Remembering over the short-term: the case against the standard model

Working memory retrieval differences between medial temporal lobe epilepsy patients and controls: a three memory layer approach

Multi-store models of working memory (WM) have given way to more dynamic approaches that conceive WM as an activated subset of long-term memory (LTM). The resulting framework considers that memory representations are governed by a hierarchy of accessibility. The activated part of LTM holds representations in a heightened state of activation, some of which can reach a state of immediate accessibility according to task demands. Recent neuroimaging studies have studied the neural basis of retrieval information with different states of accessibility. It was found that the medial temporal lobe (MTL) was involved in retrieving information within immediate access store and outside this privileged zone. In the current study we further explored the contribution of MTL to WM retrieval by analyzing the consequences of MTL damage to this process considering the state of accessibility of memory representations. The performance of a group of epilepsy patients with left hippocampal sclerosis in a 12-item recognition task was compared with that of a healthy control group. We adopted an embedded model of WM that distinguishes three components: the activated LTM, the region of direct access, and a single-item focus of attention. Groups did not differ when retrieving information from single-item focus, but patients were less accurate retrieving information outside focal attention, either items from LTM or items expected to be in the WM range. Analyses focused on items held in the direct access buffer showed that consequences of MTL damage were modulated by the level of accessibility of memory representations, producing a reduced capacity.

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.

No evidence for feature overwriting in visual working memory

The effect of repeating features in a short-term memory task was tested in three experiments. Experiments 1 and 2 used a recognition paradigm. Participants encoded four serially presented objects and then decided whether a probe matched one of them with regard to all three features. In the control condition no feature was repeated; in the experimental condition features were repeated in two memory objects. Experiment 3 used a cued recall paradigm with the same list design. After list presentation one feature was used as a cue uniquely indicating one of the memory objects. Participants recalled the remaining two features of the probed object. Feature overwriting as one component of the interference model of Oberauer and Kliegl (2006) predicts worse performance in the experimental compared to the control condition. Results of all three experiments did not support this hypothesis. Recognition performances in Experiments 1 and 2 were not impaired by repeating features. Recall performance in Experiment 3 was better for repeated features, contrary to the predictions of feature overwriting. Predictions from feature overwriting for the shape of serial position curves were also not confirmed.

Grapheme-color synesthesia can enhance immediate memory without disrupting the encoding of relational cues

Previous evidence has suggested that grapheme-color synesthesia can enhance memory for words, but little is known about how these photisms cue retrieval. Often, the encoding of specific features of individual words can disrupt the encoding of ordered relations between words, resulting in an overall decrease in recall accuracy. Here we show that the photisms arising from grapheme-color synesthesia do not function like these item-specific cues. The influences of high and low word frequency on the encoding of ordered relations and the accuracy of immediate free recall were compared across a group of 10 synesthetes and 48 nonsynesthetes. The main findings of Experiment 1 showed that the experience of synesthesia had no adverse effect on the encoding of ordered relations (as measured by input-output correspondence); furthermore, it enhanced recall accuracy in a strictly additive fashion across the two word frequency conditions. Experiment 2 corroborated these findings by showing that the synesthetes only outperformed the nonsynesthetes when the materials involved words and letters, not when they involved digits and spatial locations. Altogether, the present findings suggest that synesthesia can boost immediate memory performance without disrupting the encoding of ordered relations.

Stuck in the Past

Neurologically intact adults perseverate in immediate serial recall, intruding items from a previous trial into the current response. We applied the electroencephalogram/event-related-potential subsequent-memory paradigm to immediate serial recall to investigate the causes of these errors. In line with previous studies using this paradigm, results revealed that words that were correctly recalled elicited a greater frontal positivity during encoding when compared with words that were either perseverated over or not produced for some other reason. More surprisingly, differences were also found at encoding between the words perseverated into the subsequent response and words that were not perseverated. These findings support a theory stating that abnormalities in both how the current target and the previous trial are processed can contribute to perseveration errors. These results inform existing theories of immediate serial recall and theories of the control of irrelevant information.

Visual Long-Term Memory Has the Same Limit on Fidelity as Visual Working Memory

Visual long-term memory can store thousands of objects with surprising visual detail, but just how detailed are these representations, and how can one quantify this fidelity? Using the property of color as a case study, we estimated the precision of visual information in long-term memory, and compared this with the precision of the same information in working memory. Observers were shown real-world objects in random colors and were asked to recall the colors after a delay. We quantified two parameters of performance: the variability of internal representations of color (fidelity) and the probability of forgetting an object’s color altogether. Surprisingly, the fidelity of color information in long-term memory was comparable to the asymptotic precision of working memory. These results suggest that long-term memory and working memory may be constrained by a common limit, such as a bound on the fidelity required to retrieve a memory representation.

