Neuronal correlates of encoding and retrieval in episodic memory during a paired-word association learning task: a functional magnetic resonance imaging study

Distinct neurocognitive mechanisms underlying learning and representations of symbols of life and death

Life and death are 2 fundamental concepts regarding existence of organisms. People often signify these concepts using symbols to facilitate communications, but how the brain learns and represents these symbols remains unclear. In the present study, we quantified behavioral and brain responses during learning associations between words ("life" or "death") with shapes as concrete referents. Behavioral responses to word-shape pairs showed an affirmative response bias to life-shape pairs but a denial response bias to death-shape pairs. Multimodal brain imaging results revealed that the right frontal and dorsal cingulate cortices monitored these response biases, respectively. Moreover, relative to unlearned shapes, life-related shapes induced increased alpha (9-14 Hz) oscillations in the right parietal cortex and precuneus, whereas death-related shapes enhanced beta (15-30 Hz) oscillations in the left parietal cortex, superior temporal sulcus, and precuneus. Our findings unraveled distinct neurocognitive mechanisms underlying learning and representations of concrete referents of life and death concepts.

Computational support, not primacy, distinguishes compensatory memory reorganization in epilepsy

Temporal lobe epilepsy is associated with impairment in episodic memory. A substantial subgroup, however, is able to maintain adequate memory despite temporal lobe pathology. Missing from prior work in cognitive reorganization is a direct comparison of temporal lobe epilepsy patients with intact status with those who are memory impaired. Little is known about the regional activations, functional connectivities and/or network reconfigurations that implement changes in primary computations or support functions that drive adaptive plasticity and compensated memory. We utilized task functional MRI on 54 unilateral temporal lobe epilepsy patients and 24 matched healthy controls during the performance of a paired-associate memory task to address three questions: (i) which regions implement paired-associate memory in temporal lobe epilepsy, and do they vary as a function of good versus poor performance, (ii) is there unique functional connectivity present during memory encoding that accounts for intact status by preservation of primary memory computations or the supportive computations that allow for intact memory responses and (iii) what features during memory encoding are most distinctive: is it the magnitude and location of regional activations, or the presence of enhanced functional connections to key structures such as the hippocampus? The study revealed non-dominant hemisphere regions (right posterior temporal regions) involving both increased regional activity and increased modulatory communication with the hippocampi as most important to intact memory in left temporal lobe epilepsy compared to impaired status. The profile involved areas that are neither contralateral homologues to left hemisphere memory areas, nor regions traditionally considered computationally primary for episodic memory. None of these areas of increased activation or functional connectivity were associated with advantaged memory in healthy controls. Our emphasis on different performance levels yielded insight into two forms of cognitive reorganization: computational primacy, where left temporal lobe epilepsy showed little change relative to healthy controls, and computational support where intact left temporal lobe epilepsy patients showed adaptive abnormalities. The analyses isolated the unique regional activations and mediating functional connectivity that implements truly compensatory reorganization in left temporal lobe epilepsy. The results provided a new perspective on memory deficits by making clear that they arise not just from the knockout of a functional hub, but from the failure to instantiate a complex set of reorganization responses. Such responses provided the computational support to ensure successful memory. We demonstrated that by keeping track of performance levels, we can increase understanding of adaptive brain responses and neuroplasticity in epilepsy.

Memory response and neuroimaging correlates of a novel cognitive rehabilitation program for memory problems in epilepsy: A pilot study

BACKGROUND

Memory deficits are very common in epilepsy, but no standard of care exists to effectively manage them.

OBJECTIVE

We assessed effectiveness of cognitive rehabilitation (CR) on memory and neural plasticity in people with epilepsy (PWE) reporting memory impairments.

METHODS

Nine PWE completed 6 weekly sessions adapted from 2 generic CR programs enriched with information regarding epilepsy. Participants completed neuropsychological, mood, and quality of life (QOLIE-31) measures prior and after completion of CR; 5/9 participants also completed pre- and post-CR fMRI while performing a verbal paired associates learning task. FMRI data were analyzed using group spatial independent components analysis methods; paired t-tests compared spatial activations for pre-/post-CR.

RESULTS

Improvements were seen in immediate recall in Rey Auditory Verbal Learning Task, QOLIE-31, and read word recognition in paired associates task (all p's≤0.05). FMRI changes comparing pre-to-post CR were noted through increased activation in the left inferior frontal gyrus (IFG) and anterior cingulate and decreased activation in the left superior temporal gyrus; also noted were decreased activations in the default mode network (DMN), right cingulate, right middle temporal gyrus, right supramarginal gyrus, and increased DMN activation in the left cuneus.

CONCLUSIONS

This study demonstrates feasibility of conducting CR program in PWE with fMRI as a mechanistic biomarker. Improvements in cognition and cortical plasticity await confirmation in larger samples.

Neural Correlates of Feedback Processing in Visuo-Tactile Crossmodal Paired-Associate Learning

Previous studies have examined the neural correlates for crossmodal paired-associate (PA) memory and the temporal dynamics of its formation. However, the neural dynamics for feedback processing of crossmodal PA learning remain unclear. To examine this process, we recorded event-related scalp electrical potentials for PA learning of unimodal visual-visual pairs and crossmodal visual-tactile pairs when participants performed unimodal and crossmodal tasks. We examined event-related potentials (ERPs) after the onset of feedback in the tasks for three effects: feedback type (positive feedback vs. negative feedback), learning (as the learning progressed) and the task modality (crossmodal vs. unimodal). The results were as follows: (1) feedback type: the amplitude of P300 decreased with incorrect trials and the P400/N400 complex was only present in incorrect trials; (2) learning: progressive positive voltage shifts in frontal recording sites and negative voltage shifts in central and posterior recording sites were identified as learning proceeded; and (3) task modality: compared with the unimodal PA learning task, positive voltage shifts in frontal sites and negative voltage shifts in posterior sites were found in the crossmodal PA learning task. To sum up, these results shed light on cortical excitability related to feedback processing of crossmodal PA learning.

