root tests, have found more favorable results, however. But, Karlsson and Löthgren (2000) and others have recently pointed out several potential pitfalls of panel unit-root tests. Thus, the panel unit-root test results are suggestive, but they are far from conclusive. Moreover, consistent individual country time series evidence that supports long-run purchasing power parity continues to be scarce. In this paper, we test for long memory using Lo's (1991) modified rescaled range test, and the rescaled variance test of Giraitis, Kokoszka, Leipus, and Teyssière (2003). Our testing procedure provides a non-parametric alternative to the parametric tests commonly used in this literature. Our data set consists of monthly observations from April 1973 to April 2001 of the G-7 countries in the OECD. Our two tests find conflicting results when we use U.S. dollar real exchange rates. However, when non-U.S. dollar real exchange rates are used, we find only two cases out of fifteen where the null hypothesis of an unit-root with short-term dependence can be rejected in favor of the alternative hypothesis of long-term dependence using the modified rescaled range test, and only one case when using the rescaled variance test. Our results therefore provide a contrast to the recent favorable panel unit-root test results.

Working memory constrains human cooperation in the Prisoner’s Dilemma

Many problems in human society reflect the inability of selfish parties to cooperate. The “Iterated Prisoner’s Dilemma” has been used widely as a model for the evolution of cooperation in societies. Axelrod’s computer tournaments and the extensive simulations of evolution by Nowak and Sigmund and others have shown that natural selection can favor cooperative strategies in the Prisoner’s Dilemma. Rigorous empirical tests, however, lag behind the progress made by theorists. Clear predictions differ depending on the players’ capacity to remember previous rounds of the game. To test whether humans use the kind of cooperative strategies predicted, we asked students to play the iterated Prisoner’s Dilemma game either continuously or interrupted after each round by a secondary memory task (i.e., playing the game “Memory”) that constrained the students’ working-memory capacity. When playing without interruption, most students used “Pavlovian” strategies, as predicted, for greater memory capacity, and the rest used “generous tit-for-tat” strategies. The proportion of generous tit-for-tat strategies increased when games of Memory interfered with the subjects’ working memory, as predicted. Students who continued to use complex Pavlovian strategies were less successful in the Memory game, but more successful in the Prisoner’s Dilemma, which indicates a trade-off in memory capacity for the two tasks. Our results suggest that the set of strategies predicted by game theorists approximates human reality.

Ensemble codes involving hippocampal neurons are at risk during delayed performance tests

Multielectrode recording techniques were used to record ensemble activity from 10 to 16 simultaneously active CA1 and CA3 neurons in the rat hippocampus during performance of a spatial delayed-nonmatch-to-sample task. Extracted sources of variance were used to assess the nature of two different types of errors that accounted for 30% of total trials. The two types of errors included ensemble “miscodes” of sample phase information and errors associated with delay-dependent corruption or disappearance of sample information at the time of the nonmatch response. Statistical assessment of trial sequences and associated “strength” of hippocampal ensemble codes revealed that miscoded error trials always followed delay-dependent error trials in which encoding was “weak,” indicating that the two types of errors were “linked.” It was determined that the occurrence of weakly encoded, delay-dependent error trials initiated an ensemble encoding “strategy” that increased the chances of being correct on the next trial and avoided the occurrence of further delay-dependent errors. Unexpectedly, the strategy involved “strongly” encoding response position information from the prior (delay-dependent) error trial and carrying it forward to the sample phase of the next trial. This produced a miscode type error on trials in which the “carried over” information obliterated encoding of the sample phase response on the next trial. Application of this strategy, irrespective of outcome, was sufficient to reorient the animal to the proper between trial sequence of response contingencies (nonmatch-to-sample) and boost performance to 73% correct on subsequent trials. The capacity for ensemble analyses of strength of information encoding combined with statistical assessment of trial sequences therefore provided unique insight into the “dynamic” nature of the role hippocampus plays in delay type memory tasks.

Hormone replacement therapy and everyday memory in mid-aged New Zealand women

While empirical research to date has generally supported positive effects of estrogen on verbal memory performance in women, the literature examining specific effects of Hormone Replacement Therapy (HRT) on cognitive functioning in mid-life women is more equivocal. The Rivermead Behavioural Memory Test-Extended Version (RBMT-E), a measure of everyday memory functioning in adults within an average range of cognitive functioning, was administered to a sample of 104 New Zealand women aged 40 to 60 years who had self-selected to either use or not use HRT (53 HRT users and 51 non-users). Self-report. measures of mood, stress, general health and menopausal symptoms were also administered. These variables, along with age and education level, were used in analyses of group differences on the everyday memory measures. Results showed significant differences between the groups for three sub-tests of the RBMT-E:'Story Immediate', 'Story Delayed', and 'Message Delayed'. Women who use HRT scored higher on these subtests than those who do not use HRT. After calculation of a total profile score (adjusting for age and IQ), HRT users score higher than HRT non-users on the RBMT-E overall measure of Everyday Memory. These pilot results suggest that HRT use in this sample-is related to enhanced verbal memory in everyday memory tasks and that the RBMT-E may be a useful tool for further work in this area of research.