Retention-error patterns in complex alphanumeric serial-recall tasks

We propose a new method based on an algorithm usually dedicated to DNA sequence alignment in order to both reliably score short-term memory performance on immediate serial-recall tasks and analyse retention-error patterns. There can be considerable confusion on how performance on immediate serial list recall tasks is scored, especially when the to-be-remembered items are sampled with replacement. We discuss the utility of sequence-alignment algorithms to compare the stimuli to the participants' responses. The idea is that deletion, substitution, translocation, and insertion errors, which are typical in DNA, are also typical putative errors in short-term memory (respectively omission, confusion, permutation, and intrusion errors). We analyse four data sets in which alphanumeric lists included a few (or many) repetitions. After examining the method on two simple data sets, we show that sequence alignment offers 1) a compelling method for measuring capacity in terms of chunks when many regularities are introduced in the material (third data set) and 2) a reliable estimator of individual differences in short-term memory capacity. This study illustrates the difficulty of arriving at a good measure of short-term memory performance, and also attempts to characterise the primary factors underpinning remembering and forgetting.

Down syndrome: the brain in trisomic mode

Down syndrome is the most common form of intellectual disability and results from one of the most complex genetic perturbations that is compatible with survival, trisomy 21. The study of brain dysfunction in this disorder has largely been based on a gene discovery approach, but we are now moving into an era of functional genome exploration, in which the effects of individual genes are being studied alongside the effects of deregulated non-coding genetic elements and epigenetic influences. Also, new data from functional neuroimaging studies are challenging our views of the cognitive phenotypes associated with Down syndrome and their pathophysiological correlates. These advances hold promise for the development of treatments for intellectual disability.

Infants' Visual Recognition Memory for a Series of Categorically Related Items

We examined the interactions between visual recognition memory, working memory, and categorization by examining 6-month-old infants' (N = 168) memory for individual items in a categorized list (e.g., images of dogs or cats). In Experiments 1 and 2, infants were familiarized with 6 different cats or dogs, presented one at a time on a series of 15-s familiarization trials. When the test occurred immediately after the sixth familiarization trial (Experiment 1), infants showed strong novelty preference for items presented on the fourth or fifth familiarization trial, but not for the items presented on the first three trials or on the sixth trial. When a brief (15-s) retention delay occurred between the end of the sixth trial and the test trials (Experiment 2), memory for the sixth item was enhanced, memory for the fourth item was impaired, and memory for the fifth was unchanged relative to when no retention delay was included. Experiment 3 confirmed that infants can form a memory for the first item presented. These results reveal how factors such as interference and time to consolidate influence infants' visual recognition memory as they categorize a series of items.

Models of verbal working memory capacity: what does it take to make them work?

Theories of working memory (WM) capacity limits will be more useful when we know what aspects of performance are governed by the limits and what aspects are governed by other memory mechanisms. Whereas considerable progress has been made on models of WM capacity limits for visual arrays of separate objects, less progress has been made in understanding verbal materials, especially when words are mentally combined to form multiword units or chunks. Toward a more comprehensive theory of capacity limits, we examined models of forced-choice recognition of words within printed lists, using materials designed to produce multiword chunks in memory (e.g., leather brief case). Several simple models were tested against data from a variety of list lengths and potential chunk sizes, with test conditions that only imperfectly elicited the interword associations. According to the most successful model, participants retained about 3 chunks on average in a capacity-limited region of WM, with some chunks being only subsets of the presented associative information (e.g., leather brief case retained with leather as one chunk and brief case as another). The addition to the model of an activated long-term memory component unlimited in capacity was needed. A fixed-capacity limit appears critical to account for immediate verbal recognition and other forms of WM. We advance a model-based approach that allows capacity to be assessed despite other important processing contributions. Starting with a psychological-process model of WM capacity developed to understand visual arrays, we arrive at a more unified and complete model.