Declarative long-term memory and the mesial temporal lobe: Insights from a 5-year postsurgery follow-up study on refractory temporal lobe epilepsy

It is largely recognized that the mesial temporal lobe and its substructure support declarative long-term memory (LTM). So far, different theories have been suggested, and the organization of declarative verbal LTM in the brain is still a matter of debate. In the current study, we retrospectively selected 151 right-handed patients with temporal lobe epilepsy with and without hippocampal sclerosis, with a homogeneous (seizure-free) clinical outcome. We analyzed verbal memory performance within a normalized scores context, by means of prose recall and word paired-associate learning tasks. Patients were tested at presurgical baseline, 6months, 2 and 5years after anteromesial temporal lobe surgery, using parallel versions of the neuropsychological tests. Our main finding revealed a key involvement of the left temporal lobe and, in particular, of the left hippocampus in prose recall rather than word paired-associate task. We also confirmed that shorter duration of epilepsy, younger age, and withdrawal of antiepileptic drugs would predict a better memory outcome. When individual memory performance was taken into account, data showed that females affected by left temporal lobe epilepsy for longer duration were more at risk of presenting a clinically pathologic LTM at 5years after surgery. Taken together, these findings shed new light on verbal declarative memory in the mesial temporal lobe and on the behavioral signature of the functional reorganization after the surgical treatment of temporal lobe epilepsy.

Awake, Offline Processing during Associative Learning

Offline processing has been shown to strengthen memory traces and enhance learning in the absence of conscious rehearsal or awareness. Here we evaluate whether a brief, two-minute offline processing period can boost associative learning and test a memory reactivation account for these offline processing effects. After encoding paired associates, subjects either completed a distractor task for two minutes or were immediately tested for memory of the pairs in a counterbalanced, within-subjects functional magnetic resonance imaging study. Results showed that brief, awake, offline processing improves memory for associate pairs. Moreover, multi-voxel pattern analysis of the neuroimaging data suggested reactivation of encoded memory representations in dorsolateral prefrontal cortex during offline processing. These results signify the first demonstration of awake, active, offline enhancement of associative memory and suggest that such enhancement is accompanied by the offline reactivation of encoded memory representations.

Age related-changes in the neural basis of self-generation in verbal paired associate learning

Verbal information is better retained when it is self-generated rather than when it is received passively. The application of self-generation procedures has been found to improve memory in healthy elderly and in individuals with impaired cognition. Overall, the available studies support the notion that active participation in verbal encoding engages memory mechanisms that supplement those used during passive observation. Thus, the objective of this study was to investigate the age-related changes in the neural mechanisms involved in the encoding of paired-associates using a self-generation method that has been shown to improve memory performance across the lifespan. Subjects were 113 healthy right-handed adults (Edinburgh Handedness Inventory >50; 67 females) ages 18–76, native speakers of English with no history of neurological or psychiatric disorders. Subjects underwent fMRI at 3 T while performing didactic learning (“read”) or self-generation learning (“generate”) of 30 word pairs per condition. After fMRI, recognition memory for the second word in each pair was evaluated outside of the scanner. On the post-fMRI testing more “generate” words were correctly recognized than “read” words (p < 0.001) with older adults recognizing the “generated” words less accurately (p < 0.05). Independent component analysis of fMRI data identified task-related brain networks. Several components were positively correlated with the task reflecting multiple cognitive processes involved in self-generated encoding; other components correlated negatively with the task, including components of the default-mode network. Overall, memory performance on generated words decreased with age, but the benefit from self-generation remained consistently significant across ages. Independent component analysis of the neuroimaging data revealed an extensive set of components engaged in self-generation learning compared with didactic learning, and identified areas that were associated with age-related changes independent of performance.

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Verbal information is better retained when self-generated vs. received passively.

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Application of self-generation is associated with better retention across ages.

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Generated words were retained better than read words.

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Several components of network for word generation were identified.

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Age-associated changes within the network are discussed.

Associative learning beyond the medial temporal lobe: many actors on the memory stage

Decades of research have established a model that includes the medial temporal lobe, and particularly the hippocampus, as a critical node for episodic memory. Neuroimaging and clinical studies have shown the involvement of additional cortical and subcortical regions. Among these areas, the thalamus, the retrosplenial cortex, and the prefrontal cortices have been consistently related to episodic memory performance. This article provides evidences that these areas are in different forms and degrees critical for human memory function rather than playing only an ancillary role. First we briefly summarize the functional architecture of the medial temporal lobe with respect to recognition memory and recall. We then focus on the clinical and neuroimaging evidence available on thalamo-prefrontal and thalamo-retrosplenial networks. The role of these networks in episodic memory has been considered secondary, partly because disruption of these areas does not always lead to severe impairments; to account for this evidence, we discuss methodological issues related to the investigation of these regions. We propose that these networks contribute differently to recognition memory and recall, and also that the memory stage of their contribution shows specificity to encoding or retrieval in recall tasks. We note that the same mechanisms may be in force when humans perform non-episodic tasks, e.g., semantic retrieval and mental time travel. Functional disturbance of these networks is related to cognitive impairments not only in neurological disorders, but also in psychiatric medical conditions, such as schizophrenia. Finally we discuss possible mechanisms for the contribution of these areas to memory, including regulation of oscillatory rhythms and long-term potentiation. We conclude that integrity of the thalamo-frontal and the thalamo-retrosplenial networks is necessary for the manifold features of episodic memory.

Age-related changes in prefrontal and hippocampal contributions to relational encoding

Age-related declines in relational encoding are well documented. It remains unclear, however, whether such declines reflect dysfunction of (1) ventrolateral prefrontal cortex (VLPFC) and deficient generation of associations; and/or (2) hippocampal dysfunction and impoverished binding of associations. In order to separate VLPFC and hippocampal contributions to relational encoding, we manipulated the generative demands of the encoding task by varying the number of semantic associations between the to-be-encoded information (three words). Thus, trials with fewer semantic associations (lower-association trials) require more generative processing during encoding, relative to trials in which more semantic associations are provided for binding (higher-association trials). Parametric modulation analyses on successfully encoded items revealed that, unlike younger adults, older adults did not show an up-regulation of VLPFC activity during lower-association trials. In contrast, hippocampal activity in both older and younger adults was greater in higher- relative to lower-association trials. Moreover, recognition accuracy improved significantly in both groups with the provision of more semantic associations, indicating that both younger and older adults benefitted from this form of encoding support. Our findings suggest that left VLPFC dysfunction may underlie relational encoding deficits in older adults, but that when provided with associations to bind, hippocampal activity in older adults is comparable to young, consistent with their increased recognition accuracy under conditions of encoding support.