What You Get Out of Memory Depends on the Question You Ask

Following study, participants received 2 tests. The 1st was a recognition test; the 2nd was designed to tap recollection. The objective was to examine performance on Test I conditional on Test 2 performance. In Experiment 1, contrary to process dissociation assumptions, exclusion errors better predicted subsequent recollection than did inclusion errors. In Experiments 2 and 3, with alternate questions posed on Test 2, words having high estimates of recollection with one question had high estimates of familiarity with the other question. Results supported the following: (a) the 2-test procedure has considerable potential for elucidating the relationship between recollection and familiarity; (b) there is substantial evidence for dependency between such processes when estimates are obtained using the process dissociation and remember-know procedures; and (c) order of information access appears to depend on the question posed to the memory system.

Executive control of working memory in schizophrenia

There is considerable evidence that working memory impairment is a common feature of schizophrenia. The present study assessed working memory and executive function in 54 participants with schizophrenia, and a group of 54 normal controls matched to the patients on age, gender and estimated premorbid IQ, using traditional and newer measures of executive function and two dual tasks-Telephone Search with Counting and the Memory Span and Tracking Task. Results indicated that participants with schizophrenia were significantly impaired on all standardised measures of executive function with the exception of a composite measure of the Trail Making Test. Results for the dual task measures demonstrated that while the participants with schizophrenia were unimpaired on immediate digit span recall over a 2-min period, they recalled fewer digit strings and performed more poorly on a tracking task (box-crossing task) compared with controls. In addition, participants with schizophrenia performed more poorly on the tracking task when they were required to simultaneously recall digits strings than when they performed this task alone. Contrary to expectation, results of the telephone search task under dual conditions were not significantly different between groups. These results may reflect the insufficient complexity of the tone-counting task as an interference task. Overall, the present study showed that participants with schizophrenia appear to have a restricted impairment of their working memory system that is evident in tasks in which the visuospatial sketchpad slave system requires central executive control. (c) 2005 Elsevier Ireland Ltd. All rights reserved.

HRT and everyday memory at menopause: A comparison of two samples of mid-aged women

Research on the effect of hormone replacement therapy (HRT) on memory in mid-aged women is equivocal although findings indicate that oestrogen may enhance verbal memory. Mood may mediate the relationship between HRT and memory. This study examined the effect of HRT on mood and everyday memory in two samples of women between ages 40 and 60 years. In the cross-sectional comparison (N = 124), HRT users performed significantly better on tests of everyday and verbal memory. A within-woman comparison of 17 women showed that everyday memory, working memory, and delayed verbal memory improved after 3 months of HRT use. The improvement in memory was not mediated by mood. These results suggest that any effect of HRT on mood may be short-term but that some aspects of everyday memory are enhanced, particularly verbal memory. The development of the everyday memory construct and future investigation are discussed.

Memory for emotional faces in naturally occurring dysphoria and induced sadness

The aim was to establish if the memory bias for sad faces, reported in clinically depressed patients (Gilboa-Schechtman, Erhard Weiss, & Jeczemien, 2002; Ridout, Astell, Reid, Glen, & O'Carroll, 2003) generalises to sub-clinical depression (dysphoria) and experimentally induced sadness. Study 1: dysphoric (n = 24) and non-dysphoric (n = 20) participants were presented with facial stimuli, asked to identify the emotion portrayed and then given a recognition memory test for these faces. At encoding, dysphoric participants (DP) exhibited impaired identification of sadness and neutral affect relative to the non-dysphoric group (ND). At memory testing, DP exhibited superior memory for sad faces relative to happy and neutral. They also exhibited enhanced memory for sad faces and impaired memory for happy relative to the ND. Study 2: non-depressed participants underwent a positive (n = 24) or negative (n = 24) mood induction (MI) and were assessed on the same tests as Study 1. At encoding, negative MI participants showed superior identification of sadness, relative to neutral affect and compared to the positive MI group. At memory testing, the negative MI group exhibited enhanced memory for the sad faces relative to happy or neutral and compared to the positive MI group. Conclusion: MCM bias for sad faces generalises from clinical depression to these sub-clinical affective states.