How to be proactive about interference: lessons from animal memory

Processes of proactive interference were explored using the pigeon as a model system of memory. This study shows that proactive interference extends back in time at least 16 trials (and as many minutes), revealing a continuum of interference and providing a framework for studying memory. Pigeons were tested in a delayed same/different task containing trial-unique pictures. On interference trials, sample pictures from previous trials reappeared as test pictures on different trials. Proactive-interference functions showed greatest interference from the most recent trial and with the longer of two delays (10 s vs. 1 s). These interference functions are accounted for by a time-estimation model based on signal detection theory. The model predicts that accuracy at test is determined solely by the ratio of the elapsed time since the offset of the current-trial sample to the elapsed time since the offset of the interfering sample. Implications for comparing memory of different species and different types of memory (e.g., familiarity vs. recollection) are discussed.

A Power-Law Model of Psychological Memory Strength in Short- and Long-Term Recognition

A classic law of cognition is that forgetting curves are closely approximated by power functions. This law describes relations between different empirical dependent variables and the retention interval, and the precise form of the functional relation depends on the scale used to measure each variable. In the research reported here, we conducted a recognition task involving both short- and long-term probes. We discovered that formal memory-strength parameters from an exemplar-recognition model closely followed a power function of the lag between studied items and a test probe. The model accounted for rich sets of response time (RT) data at both individual-subject and individual-lag levels. Because memory strengths were derived from model fits to choices and RTs from individual trials, the psychological power law was independent of the scale used to summarize the forgetting functions. Alternative models that assumed different functional relations or posited a separate fixed-strength working memory store fared considerably worse than the power-law model did in predicting the data.

Attention and visuospatial working memory share the same processing resources

Attention and visuospatial working memory (VWM) share very similar characteristics; both have the same upper bound of about four items in capacity and they recruit overlapping brain regions. We examined whether both attention and VWM share the same processing resources using a novel dual-task costs approach based on a load-varying dual-task technique. With sufficiently large loads on attention and VWM, considerable interference between the two processes was observed. A further load increase on either process produced reciprocal increases in interference on both processes, indicating that attention and VWM share common resources. More critically, comparison among four experiments on the reciprocal interference effects, as measured by the dual-task costs, demonstrates no significant contribution from additional processing other than the shared processes. These results support the notion that attention and VWM share the same processing resources.

Memory Interference as a Determinant of Language Comprehension

The parameters of the human memory system constrain the operation of language comprehension processes. In the memory literature, both decay and interference have been proposed as causes of forgetting; however, while there is a long history of research establishing the nature of interference effects in memory, the effects of decay are much more poorly supported. Nevertheless, research investigating the limitations of the human sentence processing mechanism typically focus on decay-based explanations, emphasizing the role of capacity, while the role of interference has received comparatively little attention. This paper reviews both accounts of difficulty in language comprehension by drawing direct connections to research in the memory domain. Capacity-based accounts are found to be untenable, diverging substantially from what is known about the operation of the human memory system. In contrast, recent research investigating comprehension difficulty using a retrieval-interference paradigm is shown to be wholly consistent with both behavioral and neuropsychological memory phenomena. The implications of adopting a retrieval-interference approach to investigating individual variation in language comprehension are discussed.

Neocortical-hippocampal dynamics of working memory in healthy and diseased brain states based on functional connectivity

Working memory (WM) is the ability to transiently maintain and manipulate internal representations beyond its external availability to the senses. This process is thought to support high level cognitive abilities and been shown to be strongly predictive of individual intelligence and reasoning abilities. While early models of WM have relied on a modular perspective of brain functioning, more recent evidence suggests that cognitive functions emerge from the interactions of multiple brain regions to generate large-scale networks. Here we will review the current research on functional connectivity of WM processes to highlight the critical role played by neural interactions in healthy and pathological brain states. Recent findings demonstrate that WM abilities are not determined solely by local brain activity, but also rely on the functional coupling of neocortical-hippocampal regions to support WM processes. Although the hippocampus has long been held to be important for long-term declarative memory, recent evidence suggests that the hippocampus may also be necessary to coordinate disparate cortical regions supporting the periodic reactivation of internal representations in WM. Furthermore, recent brain imaging studies using connectivity measures, have shown that changes in cortico-limbic interactions can be useful to characterize WM impairments observed in different neuropathological conditions. Recent advances in electrophysiological and neuroimaging techniques to model network activity has led to important insights into how neocortical and hippocampal regions support WM processes and how disruptions along this network can lead to the memory impairments commonly reported in many neuropathological populations.

