Jeroen G. W. Raaijmakers

SAM: A theory of probabilistic search in associative memory.

Publisher Summary This chapter discusses probabilistic search of associative memory. The chapter introduces a theory of retrieval from long-term memory and presents a number of applications to data from paradigms involving free recall, categorized free recall, and paired-associate recall. Long-term store (LTS) is held to be a richly interconnected network, with numerous levels, stratifications, categories, and trees, containing varieties of relationships, schemata, frames, and associations. The retrieval system is noisy and inherently probabilistic; for a given memory structure and set of probe cues, the image selected from memory is a random variable. The retrieval process concern sampling and recovery. The relatively small set of images with non-negligible sampling probabilities is denoted as the “search-set.” When an image is sampled, its features will tend to become activated. There are subject controlled strategies in the theory, such as search termination rules, and choice of cues at various stages of the search.

Statistical analysis of the Michaelis-Menten equation.

An application of the method of maximum likelihood (ML) is described for analysing the results of enzyme kinetic experiments in which the Michaelis-Menten equation is obeyed. Accurate approximate solutions to the ML equations for the parameter estimates are presented for the case in which the experimental errors are of constant relative magnitude. Formulae are derived that approximate the standard errors of these estimates. The estimators are shown to be asymptotically unbiased and the standard errors observed in simulated data rapidly approach the theoretical lower bound as the sample size increases. The results of a large-scale Monte Carlo simulation study indicate that for data with a constant coefficient of variation, the present method is superior to other published methods, including the conventional transformations to linearity and the nonparametric technique proposed by Eisenthal and Cornish-Bowden (1974, Biochemical Journal 139, 715-720). Finally, the present results are extended to the analysis of simple receptor binding experiments using the general approach described by Munson and Rodbard (1980, Analytical Biochemistry 107, 220-239).

Similar network activated by young and old adults during the acquisition of a motor sequence

In this functional MRI (fMRI) study, we investigated ageing effects on motor skill learning. We applied an adapted version of the serial reaction time (SRT) task to extensive groups of young (N=26) and elderly (N=40) subjects. Since indications have been provided for age-related shrinkage of brain regions assumed to be critical to motor skill learning, we tested the hypothesis that age effects on implicit sequence learning are larger on a neurofunctional level than on a behavioural level. The SRT task consisted of two identical scan sessions, in which subjects had to manually trail an asterisk appearing serially in one of four spatial positions by means of button-pressing. Reliable response time reductions were already found in the first session for both the young and the elderly groups, when comparing a fixed sequence condition to a random sequence, but the learning effect was greater for the young subjects. In the second session, though, both groups showed a similar degree of learning. This indicates that implicit sequence learning is still intact in elderly adults, but that the rate of learning is somewhat slower. Reliable learning-related changes in brain activity were also observed. A similar network of brain regions was recruited by both groups during the fixed compared to the random sequence, involving several regions that have been previously associated with implicit sequence learning, including bilateral parietal, and frontal regions, the supplementary motor area (SMA), cerebellum and the basal ganglia. The direct group comparison did not reveal any differences in brain activity. In addition, we did not observe any significant differences in activity when comparing the different sessions either, neither for the young nor for the elderly subjects. Hence, we did not find indications for an age-related functional reorganisation of neural networks involved in motor sequence learning. In view of earlier reports of pronounced ageing effects on the performance on declarative memory tasks, our finding of age-related sparing of processes that sustain motor skill learning, provides further support for the proposition of different memory systems relying on different brain substrates.

Deep processing activates the medial temporal lobe in young but not in old adults

Age-related impairments in episodic memory have been related to a deficiency in semantic processing, based on the finding that elderly adults typically benefit less than young adults from deep, semantic as opposed to shallow, nonsemantic processing of study items. In the present study, we tested the hypothesis that elderly adults are not able to perform certain cognitive operations under deep processing conditions. We further hypothesised that this inability does not involve regions commonly associated with lexical/semantic retrieval processes, but rather involves a dysfunction of the medial temporal lobe (MTL) memory system. To this end, we used functional MRI on rather extensive groups of young and elderly adults to compare brain activity patterns obtained during a deep (living/nonliving) and a shallow (uppercase/lowercase) classification task. Common activity in relation to semantic classification was observed in regions that have been previously related to semantic retrieval, including mainly left-lateralised activity in the inferior prefrontal, middle temporal, and middle frontal/anterior cingulate gyrus. Although the young adults showed more activity in some of these areas, the finding of mainly overlapping activation patterns during semantic classification supports the idea that lexical/semantic retrieval processes are still intact in elderly adults. This received further support by the finding that both groups showed similar behavioural performances as well on the deep and shallow classification tasks. Importantly, though, the young revealed significantly more activity than the elderly adults in the left anterior hippocampus during deep relative to shallow classification. This finding is in line with the idea that age-related impairments in episodic encoding are, at least partly, due to an under-recruitment of the medial temporal lobe memory system.