Functional imaging of the thalamus in language

Herein, the literature regarding functional imaging of the thalamus during language tasks is reviewed. Fifty studies met criteria for analysis. Two of the most common task paradigms associated with thalamic activation were generative tasks (e.g. word or sentence generation) and naming, though activation was also seen in tasks that involve lexical decision, reading and working memory. Typically, thalamic activation was seen bilaterally, left greater than right, along with activation in frontal and temporal cortical regions. Thalamic activation was seen with perceptually challenging tasks, though few studies rigorously correlated thalamic activation with measures of attention or task difficulty. The peaks of activation loci were seen in virtually all thalamic regions, with a bias towards left-sided and midline activation. These analyses suggest that the thalamus may be involved in processes that involve manipulations of lexical information, but point to the need for more systematic study of the thalamus using language tasks.

Segregating cognitive functions within hippocampal formation: a quantitative meta-analysis on spatial navigation and episodic memory

The most important cognitive domains where hippocampal formation is crucially involved are navigation and memory. Some evidence suggests that different hippocampal subregions mediate these domains. However, a quantitative meta-analysis on neuroimaging studies of spatial navigation versus memory is lacking. By means of activation likelihood estimation (ALE), we investigate concurrence of brain regions activated during spatial navigation encoding and retrieval as well as during episodic memory encoding and retrieval tasks in humans. During encoding in spatial navigation, activity was located in more posterior regions of the hippocampal formation, whereas episodic memory encoding was located in more anterior regions. Retrieval in spatial navigation was more strongly lateralized to the right compared to episodic memory retrieval. Within studies on spatial navigation retrieval, immediate recall was located more posterior and delayed recall more anterior. Overlap between concurrence of activation in spatial navigation and episodic memory was rather limited in comparison to uniquely involved regions. This argues in favor of two distinct networks, one for spatial navigation the other for episodic memory within the hippocampal formation.

Cortical correlates of self-generation in verbal paired associate learning

Behavioral studies have shown that verbal information is better retained when it is self-generated rather than read (learned passively). We used fMRI and a paired associates task to examine brain networks underlying self-generated memory encoding. Subjects were 49 healthy English speakers ages 19-62 (30 female). In the fMRI task, related word pairs were presented in a "read" condition, where subjects viewed both words and read the second word aloud, or a "generate" condition, where the second word was presented with only the first letter and the subject was required to generate the word. Thirty word pairs were presented in each condition. After the fMRI scan, words that were read or generated were presented, each with two foils, in a forced-choice recognition task. On the recognition post-test, words from the "generate" condition were more correctly recognized than from the "read" condition (80.0% for generated words versus 72.0% for read words; t(48)=5.17, p<0.001). FMRI revealed increased activation for generate>read in inferior/middle frontal gyri bilaterally (L>R), anterior cingulate, and caudate nucleus and the temporo-parietal-occipital junction bilaterally. For the "read" condition, better subsequent memory performance across individual subjects was positively correlated with activation in the cuneus bilaterally. In the "generate" condition, better subsequent memory performance was positively correlated with activation in the left superior temporal gyrus. These results suggest that self-generation improves memory performance, that enhanced cortical activation accompanies self-generated encoding, and that recruitment of a specific brain network underlies self-generated encoding. The findings may have implications for the development of procedures to enhance memory performance.

Reduced specificity of hippocampal and posterior ventrolateral prefrontal activity during relational retrieval in normal aging

Neuroimaging studies of episodic memory in young adults demonstrate greater functional neural activity in ventrolateral pFC and hippocampus during retrieval of relational information as compared with item information. We tested the hypothesis that healthy older adults--individuals who exhibit behavioral declines in relational memory--would show reduced specificity of ventrolateral prefrontal and hippocampal regions during relational retrieval. At study, participants viewed two nouns and were instructed to covertly generate a sentence that related the words. At retrieval, fMRIs were acquired during item and relational memory tasks. In the relational task, participants indicated whether the two words were previously seen together. In the item task, participants indicated whether both items of a pair were previously seen. In young adults, left posterior ventrolateral pFC and bilateral hippocampal activity was modulated by the extent to which the retrieval task elicited relational processing. In older adults, activity in these regions was equivalent for item and relational memory conditions, suggesting a reduction in ventrolateral pFC and hippocampal specificity with normal aging.

Involvement of the parietal cortex in perceptual learning (Eureka effect): an interference approach using rTMS

The neural mechanisms underlying perceptual learning are still under investigation. Eureka effect is a form of rapid, long-lasting perceptual learning by which a degraded image, which appears meaningless when first seen, becomes recognizable after a single exposure to its undegraded version. We used online interference by focal 10-Hz repetitive transcranial magnetic stimulation (rTMS) to evaluate whether the parietal cortex (PC) is involved in Eureka effect, as suggested by neuroimaging data. RTMS of the PC did not affect recognition of degraded pictures when displayed 2s after the presentation of their undegraded version (learning phase). However, rTMS delivered over either right or left intraparietal sulcus simultaneously to the undegraded image presentation, disrupted identification of the degraded version of the same pictures when displayed 30 min after the learning phase. In contrast, recognition of degraded images was unaffected by rTMS over the vertex or by sham rTMS, or when rTMS of either PC was delivered 2s after the presentation of the undegraded image. Findings strongly support the hypothesis that both PC at the level of the intraparietal sulcus play a pivotal role in the Eureka effect particularly in consolidation processes, and contribute to elucidate the neural network underlying rapid perceptual learning.

Comparison of the disparity between Talairach and MNI coordinates in functional neuroimaging data: validation of the Lancaster transform

Spatial normalization of neuroimaging data is a standard step when assessing group effects. As a result of divergent analysis procedures due to different normalization algorithms or templates, not all published coordinates refer to the same neuroanatomical region. Specifically, the literature is populated with results in the form of MNI or Talairach coordinates, and their disparity can impede the comparison of results across different studies. This becomes particularly problematic in coordinate-based meta-analyses, wherein coordinate disparity should be corrected to reduce error and facilitate literature reviews. In this study, a quantitative comparison was performed on two corrections, the Brett transform (i.e., "mni2tal"), and the Lancaster transform (i.e., "icbm2tal"). Functional magnetic resonance imaging (fMRI) data acquired during a standard paired associates task indicated that the disparity between MNI and Talairach coordinates was better reduced via the Lancaster transform, as compared to the Brett transform. In addition, an activation likelihood estimation (ALE) meta-analysis of the paired associates literature revealed that a higher degree of concordance was obtained when using the Lancaster transform in the form of fewer, smaller, and more intense clusters. Based on these results, we recommend that the Lancaster transform be adopted as the community standard for reducing disparity between results reported as MNI or Talairach coordinates, and suggest that future spatial normalization strategies be designed to minimize this variability in the literature.