Phonological similarity effects in simple and complex span tasks

Three experiments were conducted to examine the effect of phonological similarity in simple and complex memory span tasks. In Experiment 1, participants performed either a simple or a complex span task, and the memoranda within lists were either phonologically similar or distinct. Phonologically similar lists consisted of words that rhymed.The simple span task was word span. There were two complex span tasks; one was the original reading span task, and the other was a variant of reading span in which all the sentences within a list were contextually related. The classic phonological similarity decrement was observed in word span. In contrast, phonological similarity facilitation was observed in both versions of reading span. This facilitation effect was further investigated in Experiment 2 using two new versions of reading span. In Experiment 2, the sentences in reading span were either short or long, and the memoranda were presented separately from, and were unrelated to, the sentences. Again, words within phonologically similar lists rhymed, and again, facilitation was observed. In Experiment 3, phonological similarity was operationalized in terms of feature overlap, rather than rhyme. The classic phonological similarity decrement was still observed in word span, but facilitation was not observed in complex span. The results suggest that phonological similarity, when operationalized using words that rhyme, serves as a list retrieval cue and that complex span tasks are more dependent on cue-driven memory retrieval mechanisms than are simple span tasks.

Resolving semantic and proactive interference in memory over the short-term

Interference is a major source of short-term errors of memory. The present investigation explores the relationship between two important forms of interference: proactive interference (PI), induced by the need to reject recently studied items no longer relevant to task performance, and semantic interference (SI), induced by the need to reject lures sharing a meaningful relationship with current memoranda. We explore the possibility that shared cognitive control processes are recruited to resolve both forms of interference. In Experiment 1, we find that the requirement to engage in articulatory suppression during the retention interval of tasks that induce either PI or SI increases both forms of interference similarly and selectively. In Experiment 2, we develop a task to examine PI and SI within the same experimental context. The results show interactive effects between factors that lead to the two forms of interference. Taken together, these findings support contextual-cuing models of short-term remembering (Nairne, Annual Review of Psychology, 53, 53-81 2002), where the context in which retrieval occurs can influence susceptibility to interference. Lastly, we discuss several theoretical hypotheses concerning the cognitive control processes that are recruited to resolve SI and PI in short-term remembering.

What's magic about magic numbers? Chunking and data compression in short-term memory

Short term memory is famously limited in capacity to Miller's (1956) magic number 7±2-or, in many more recent studies, about 4±1 "chunks" of information. But the definition of "chunk" in this context has never been clear, referring only to a set of items that are treated collectively as a single unit. We propose a new more quantitatively precise conception of chunk derived from the notion of Kolmogorov complexity and compressibility: a chunk is a unit in a maximally compressed code. We present a series of experiments in which we manipulated the compressibility of stimulus sequences by introducing sequential patterns of variable length. Our subjects' measured digit span (raw short term memory capacity) consistently depended on the length of the pattern after compression, that is, the number of distinct sequences it contained. The true limit appears to be about 3 or 4 distinct chunks, consistent with many modern studies, but also equivalent to about 7 uncompressed items of typical compressibility, consistent with Miller's famous magical number.

Cue-dependent interference in comprehension

The role of interference as a primary determinant of forgetting in memory has long been accepted, however its role as a contributor to poor comprehension is just beginning to be understood. The current paper reports two studies, in which speed-accuracy tradeoff and eye-tracking methodologies were used with the same materials to provide converging evidence for the role of syntactic and semantic cues as mediators of both proactive (PI) and retroactive interference (RI) during comprehension. Consistent with previous work (e.g., Van Dyke & Lewis, 2003), we found that syntactic constraints at the retrieval site are among the cues that drive retrieval in comprehension, and that these constraints effectively limit interference from potential distractors with semantic/pragmatic properties in common with the target constituent. The data are discussed in terms of a cue-overload account, in which interference both arises from and is mediated through a direct-access retrieval mechanism that utilizes a linear, weighted cue-combinatoric scheme.

Neural evidence for a distinction between short-term memory and the focus of attention

It is widely assumed that the short-term retention of information is accomplished via maintenance of an active neural trace. However, we demonstrate that memory can be preserved across a brief delay despite the apparent loss of sustained representations. Delay period activity may, in fact, reflect the focus of attention, rather than STM. We unconfounded attention and memory by causing external and internal shifts of attention away from items that were being actively retained. Multivariate pattern analysis of fMRI indicated that only items within the focus of attention elicited an active neural trace. Activity corresponding to representations of items outside the focus quickly dropped to baseline. Nevertheless, this information was remembered after a brief delay. Our data also show that refocusing attention toward a previously unattended memory item can reactivate its neural signature. The loss of sustained activity has long been thought to indicate a disruption of STM, but our results suggest that, even for small memory loads not exceeding the capacity limits of STM, the active maintenance of a stimulus representation may not be necessary for its short-term retention.