How to quantify support for and against the null hypothesis: a flexible WinBUGS implementation of a default Bayesian t test.

We propose a sampling-based Bayesian t test that allows researchers to quantify the statistical evidence in favor of the null hypothesis. This Savage—Dickey (SD) t test is inspired by the Jeffreys—Zellner—Siow (JZS) t test recently proposed by Rouder, Speckman, Sun, Morey, and Iverson (2009). The SD test retains the key concepts of the JZS test but is applicable to a wider range of statistical problems. The SD test allows researchers to test order restrictions and applies to two-sample situations in which the different groups do not share the same variance.

Alzheimer's Disease Versus Normal Ageing: A Review of the Efficiency of Clinical and Experimental Memory Measures

This paper reviews research findings concerning memory performance in Alzheimers disease (AD) and normal ageing. Studies using clinical (i.e., episodic) memory tests are compared with studies using various experimental memory paradigms (semantic memory, implicit memory, working memory), in order to determine their efficiency to differentiate between AD and normal ageing. In addition, attention is focused on early and preclinical AD. It is argued that traditional clinical memory tests alone are not best able at detecting AD at an early stage. More specifically, tasks calling upon semantic knowledge may aid to an earlier and more efficient assessment of AD.

The role of item strength in retrieval-induced forgetting.

In 3 experiments, the role of item strength in the retrieval-induced forgetting paradigm was tested. According to the inhibition theory of forgetting proposed by M. C. Anderson, R. A. Bjork, and E. L. Bjork (1994), retrieval-induced forgetting should be larger for items that are more strongly associated with the category cue. In the present experiments, the authors varied item strength on the study list by manipulating the position of an item within its category (Experiments 1 and 2) and by the number of presentations in the study phase (Experiment 3). Contrary to the predictions from inhibition theory, in all 3 experiments, stronger items did not show more retrieval-induced forgetting than weaker items.

Parahippocampal Activation during Successful Recognition of Words: A Self-Paced Event-Related fMRI Study

In this study, we investigated retrieval from verbal episodic memory using a self-paced event-related fMRI paradigm, similar to the designs typically used in behavioral studies of memory function. We tested the hypothesis that the medial temporal lobe (MTL) is involved in the actual recovery of verbal information (retrieval success) rather than in the attempt to retrieve information (retrieval attempt). To this end, we used a verbal recognition task, distinguishing correctly recognized words, correctly rejected words, and a low-level baseline condition. Directly contrasting correct recognition with correct rejection of words, we found activation in the left fusiform/parahippocampal gyrus, indicating that this region has a distinct role in the successful retrieval of verbal information. Furthermore, our results were in agreement with those of previous imaging studies that compared a fixed-paced verbal recognition task to a baseline condition, showing activation in bilateral inferior frontal cortex, left dorsolateral prefrontal cortex, left anterior insular cortex, and anterior cingulate. This demonstrates the applicability of a self-paced event-related design within imaging studies of memory function.

Early assessment of dementia : the contribution of different memory components

A broad memory test battery (reflecting explicit and implicit memory functioning) was administered to a heterogeneous sample of initially nondemented, community-dwelling elderly subjects. To examine the profile of preclinical dementia, subjects were tested twice: At baseline, all subjects were nondemented according to Diagnostic and Statistical Manual of Mental Disorders (4th ed.; American Psychiatric Association, 1994) criteria; 2 years later, a subgroup had developed dementia. Performance of the preclinically demented subjects was best characterized, relative to that of cognitively impaired subjects who did not develop dementia 2 years later, by an inability to benefit at recall from semantic relations and by absent repetition priming effects. The authors conclude that in addition to testing episodic memory functioning, it is important to be aware of semantic and implicit memory deficits in the early assessment of dementia.

Retrieval-induced forgetting without competition: testing the retrieval specificity assumption of the inhibition theory

According to the inhibition theory of forgetting (Anderson, Journal of Memory and Language 49:415–445, 2003; Anderson, Bjork, & Bjork, Psychonomic Bulletin & Review 7:522-530, 2000), retrieval practice on a subset of target items leads to forgetting for the other, nontarget items, due to the fact that these other items interfere during the retrieval process and have to be inhibited in order to resolve the interference. In this account, retrieval-induced forgetting occurs only when competition takes place between target and nontarget items during target item practice, since only in such a case is inhibition of the nontarget items necessary. Strengthening of the target item without active retrieval should not lead to such an impairment. In two experiments, we investigated this assumption by using noncompetitive retrieval during the practice phase. We strengthened the cue–target item association during practice by recall of the category name instead of the target item, and thus eliminated competition between the different item types (as in Anderson et al., Psychonomic Bulletin & Review 7:522-530 2000). In contrast to the expectations of the inhibition theory, retrieval-induced forgetting occurred even without competition, and thus the present study does not support the retrieval specificity assumption.