Functional imaging of memory processes in humans: positron emission tomography and functional magnetic resonance imaging

Human memory is not a unitary function; it consists of multiple memory systems, with different characteristics and specialisations that are implemented in the brain. The cognitive neuroscience of human memory tries to comprehend how we encode, store, and retrieve memory items within and across those systems. The emergence of functional neuroimaging techniques offered the unprecedented opportunity to directly observe the brain regions engaged in memory functions. Brain imaging techniques can roughly be divided into those measuring the electric or magnetic fields generated by neuronal activity (EEG, magnetencephalography [MEG]) and those measuring the haemodynamic or metabolic sequelae of neuronal activity (positron emission tomography [PET], functional magnetic resonance imaging [fMRI]). Out of these techniques, the following two will be discussed in detail: fMRI and PET. Although functional neuroimaging is able to acquire images of the brain engaged in consolidating or retrieving memories, these processes are not clearly visible in the data. Statistical techniques are needed to reduce the complexity of the data and to extract the processes of interest. This article outlines the experimental and analytical procedures of neuroimaging studies with PET and fMRI. We will use a PET-study on episodic memory in human volunteers to illustrate design, analysis, and interpretation of functional imaging studies on memory.

Learning in trance: functional brain imaging studies and neuropsychology

This study examined the fundamental question, whether verbal memory processing in hypnosis and in the waking state is mediated by a common neural system or by distinct cortical areas. Seven right-handed volunteers (25.4 years, sd 3.1) with high-hypnotic susceptibility scores were PET-scanned while encoding/retrieving word associations either in hypnosis or in the waking state. Word-pairs were visually presented and highly imaginable, but not semantically related (e.g. monkey-street). The presentation of pseudo-words served as a reference condition. An emission scan was recorded after each intravenous administration of O-15 water. Encoding under hypnosis was associated with more pronounced bilateral activations in the occipital cortex and the prefrontal areas as compared to learning in the waking state. During memory retrieval of word-pairs which had been previously learned under hypnosis, activations were found in the occipital lobe and the cerebellum. Under both experimental conditions precuneus and prefrontal cortex showed a consistent bilateral activation which was most distinct when the learning had taken place under hypnosis. In order to further analyze the effect of hypnosis on imagery-mediated learning, we administered sets of high-imagery word-pairs and sets of abstract words. In the first experimental condition word-pair associations were presented visually. In the second condition it was found that highly hypnotisable persons recalled significantly more high-imagery words under hypnosis as compared to low-hypnotisables both in the visual and auditory modality. Furthermore, high-imagery words were also better recalled by the highly hypnotisable subjects during the non-hypnotic condition. The memory effect was consistently present under both, immediate and delayed recall conditions. Taken together, the findings advance our understanding of the neural representation that underlies hypnosis and the neuropsychological correlates of hypnotic susceptibility.

Bilingual and multilingual language processing

This chapter addresses the interesting question on the neurolinguistics of bilingualism and the representation of language in the brain in bilingual and multilingual subjects. A fundamental issue is whether the cerebral representation of language in bi- and multilinguals differs from that of monolinguals, and if so, in which specific way. This is an interdisciplinary question which needs to identify and differentiate different levels involved in the neural representation of languages, such as neuroanatomical, neurofunctional, biochemical, psychological and linguistic levels. Furthermore, specific factors such as age, manner of acquisition and environmental factors seem to affect the neural representation. We examined the question whether verbal memory processing in two unrelated languages is mediated by a common neural system or by distinct cortical areas. Subjects were Finnish-English adult multilinguals who had acquired the second language after the age of ten. They were PET-scanned whilst either encoding or retrieving word pairs in their mother tongue (Finnish) or in a foreign language (English). Within each language, subjects had to encode and retrieve four sets of 12 visually presented paired word associates which were not semantically related. Two sets consisted of highly imaginable words and the other two sets of abstract words. Presentation of pseudo-words served as a reference condition. An emission scan was recorded after each intravenous administration of O-15 water. Encoding was associated with prefrontal and hippocampal activation. During memory retrieval, precuneus showed a consistent activation in both languages and for both highly imaginable and abstract words. Differential activations were found in Broca's area and in the cerebellum as well as in the angular/supramarginal gyri according to the language used. The findings advance our understanding of the neural representation that underlies multiple language functions. Further studies are needed to elucidate the neuronal mechanisms of bi/multilingual language processing. A promising perspective for future bi/multilingual research is an integrative approach using brain imaging studies with a high spatial resolution such as fMRI, combined with techniques with a high temporal resolution, such as magnetoencephalography (MEG).

Learning related interactions among neuronal systems involved in memory processes

Functional neuroimaging techniques using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have provided new insights in our understanding of brain function from the molecular to the systems level. While subtraction strategy based data analyses have revealed the involvement of distributed brain regions in memory processes, covariance analysis based data analysis strategies allow functional interactions between brain regions of a neuronal network to be assessed. The focus of this chapter is to (1) establish the functional topography of episodic and working memory processes in young and old normal volunteers, (2) to assess functional interactions between modules of networks of brain regions by means of covariance based analyses and systems level modelling and (3) to relate neuroimaging data to the underpinning neural networks. Male normal young and old volunteers without neurological or psychiatric illness participated in neuroimaging studies (PET, fMRI) on working and episodic memory. Distributed brain areas are involved in memory processes (episodic and working memory) in young volunteers and show much of an overlap with respect to the network components. Systems level modelling analyses support the hypothesis of bihemispheric, asymmetric networks subserving memory processes and revealed both similarities in general and differences in the interactions between brain regions during episodic encoding and retrieval as well as working memory. Changes in memory function with ageing are evident from studies in old volunteers activating more brain regions compared to young volunteers and revealing more and stronger influences of prefrontal regions. We finally discuss the way in which the systems level models based on PET and fMRI results have implications for the understanding of the underlying neural network functioning of the brain.