Working memory: theories, models, and controversies

I present an account of the origins and development of the multicomponent approach to working memory, making a distinction between the overall theoretical framework, which has remained relatively stable, and the attempts to build more specific models within this framework. I follow this with a brief discussion of alternative models and their relationship to the framework. I conclude with speculations on further developments and a comment on the value of attempting to apply models and theories beyond the laboratory studies on which they are typically based.

Event-related brain potentials index cue-based retrieval interference during sentence comprehension

Successful language use requires access to products of past processing within an evolving discourse. A central issue for any neurocognitive theory of language then concerns the role of memory variables during language processing. Under a cue-based retrieval account of language comprehension, linguistic dependency resolution (e.g., retrieving antecedents) is subject to interference from other information in the sentence, especially information that occurs between the words that form the dependency (e.g., between the antecedent and the retrieval site). Retrieval interference may then shape processing complexity as a function of the match of the information at retrieval with the antecedent versus other recent or similar items in memory. To address these issues, we studied the online processing of ellipsis in Castilian Spanish, a language with morphological gender agreement. We recorded event-related brain potentials while participants read sentences containing noun-phrase ellipsis indicated by the determiner otro/a ('another'). These determiners had a grammatically correct or incorrect gender with respect to their antecedent nouns that occurred earlier in the sentence. Moreover, between each antecedent and determiner, another noun phrase occurred that was structurally unavailable as an antecedent and that matched or mismatched the gender of the antecedent (i.e., a local agreement attractor). In contrast to extant P600 results on agreement violation processing, and inconsistent with predictions from neurocognitive models of sentence processing, grammatically incorrect determiners evoked a sustained, broadly distributed negativity compared to correct ones between 400 and 1000ms after word onset, possibly related to sustained negativities as observed for referential processing difficulties. Crucially, this effect was modulated by the attractor: an increased negativity was observed for grammatically correct determiners that did not match the gender of the attractor, suggesting that structurally unavailable noun phrases were at least temporarily considered for grammatically correct ellipsis. These results constitute the first ERP evidence for cue-based retrieval interference during comprehension of grammatical sentences.

Retroactive interference in short-term memory and the word-length effect

Two experiments investigated the possibility that the word-length effect in short-term memory (STM) is a consequence of long words generating a greater level of retroactive interference than shorter words. In Experiment 1, six-word lists were auditorily presented under articulatory suppression for immediate serial reconstruction of only the first three words. These three words were always drawn from a single set of middle-length words, whereas the last three positions were occupied by either short or long interfering words. The results showed worse memory performance when the to-be-remembered words were followed by long words. In Experiment 2, a recent-probes task was used, in which recent negative probes matched a target word in trial n-2. The results showed lower levels of proactive interference when trial n-1 involved long words instead of short words, suggesting that long words displaced previous STM content to a greater extent. By two different experimental approaches, therefore, this study shows that long words produce more retroactive interference than short words, supporting an interference-based account for the word-length effect.

Reversing the course of forgetting

Forgetting functions were generated for pigeons in a delayed matching-to-sample task, in which accuracy decreased with increasing retention-interval duration. In baseline training with dark retention intervals, accuracy was high overall. Illumination of the experimental chamber by a houselight during the retention interval impaired performance accuracy by increasing the rate of forgetting. In novel conditions, the houselight was lit at the beginning of a retention interval and then turned off partway through the retention interval. Accuracy was low at the beginning of the retention interval and then increased later in the interval. Thus the course of forgetting was reversed. Such a dissociation of forgetting from the passage of time is consistent with an interference account in which attention or stimulus control switches between the remembering task and extraneous events.

Reversing the signaled magnitude effect in delayed matching to sample: delay-specific remembering

Pigeons performed a delayed matching-to-sample task in which large or small reinforcers for correct remembering were signaled during the retention interval. Accuracy was low when small reinforcers were signaled, and high when large reinforcers were signaled (the signaled magnitude effect). When the reinforcer-size cue was switched from small to large partway through the retention interval, accuracy accordingly changed from low to high. The opposite happened when the cue was switched from large to small. This dissociation of forgetting from the passage of time raises the possibility that remembering is delay-specific. The reversal of the signaled magnitude effect during the retention interval is consistent with an attentional account in which the stimulus control of remembering is influenced by extraneous events.

Recency Effects in the Inferior Parietal Lobe during Verbal Recognition Memory

The most recently encountered information is often most easily remembered in psychological tests of memory. Recent investigations of the neural basis of such “recency effects” have shown that activation in the lateral inferior parietal cortex (LIPC) tracks the recency of a probe item when subjects make recognition memory judgments. A key question regarding recency effects in the LIPC is whether they fundamentally reflect the storage (and strength) of information in memory, or whether such effects are a consequence of task difficulty or an upswing in resting state network activity. Using functional magnetic resonance imaging we show that recency effects in the LIPC are independent of the difficulty of recognition memory decisions, that they are not a by-product of an increase in resting state network activity, and that they appear to dissociate from regions known to be involved in verbal working memory maintenance. We conclude with a discussion of two alternative explanations – the memory strength and “expectancy” hypotheses, respectively – of the parietal lobe recency effect.

Does working memory training work? The promise and challenges of enhancing cognition by training working memory

A growing body of literature shows that one's working memory (WM) capacity can be expanded through targeted training. Given the established relationship between WM and higher cognition, these successful training studies have led to speculation that WM training may yield broad cognitive benefits. This review considers the current state of the emerging WM training literature, and details both its successes and limitations. We identify two distinct approaches to WM training, strategy training and core training, and highlight both the theoretical and practical motivations that guide each approach. Training-related increases in WM capacity have been successfully demonstrated across a wide range of subject populations, but different training techniques seem to produce differential impacts upon the broader landscape of cognitive abilities. In particular, core WM training studies seem to produce more far-reaching transfer effects, likely because they target domain-general mechanisms of WM. The results of individual studies encourage optimism regarding the value of WM training as a tool for general cognitive enhancement. However, we discuss several limitations that should be addressed before the field endorses the value of this approach.

Memory profiles of Down, Williams, and fragile X syndromes: implications for reading development

The purpose of this review was to understand the types of memory impairments that are associated with intellectual disability (ID, formerly called mental retardation) and the implications of these impairments for reading development. Specifically, studies on working memory, delayed memory and learning, and semantic/conceptual memory in Down syndrome, Williams syndrome, and fragile X syndrome were examined. A distinct memory profile emerged for each of the 3 etiologies of ID. Memory profiles are discussed in relation to strengths and weaknesses in reading skills in these three etiologies. We suggest that reading instruction be designed to capitalize on relatively stronger memory skills while providing extra support for especially challenging aspects of reading.

A review of visual memory capacity: Beyond individual items and toward structured representations

Traditional memory research has focused on identifying separate memory systems and exploring different stages of memory processing. This approach has been valuable for establishing a taxonomy of memory systems and characterizing their function but has been less informative about the nature of stored memory representations. Recent research on visual memory has shifted toward a representation-based emphasis, focusing on the contents of memory and attempting to determine the format and structure of remembered information. The main thesis of this review will be that one cannot fully understand memory systems or memory processes without also determining the nature of memory representations. Nowhere is this connection more obvious than in research that attempts to measure the capacity of visual memory. We will review research on the capacity of visual working memory and visual long-term memory, highlighting recent work that emphasizes the contents of memory. This focus impacts not only how we estimate the capacity of the system--going beyond quantifying how many items can be remembered and moving toward structured representations--but how we model memory systems and memory processes.

Direct-access retrieval during sentence comprehension: Evidence from Sluicing

Language comprehension requires recovering meaning from linguistic form, even when the mapping between the two is indirect. A canonical example is ellipsis, the omission of information that is subsequently understood without being overtly pronounced. Comprehension of ellipsis requires retrieval of an antecedent from memory, without prior prediction, a property which enables the study of retrieval in situ (Martin & McElree, 2008, 2009). Sluicing, or inflectional phrase ellipsis, in the presence of a conjunction, presents a test case where a competing antecedent position is syntactically licensed, in contrast with most cases of nonadjacent dependency, including verb phrase ellipsis. We present speed-accuracy tradeoff and eye-movement data inconsistent with the hypothesis that retrieval is accomplished via a syntactically guided search, a particular variant of search not examined in past research. The observed timecourse profiles are consistent with the hypothesis that antecedents are retrieved via a cue-dependent direct-access mechanism susceptible to general memory variables.