Prefrontal and parietal cortex in human episodic memory: an interference study by repetitive transcranial magnetic stimulation

Neuroimaging findings, including repetitive transcranial magnetic stimulation (rTMS) interference, point to an engagement of prefrontal cortex (PFC) in learning and memory. Whether parietal cortex (PC) activity is causally linked to successful episodic encoding and retrieval is still uncertain. We compared the effects of event-related active or sham rTMS (a rapid-rate train coincident to the very first phases of memoranda presentation) to the left or right intraparietal sulcus, during a standardized episodic memory task of visual scenes, with those obtained in a fully matched sample of subjects who received rTMS on left or right dorsolateral PFC during the same task. In these subjects, specific hemispheric effects of rTMS included interference with encoding after left stimulation and disruption of retrieval after right stimulation. The interference of PC-rTMS on encoding/retrieval performance was negligible, lacking specificity even when higher intensities of stimulation were applied. However, right PC-rTMS of the same intensity lengthened reaction times in the context of a purely attentive visuospatial task. These results suggest that the activity of intraparietal sulci shown in several functional magnetic resonance studies on memory, unlike that of the dorsolateral PFC, is not causally engaged to a useful degree in memory encoding and retrieval of visual scenes. The parietal activations accompanying the memorization processes could reflect the engagement of a widespread brain attentional network, in which interference on a single 'node' is insufficient for an overt disruption of memory performance.

Precuneus is involved in allocentric spatial location encoding and recognition

Using a declarative memory paradigm, the anatomical correlates of spatial location encoding and retrieval in the healthy human brain as reflected by BOLD fMRI were investigated. During encoding, subjects were instructed to view and keep in mind different locations of an object on a platform seen from different viewpoints in virtual 3D. In retrieval trials, subjects had to recognize previously learned object locations. Comparing activation patterns associated with encoding and recognition on a voxel-by-voxel basis, we found regions in the precuneus bilaterally activated by both processes. To our knowledge, this is the first study that directly compared human brain activation patterns associated with allocentric encoding and retrieval of spatial locations in virtual 3D. Our results provide further information concerning the role of the precuneus in declarative memory processes, pointing to precuneus involvement in encoding and retrieval of spatial locations.

Prefrontal and hippocampal contributions to visual associative recognition: Interactions between cognitive control and episodic retrieval

The ability to recover episodic associations is thought to depend on medial-temporal lobe mnemonic mechanisms and frontal lobe cognitive control processes. The present study examined the neural circuitry underlying non-verbal associative retrieval, and considered the consequences of successful retrieval on cognitive control demands. Event-related fMRI data were acquired while subjects retrieved strongly or weakly associated pairs of novel visual patterns in a two-alternative forced choice associative recognition paradigm. Behaviorally, successful retrieval of strongly associated relative to weakly associated pairs was more likely to be accompanied by conscious recollection of the pair's prior co-occurrence. At the neural level, right ventrolateral prefrontal cortex (VLPFC) and hippocampus were more active during successful retrieval of Strong than of Weak associations, consistent with a role in visual associative recollection. By contrast, Weak trials elicited greater activation in right anterior cingulate cortex (ACC), which may detect conflict between the similarly familiar target and foil stimuli in the absence of recollection. Consistent with this interpretation, stronger ACC activity was associated with weaker hippocampal and stronger right dorsolateral PFC (DLPFC) responses. Thus, recollection of relevant visual associations (hippocampus and VLPFC) results in lower levels of mnemonic conflict (ACC) and decreased familiarity-based monitoring demands (DLPFC). These findings highlight the interplay between cognitive control and episodic retrieval.

Functional MR imaging: paradigms for clinical preoperative mapping

Clinical applications of functional MR imaging include mapping of brain functions in relationship to intracranial tumors, seizure foci, or vascular malformations to determine the risk for performing surgical excision, the need for intraoperative mapping during excision, and selecting the optimal surgical approach to a lesion. A variety of paradigms are used to produce a blood-oxygen-level-dependent response in various brain regions, which can be identified with functional MR imaging. The paradigms used include active motor, language, or cognitive tasks, and passive tactile, auditory, or visual stimuli. Activation usually indicates the location of eloquent cortex. Lack of function in a region cannot be assumed when functional MR imaging shows absence of activation within the region.

Verbal and Visuospatial Learning and Memory Function in Children With Moderate Prenatal Alcohol Exposure

BACKGROUND

This study investigated the effects of moderate prenatal alcohol exposure on learning and memory in 14-year-old adolescents. The Children's Memory Scale was used to assess learning and memory function in the verbal/auditory and visual/spatial domains. In addition, both short- and long-term memory function were assessed.

METHODS

Data were collected as part of the Maternal Health Practices and Child Development Project, a longitudinal study including 580 children and their mothers. Women were assessed during each trimester of pregnancy and with their children from birth to 16 years of age. At age 14, memory function was evaluated using the Children's Memory Scale, an assessment tool that measures learning and immediate and delayed memory function in the verbal and visual-spatial domains.

RESULTS

Prenatal alcohol exposure during the first trimester predicted deficits in learning, short-term memory, and long-term memory, specifically in the verbal domain. Deficits in performance were specific to learning and memory of word-pairs. In addition, deficits in memory were mediated by learning performance.

CONCLUSIONS

Results demonstrated that prenatal alcohol exposure lead to deficits in encoding processes as indicated by deficits in verbal learning. Initial deficits in acquisition were responsible for deficits in immediate and delayed recall of verbal information in children who were exposed to alcohol during pregnancy but did not have fetal alcohol syndrome.

fMRI differences in encoding and retrieval of pictures due to encoding strategy in the elderly

Functional MRI (fMRI) was used to examine the neural correlates of depth of processing during encoding and retrieval of photographs in older normal volunteers (n = 12). Separate scans were run during deep (natural vs. man-made decision) and shallow (color vs. black-and-white decision) encoding and during old/new recognition of pictures initially presented in one of the two encoding conditions. A baseline condition consisting of a scrambled, color photograph was used as a contrast in each scan. Recognition accuracy was greater for the pictures on which semantic decisions were made at encoding, consistent with the expected levels of processing effect. A mixed-effects model was used to compare fMRI differences between conditions (deep-baseline vs. shallow-baseline) in both encoding and retrieval. For encoding, this contrast revealed greater activation associated with deep encoding in several areas, including the left parahippocampal gyrus (PHG), left middle temporal gyrus, and left anterior thalamus. Increased left hippocampal, right dorsolateral, and inferior frontal activations were found for recognition of items that had been presented in the deep relative to the shallow encoding condition. We speculate that the modulation of activity in these regions by the depth of processing manipulation shows that these regions support effective encoding and successful retrieval. A direct comparison between encoding and retrieval revealed greater activation during retrieval in the medial temporal (right hippocampus and bilateral PHG), anterior cingulate, and bilateral prefrontal (inferior and dorsolateral). Most notably, greater right posterior PHG was found during encoding compared to recognition. Focusing on the medial temporal lobe (MTL) region, our results suggest a greater involvement of both anterior MTL and prefrontal regions in retrieval compared to encoding.