At the intersection of attention and memory: the mechanistic role of the posterior parietal lobe in working memory

Portions of the posterior parietal cortex (PPC) play a role in working memory (WM) yet the precise mechanistic function of this region remains poorly understood. The pure storage hypothesis proposes that this region functions as a short-lived modality-specific memory store. Alternatively, the internal attention hypothesis proposes that the PPC functions as an attention-based storage and refreshing mechanism deployable as an alternative to material-specific rehearsal. These models were tested in patients with bilateral PPC lesions. Our findings discount the pure storage hypothesis because variables indexing storage capacity and longevity were not disproportionately affected by PPC damage. Instead, our data support the internal attention account by showing that (a) normal participants tend to use a rehearsal-based WM maintenance strategy for recall tasks but not for recognition tasks; (b) patients with PPC lesions performed normally on WM tasks that relied on material-specific rehearsal strategies but poorly on WM tasks that relied on attention-based maintenance strategies and patient strategy usage could be shifted by task or instructions; (c) patients' memory deficits extended into the long-term domain. These findings suggest that the PPC maintains or shifts internal attention among the representations of items in WM.

The focus of attention as observed in visual working memory tasks: making sense of competing claims

Recent behavioral and neuroscientific evidence speaks to the question of whether the human focus of attention is limited to a single item or can accommodate several items. This issue is fundamental to an understanding of the nature of human cognition and brain function. Here I review evidence from visual working memory tasks and suggest that it supports the concept of a focus of attention that can include several items at once as a core vehicle of working memory, regardless of the stimulus modality. One brain area in particular, the left intraparietal sulcus (IPS), seems critically important in the network underlying the focus of attention as a working memory storage mechanism. This view is reconciled with evidence previously taken to indicate that the focus of attention only includes a single item at a time, which is reinterpreted here.

Comparisons of Complex Sounds across Extended Retention Intervals Survives Reading Aloud

A simple experimental arrangement is designed to foil verbal rehearsal during an extended (from 5 to 30 s) retention interval across which participants attempt to discriminate two periodic complex sounds. Sounds have an abstract timbre that does not lend itself to verbal labeling, they differ across trials so that no ‘standard’ comparison stimulus is built up by the participants, and the spectral change to be discriminated is very slight and therefore does not shift the stimulus into a new verbal category. And, crucially, in one experimental condition, participants read aloud during most of the retention interval. Despite these precautions, performance is robust across the extended retention interval. The inference is that one form of auditory memory does not require verbal rehearsal. Nevertheless, modest forgetting occurred. Whatever form memory takes in this situation, it is not totally secure from disruption.

A single brief burst induces GluR1-dependent associative short-term potentiation: a potential mechanism for short-term memory

Recent work showed that short-term memory (STM) is selectively reduced in GluR1 knockout mice. This raises the possibility that a form of synaptic modification dependent on GluR1 might underlie STM. Studies of synaptic plasticity have shown that stimuli too weak to induce long-term potentiation induce short-term potentiation (STP), a phenomenon that has received little attention. Here we examined several properties of STP and tested the dependence of STP on GluR1. The minimal requirement for inducing STP was examined using a test pathway and a conditioning pathway. Several closely spaced stimuli in the test pathway, forming a single brief burst, were sufficient to induce STP. Thus, STP is likely to be induced by the similar bursts that occur in vivo. STP induction is associative in nature and dependent on the NMDAR. STP decays with two components, a fast component (1.6 +/- 0.26 min) and a slower one (19 +/- 6.6 min). To test the role of GluR1 in STP, experiments were conducted on GluR1 knockout mice. We found that STP was greatly reduced. These results, taken together with the behavioral work of D. Sanderson et al. [Sanderson, D., Good, M. A., Skelton, K., Sprengel, R., Seeburg, P. H., Nicholas, J., et al. Enhanced long-term and impaired short-term spatial memory in GluA1 AMPA receptor subunit knockout mice: Evidence for a dual-process memory model. Learning and Memory, 2009], provide genetic evidence that STP is a likely mechanism of STM.

The phonological similarity effect in serial recognition

An experiment is reported examining the effect of consistent phonemic similarity among list items on memory retention in a task that is independent of overt speech production, the serial recognition task. Lists of consonant-vowel-consonant (CVC) words and nonwords were constructed such that although phoneme overlap was manipulated (i.e., shared vowel and final consonant [_VC], initial consonant and vowel [CV_], or the two consonants [C_C]), similarity remained constant. The results show that the influence of sub-syllabic mechanisms on STM performance is independent of speech production processes, and similarity in the pattern of results suggests that the same mechanisms subserve the recall of words and nonwords in STM. It is argued that the results are more consistent with psycholinguistic models than nonlinguistic models of STM, and implications for current STM models are discussed.