Left orbital frontal cortex volume and performance on the benton visual retention test in older depressives and controls

Changes within the prefrontal cortex (PFC) have been associated with both mood disorders and with specific impairments in cognitive testing. The left PFC has been implicated in relational processing, that is, binding different pieces of information. We hypothesized that among older depressives and elderly controls, lower performance on one test of relational processing would be associated with smaller volume of the orbital frontal cortex (OFC). A total of 30 depressed and 40 control subjects were included in the study. All subjects were administered the Benton Visual Retention Test (BVRT). Subjects received a standardized magnetic resonance imaging, for which volumes of the OFC and total brain were calculated. We found that, controlling for age and education, total correct on BVRT was associated with left OFC volume normalized for total brain volume among the entire sample. For the depressed sample only, the number of perseverative errors was negatively associated with left OFC volume normalized for total brain volume after controlling for age and education. These results add to the literature linking mood and cognitive disturbances to the PFC. Future studies with a larger sample employing functional measures are warranted.

The Role of Prefrontal Cortex in Verbal Episodic Memory: rTMS Evidence

Long-term, episodic memory processing is supposed to involve the prefrontal cortex asymmetrically. Here we investigate the role of the dorsolateral prefrontal cortex (DLPFC) in encoding and retrieval of semantically related or unrelated word pairs. Subjects were required to perform a task consisting of two parts: a study phase (encoding), in which word pairs were presented, and a test phase (retrieval), during which stimuli previously presented had to be recognized among other stimuli. Consistently with our previous findings using pictures, repetitive transcranial magnetic stimulation (rTMS) had a significant impact on episodic memory. The performance was significantly disrupted when rTMS was applied to the left or right DLPFC during encoding, and to the right DLPFC in retrieval, but only for unrelated word pairs. These results indicate that the nature of the material to be remembered interacts with the encoding–retrieval DLPFC asymmetry; moreover, the crucial role of DLPFC is evident only for novel stimuli.

Role of medial temporal lobe in extensive retrieval of task-related knowledge

The role of medial temporal lobe (MTL) in deep semantic processing was examined in a triple semantic judgment task in which subjects were asked to decide which one of the two bottom words was more semantically fit to the top word. By changing the number of bottom words that are semantically related to the top word, we can disassociate effects of reactivating the "old" semantic associations and effects of establishing "new" semantic associations on the MTL. The results of event-related fMRI analysis indicated that MTL was more activated in the retrieval of old semantic associations than in the establishment of new semantic associations. The function of MTL in this semantic judgment task was explained as subserving the process of extensive retrieval of task-related knowledge.

Retrieval of relational information: a role for the left inferior prefrontal cortex

Neuroimaging studies have implicated different areas of prefrontal cortex and medial temporal lobe structures (MTL) in episodic retrieval tasks. However, the role of specific regions in particular aspects of episodic memory is still unclear. In this experiment we studied changes in regional cerebral blood flow (rCBF) associated with relational and nonrelational retrieval of studied pairs of words. For relational retrieval, a list of either studied or rearranged pairs was presented and subjects (n = 8) were asked to indicate whether pairs had appeared on the study list. Under the nonrelational retrieval condition they indicated whether one or both words of the pair had appeared on the study list. As compared to the baseline condition (looking at a cross-mark), increased rCBF was observed in the left inferior prefrontal cortex (LIPFC) for both studied pairs and rearranged pairs under the relational retrieval condition. Under the nonrelational condition, an increase was observed in right inferior frontal gyrus. The MTL showed a trend for increased rCBF in the rearranged-pair condition. This increase was probably associated with the encoding that accompanies retrieval of novel stimuli. Results suggest that the lateralized activation of prefrontal cortex observed in episodic memory tasks may be related to the degree of relational processing involved. The LIPFC appears to be associated with relational retrieval and the right prefrontal cortex with nonrelational retrieval.

Neuropsychological processes in post-traumatic stress disorder

The unique constellation of symptoms that constitute post-traumatic stress disorder (PTSD) may reflect persistent alterations in attention and memory that are fundamental to this disorder. This article reviews the literature on the neuropsychology of PTSD and its relevance to phenomenological, clinical, and biological aspects of this disorder. It addresses the nature and magnitude of the neuropsychological alterations in PTSD and the extent to which they involve the processing of trauma-related or neutral stimuli. This article summarizes the types of deficits in attention and memory that have been found in a broad range of trauma survivors with PTSD and explores the extent to which the deficits may be risk factors for or consequences of trauma and PTSD. The implications of these findings for the course and pathophysiology of PTSD are also discussed.

PET studies on the memory processing of word pairs in bilingual Finnish-English subjects

This study examined the fundamental question whether verbal memory processing in two unrelated languages is mediated by a common neural system or by distinct cortical areas. Ten right-handed, male Finnish--English adult late bilinguals who had acquired the second language after the age of 10 were scanned whilst either encoding/retrieving word pairs in their mother tongue (Finnish) or in a foreign language (English). Within each language, subjects had to encode and retrieve four sets of 12 visually presented paired word associates which were not semantically related. Two sets consisted of highly imageable words (e.g. monkey-table; koira-lasi) and the other two sets of abstract word pairs (e.g. freedom-moral; uhka-suure). Presentation of pseudowords served as a reference condition. An emission scan was recorded after each intravenous administration of O-15 water. Encoding was associated with prefrontal and hippocampal activation. During memory retrieval, precuneus showed a consistent activation in both languages and for both highly imageable and abstract words. Although the brain mechanisms of the two languages share common components, differential activations were found in Broca's area and in the cerebellum as well as in the angular/supramarginal gyri according to the language used.

Object activation from features in the semantic system

The human brain is thought to elicit an object representation via co-activation of neural regions that encode various object features. The cortical regions and mechanisms involved in this process have never been elucidated for the semantic system. We used functional magnetic resonance imaging (fMRI) to evaluate regions activated during a task designed to elicit object activation within the semantic system (e.g., presenting the words "desert" and "humps" with the task to determine if they combine to form an object, in this case a "camel"). There were signal changes in the thalamus for word pairs that activated an object, but not for pairs that (a) failed to activate an object, (b) were simply semantically associated, or (c) were members of the same category. These results suggest that the thalamus has a critical role in coordinating the cortical activity required for activating an object concept in the semantic system.

Stress, memory, and the hippocampus: can't live with it, can't live without it

Since the 1968s discovery of receptors for stress hormones (corticosteroids) in the rodent hippocampus, a tremendous amount of data has been gathered on the specific and somewhat isolated role of the hippocampus in stress reactivity. The hippocampal sensitivity to stress has also been extended in order to explain the negative impact of stress and related stress hormones on animal and human cognitive function. As a consequence, a majority of studies now uses the stress-hippocampus link as a working hypothesis in setting up experimental protocols. However, in the last decade, new data were gathered showing that stress impacts on many cortical and subcortical brain structures other than the hippocampus. The goal of this paper is to summarize the four major arguments previously used in order to confirm the stress-hippocampus link, and to describe new data showing the implication of other brain regions for each of these previously used arguments. The conclusion of this analysis will be that scientists should gain from extending the impact of stress hormones to other brain regions, since hormonal functions on the brain are best explained by their modulatory role on various brain structures, rather than by their unique impact on one particular brain region.

An examination of the effects of stimulus type, encoding task, and functional connectivity on the role of right prefrontal cortex in recognition memory

Right anterior prefrontal cortex and other brain areas are active during memory retrieval but the role of prefrontal cortex and how it interacts with these other regions to mediate memory function remain unclear. To explore these issues we used positron emission tomography to examine the effects of stimulus material and encoding task on brain activity during visual recognition, assessing both task-related changes and functional connectivity. Words and pictures of objects were encoded using perceptual and semantic strategies, resulting in better memory for semantically encoded items. There was no significant effect of prior encoding strategy on brain activity during recognition. Right anterior prefrontal cortex was equally active during recognition of both types of stimuli irrespective of initial encoding strategy. Regions whose activity was positively correlated with activity in right anterior prefrontal cortex included widespread areas of prefrontal and inferior temporal cortices bilaterally. Activity in this entire network of regions was negatively correlated with recognition accuracy of semantically encoded items. These results suggest that initial encoding task has little impact on the set of brain regions that is active during subsequent recognition. Right anterior prefrontal cortex appears to be involved in retrieval mode, reflected in its equivalent activity across conditions differing in both stimulus type and encoding task, and also in retrieval effort, shown by the negative correlation between its functional connectivity and individual differences in recognition accuracy.

Statistical analysis of hippocampal asymmetry in schizophrenia

The asymmetry of brain structures has been studied in schizophrenia to better understand its underlying neurobiology. Brain regions of interest have previously been characterized by volumes, cross-sectional and surface areas, and lengths. Using high-dimensional brain mapping, we have developed a statistical method for analyzing patterns of left-right asymmetry of the human hippocampus taken from high-resolution MR scans. We introduce asymmetry measures that capture differences in the patterns of high-dimensional vector fields between the left and right hippocampus surfaces. In 15 pairs of subjects previously studied (J. G. Csernansky et al., 1998, Proc. Natl. Acad. Sci. USA 95, 11406-11411). we define the difference in hippocampal asymmetry patterns between the groups. Volume analysis indicated a large normative asymmetry between left and right hippocampus (R > L), and shape analysis allowed us to visualize the normative asymmetry pattern of the hippocampal surfaces. We observed that the right hippocampus was wider along its lateral side in both schizophrenia and control subjects. Also, while patterns of hippocampal asymmetry were generally similar in the schizophrenia and control groups, a principal component analysis based on left-right asymmetry vector fields detected a statistically significant difference between the two groups, specifically related to the subiculum.

Transperceptual encoding and retrieval processes in memory: a PET study of visual and haptic objects

An important objective of functional neuroimaging research is to identify neuroanatomical correlates of memory processes such as encoding and retrieval. In typical studies directed at this goal, however, the to-be-remembered information has been presented in a single perceptual modality. Under these conditions it is not known whether the observed brain activity reflects the studied memory process as such or only the memory process in the given modality. The positron emission tomography (PET) study reported here was designed to identify brain regions involved in encoding and retrieval processes specific to visual and haptic modalities, as well as those common to the two modalities. These latter, common regions, were assumed to be associated with "transperceptual" encoding and retrieval processes. Abstract three-dimensional objects, difficult to describe verbally, served as to-be-remembered materials. A multivariate partial least squares analysis of the PET data revealed that transperceptual encoding processes activated right medial temporal lobe, superior prefrontal cortex bilaterally, and posterior inferior temporal gyrus bilaterally. Transperceptual recognition activations were observed in two right orbitofrontal regions and in anterior cingulate. These results provide initial evidence that some processes involved in memory encoding and retrieval operate beyond perceptual processes and in that sense are transperceptual.

Auditory memory in congenitally blind adults: a behavioral-electrophysiological investigation

Blind people must rely more than sighted people on auditory input in order to acquire information about the world. The present study was designed to test the hypothesis that blind people have better memory than sighted individuals for auditory verbal material and specifically to determine whether memory encoding and/or retrieval are improved in blind adults. An incidental memory paradigm was employed in which 11 congenitally blind people and 11 matched sighted controls first listened to 80 sentences which ended either with a semantically appropriate or inappropriate word. Immediately following, the recognition phase occurred, in which all sentence terminal words were presented again randomly intermixed with the same number of new words. Participants indicated whether or not they had heard the word in the initial study phase. Event-related brain potentials (ERPs) were recorded from 28 electrode positions during both the encoding and the retrieval phase. Blind participants' memory performance was superior to that of sighted controls. In addition, during the recognition phase, previously presented words elicited ERPs with larger positive amplitudes than new words, particularly over the right hemisphere. During the study phase, words that would subsequently be recognized elicited a more pronounced late positive potential than words that were not subsequently recognized. These effects were reliable in the congenitally blind participants but could only be obtained in the subgroup of sighted participants who had the highest memory performance. These results imply that blind people encode auditory verbal material more efficiently than matched sighted controls and that this in turn allows them to recognize these items with a higher probability.