Are representations in working memory distinct from representations in long-term memory? Neural evidence in support of a single store

Neural activation in a 12-item probe-recognition task was examined to investigate the contribution of the hippocampus to long-term memory (LTM) retrieval and working memory (WM) retrieval. Results indicated a dissociation between the last item that participants studied and other items of the study list: Compared with all other serial positions, activation was reduced for the item in the most recent position (for which no items intervened between study and test). This finding suggests that this last item was in focal attention at test time, and, therefore, no retrieval operation was required to access it. However, contra the assertion that the hippocampus should selectively support LTM, activation of the medial temporal lobe was observed for all serial positions other than the last position, and activation level could be predicted from the underlying memory strength. Collectively, these findings support single-store accounts that assume there are similar operating principles across WM and LTM representations, and the focus of attention is limited.

Rapid Communication: Interference in Short-Term Auditory Memory

Although interference is a well-established forgetting function in short-term auditory memory, an adequate understanding of its underlying mechanisms and time course has yet to be attained. The present study therefore aimed to explore these issues in memory for timbre. Listeners compared standard and comparison complex tones, having distinct timbres (four components varying in frequency), over a 4.7-s retention interval and made a same–different response. This interval either was silent or included one of 15 distractor tones occurring 0 ms, 100 ms, or 1,200 ms after the standard. These distractors varied in the extent to which the frequencies of their component tones were shared with the standard. Performance in comparing the two tones was significantly impaired by distractors composed of novel frequencies, regardless of the temporal position at which the distractor occurred. These results were fully compatible with the recent timbre memory model (McKeown & Wellsted, 2009) and suggested that interference in auditory memory operates via a feature-overwriting mechanism.

On the division of working memory and long-term memory and their relation to intelligence: A latent variable approach

The present study examined the extent to which working (WM) and long-term memory (LTM) reflect the same, related, or completely different constructs and how they relate to other cognitive ability constructs. Participants performed various WM, recall, recognition, general fluid (gF) and general crystallized intelligence (gC) measures. Confirmatory factor analyses suggested that the memory measures could be grouped into three separate yet correlated factors (WM, recall, and recognition) and that these factors were strongly related to gF, but were related less so with gC. Furthermore, it was found that the common variance from the three memory factors could be accounted for by a higher-order memory factor which was strongly related to gF, but less so with gC. Finally, structural equation modeling suggested that both the variance common to the WM tasks and the variance common to all the memory tasks accounted for a unique variance in gF. These results are interpreted within an embedded process model of memory and suggest that WM and LTM tasks measure both shared and unique processes, which are important for intelligence.

Long-term versus short-term memory deficits for faces in temporal lobe and generalized epilepsy patients

It is still an open question whether short-term and long-term memory are two anatomically dissociable memory systems working in parallel or whether they are represented by neural circuits within similar cortical areas. Epilepsy may be used as a model to study these memory processes. We hypothesized that a double dissociation of short-term and long-term memory exists in temporal lobe epilepsy (TLE) and idiopathic generalized epilepsy (IGE). Immediate and 24-hour face recognition was tested in 10 TLE patients, 9 IGE patients, and 10 healthy controls. TLE patients' immediate recognition was unimpaired, but their memory scores were reduced as compared to healthy controls after 24 hours. In IGE patients, memory was already reduced during immediate recognition. These results are in line with the idea that short-term memory is a transient trace that requires consolidation supported by the medial temporal lobe to change into a more stable status of long-term memory.

Evaluating models of working memory through the effects of concurrent irrelevant information

Working memory is believed to play a central role in almost all domains of higher cognition, yet the specific mechanisms involved in working memory are still fiercely debated. We describe a neuroimaging experiment with functional magnetic resonance imaging (fMRI) and a companion behavioral experiment, and in both we seek to adjudicate between alternative theoretical models of working memory on the basis of the effects of interference from articulatory suppression, irrelevant speech, and irrelevant nonspeech. In Experiment 1 we examined fMRI signal changes induced by each type of irrelevant information while subjects performed a probed recall task. Within a principally frontal and left-lateralized network of brain regions, articulatory suppression caused an increase in activity during item presentation, whereas both irrelevant speech and nonspeech caused relative activity reductions during the subsequent delay interval. In Experiment 2, the specific timing of interference was manipulated in a delayed serial recall task. Articulatory suppression was found to be most consequential when it coincided with item presentation, whereas both irrelevant speech and irrelevant nonspeech effects were strongest when limited to the subsequent delay period. Taken together, these experiments provide convergent evidence for a dissociation of articulatory suppression from the 2 irrelevant sound conditions. Implications of these findings are considered for 4 prominent theories of working memory.