Functionally activated brain imaging (O-15 PET and fMRI) in the study of learning and memory after traumatic brain injury

Advances in functional imaging technology and cognitive neuropsychology have resulted in paradigms in which participants can perform cognitive tasks during functional image acquisition. We will discuss the application of two approaches (oxygen-15 positron emission tomography and functional magnetic resonance imaging) that have recently been used to examine components of learning and memory following traumatic brain injury (TBI). Activated functional brain imaging findings that we will discuss may suggest possible functional reallocation and reorganization of brain substrates involved in verbal learning and memory following brain injury. The findings also are clearly in line with other research that indicates a prominent role for the frontal lobes in learning and memory functioning, and support the concept of distributed neural networks for memory-related functions, cognitive load, and the potential for examining brain re-organization after injury.

Cognitive association formation in human memory revealed by spatiotemporal brain imaging

Cognitive theory posits association by juxtaposition or by fusion. We employed the measurement of event-related brain potentials (ERPs) to a concept fusion task in order to explore memory encoding of these two types of associations between word pairs, followed by a memory test for original pair order. Encoding processes were isolated by subtracting fusion task ERPs corresponding to pairs later retrieved quickly from ERPs corresponding to pairs later retrieved slowly, separately for pairs fused successfully and unsuccessfully (i.e., juxtaposed). Analyses revealed that the encoding of these two types of associations yields different ERP voltage polarities, scalp topographies, and brain sources extending over the entire time course of processing.

Imaging and neural modelling in episodic and working memory processes

Neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have revealed the involvement of distributed brain regions in memory processes mainly by the use of subtraction strategy based data analyses. Covariance analysis based data analysis strategies have been introduced more recently which allow functional interactions between brain regions of a neuronal network to be assessed. This contribution focuses on studies aiming to (1) establish the functional topography of episodic and working memory processes in young and old normal volunteers, (2) to assess functional interactions between modules of networks of brain regions by means of covariance based analyses and systems level modelling, (3) to characterise the temporal dynamics by the use of magnetoencephalography (MEG) and (4) to relate neuroimaging data to the underpinning neural networks. Male normal young and old volunteers without neurological or psychiatric illness participated in neuroimaging studies (PET, fMRI, MEG). Studies were approved by the ethical committee and federal authorities. Our results in young volunteers show distributed brain areas that are involved in memory processes (episodic and working memory) and show much of an overlap with respect to the network components. Systems level modelling analyses support the hypothesis of bihemispheric, asymmetric networks subserving memory processes and revealed both similarities in general and differences in the interactions between brain regions during episodic encoding and retrieval as well as working memory. Changes in memory function with ageing are evident from functional topographic studies in old volunteers activating more brain regions as compared to young volunteers. There are more and stronger influences of prefrontal regions in elderly volunteers comparing the functional models between old and young subjects. We discuss the way that the systems level models of the PET and fMRI results have implications for the underlying neural network functioning of the brain. This is done by developing simplifying assumptions, which lead from the equations describing the activities of the coupled neural modules to the systems level model equations. The resulting implications for the neural interactions are then discussed, in terms of a set of synaptically coupled neural modules. Finally, we consider how a similar analysis could be extended from the spatial to the temporal domain thus including the EEG and MEG results. The implication of preliminary MEG results presented here for the temporality arising in the interaction between the coupled neural modules in a working memory paradigm is discussed in terms of the previously developed neural network models arising from the PET and fMRI data.

Neural correlates of semantic associative encoding in episodic memory

Associations between individual items are the basic building blocks of learning and memory. Functional neuroimaging has now made it possible to study neural correlates of such associations. The present PET study examined three associative encoding conditions differing in the number of words (0, 1, or 2) semantically related to a third word representing the name of a semantic category. A recall task consisting in the presentation of the category names as cues for retrieving the other two members of the triads followed each encoding condition. As expected, retrieval performance increased as the number of semantic exemplars at encoding increased (10%, 43%, 70% items recalled, respectively). A first analysis (partial least squares, PLS) of the PET data identified task-related patterns of activity for associative encoding and cued-recall tasks. A second analysis identified brain regions whose activity was modulated by the number of semantic exemplars at encoding. Some of the task-related brain regions also showed modulated activity by semantic relatedness and consisted in the left inferior prefrontal cortex, right medial temporal lobe, fusiform gyrus and inferior temporal gyrus bilaterally. Some of these regions showed greater activity when words in a triad were unrelated, whereas others did so when the three words were semantically related. These regions have been consistently reported in previous functional neuroimaging studies of associative encoding and may constitute key structures in association formation.

Network analysis in episodic encoding and retrieval of word-pair associates: a PET study

The involvement of distributed brain regions in declarative memory has been hypothesized based on studies with verbal memory tasks. To characterize episodic declarative memory function further, 14 right-handed volunteers performed a visual verbal learning task using paired word associates. The volunteers underwent positron emission tomography. 15O-butanol was used as a tracer of regional cerebral blood flow (rCBF). Inter-regional functional interactions were assessed based on within-task, across-subject inter-regional rCBF correlations. Anatomical connections between brain areas were based on known anatomy. Structural equation modelling was used to calculate the path coefficients representing the magnitudes of the functional influences of each area on the ones to which it is connected by anatomical pathways. The encoding and the retrieval network elicit similarities in a general manner but also differences. Strong functional linkages involving visual integration areas, parahippocampal regions, left precuneus and cingulate gyrus were found in both encoding and retrieval; the functional linkages between posterior regions and prefrontal regions were more closely linked during encoding, whereas functional linkages between the left parahippocampal region and posterior cingulate as well as extrastriate areas and posterior cingulate gyrus were stronger during retrieval. In conclusion, these findings support the idea of a global bihemispheric, asymmetric encoding/retrieval network subserving episodic declarative memory. Our results further underline the role of the precuneus in episodic memory, not only during retrieval but also during encoding.