Direct and indirect measures of verbal relational memory following anterior temporal lobectomy

Effects of risperidone/paliperidone versus placebo on cognitive functioning over the first 6 months of treatment for psychotic disorder: secondary analysis of a triple-blind randomised clinical trial

The drivers of cognitive change following first-episode psychosis remain poorly understood. Evidence regarding the role of antipsychotic medication is primarily based on naturalistic studies or clinical trials without a placebo arm, making it difficult to disentangle illness from medication effects. A secondary analysis of a randomised, triple-blind, placebo-controlled trial, where antipsychotic-naive patients with first-episode psychotic disorder were allocated to receive risperidone/paliperidone or matched placebo plus intensive psychosocial therapy for 6 months was conducted. A healthy control group was also recruited. A cognitive battery was administered at baseline and 6 months. Intention-to-treat analysis involved 76 patients (antipsychotic medication group: 37; 18.6_Mage [2.9] years; 21 women; placebo group: 39; 18.3_Mage [2.7]; 22 women); and 42 healthy controls (19.2_Mage [3.0] years; 28 women). Cognitive performance predominantly remained stable (working memory, verbal fluency) or improved (attention, processing speed, cognitive control), with no group-by-time interaction evident. However, a significant group-by-time interaction was observed for immediate recall (p = 0.023), verbal learning (p = 0.024) and delayed recall (p = 0.005). The medication group declined whereas the placebo group improved on each measure (immediate recall: p = 0.024; η_p² = 0.062; verbal learning: p = 0.015; η_p² = 0.072 both medium effects; delayed recall: p = 0.001; η_p² = 0.123 large effect). The rate of change for the placebo and healthy control groups was similar. Per protocol analysis (placebo n = 16, medication n = 11) produced similar findings. Risperidone/paliperidone may worsen verbal learning and memory in the early months of psychosis treatment. Replication of this finding and examination of various antipsychotic agents are needed in confirmatory trials. Antipsychotic effects should be considered in longitudinal studies of cognition in psychosis.Trial registration: Australian New Zealand Clinical Trials Registry ( http://www.anzctr.org.au/ ; ACTRN12607000608460).

Larger capacity for unconscious versus conscious episodic memory

Episodic memory is the memory for experienced events. A peak competence of episodic memory is the mental combination of events to infer commonalities. Inferring commonalities may proceed with and without consciousness of events. Yet what distinguishes conscious from unconscious inference? This question inspired nine experiments that featured strongly and weakly masked cartoon clips presented for unconscious and conscious inference. Each clip featured a scene with a visually impenetrable hiding place. Five animals crossed the scene one-by-one consecutively. One animal trajectory represented one event. The animals moved through the hiding place, where they might linger or not. The participants' task was to observe the animals' entrances and exits to maintain a mental record of which animals hid simultaneously. We manipulated information load to explore capacity limits. Memory of inferences was tested immediately, 3.5 or 6 min following encoding. The participants retrieved inferences well when encoding was conscious. When encoding was unconscious, the participants needed to respond intuitively. Only habitually intuitive decision makers exhibited a significant delayed retrieval of inferences drawn unconsciously. Their unconscious retrieval performance did not drop significantly with increasing information load, while conscious retrieval performance dropped significantly. A working memory network, including hippocampus, was activated during both conscious and unconscious inference and correlated with retrieval success. An episodic retrieval network, including hippocampus, was activated during both conscious and unconscious retrieval of inferences and correlated with retrieval success. Only conscious encoding/retrieval recruited additional brain regions outside these networks. Hence, levels of consciousness influenced the memories' behavioral impact, memory capacity, and the neural representational code.

Cognitive ability and metabolic physical health in first-episode psychosis

Cognitive impairments are a core feature of first-episode psychosis (FEP), arising before illness onset and antipsychotic exposure. Individuals with chronic psychosis experience poorer physical health while taking antipsychotic medication, but health disparities may be evident at FEP onset, prior to antipsychotic exposure. Given the links between cognition and physical health in healthy populations, the aim was to explore whether cognition and physical health are associated in FEP, which could inform early physical health interventions for cognition in FEP. Participants were aged 15 to 25 and included 86 individuals experiencing FEP with limited antipsychotic exposure and duration of untreated psychosis of ≤six months, and 43 age- and sex-matched controls. Individuals with FEP performed significantly poorer than controls in most cognitive domains (Cohen's d = 0.38 to 1.59). Groups were similar in metabolic health measures, excluding a significantly faster heart rate in FEP (d = 0.68). Through hierarchical regression analyses, we found that in the overall sample, BMI was negatively related to current IQ after controlling for education and group (FEP/control). Relationships between BMI and cognition were consistent across the FEP and healthy control groups. In FEP, current IQ and working memory were negatively correlated with lipid profiles. Findings suggest that in FEP, impaired cognition is exhibited earlier than physical health problems, and that compared to controls, similar relationships with cognition are demonstrated. Causal pathways and trajectories of relationships between health and cognition in FEP require investigation, especially as antipsychotic medications are introduced. The findings have implications for cognitive and health interventions.

Structural and Functional Alterations in the Contralesional Medial Temporal Lobe in Glioma Patients

Background

The human brain has an extraordinary ability to functionally change or reorganize its structure in response to disease. The aim of this study is to assess the structural and functional plasticity of contralesional medial temporal lobe (MTL) in patients with unilateral MTL glioma.

Methods

Sixty-eight patients with unilateral MTL glioma (left MTL glioma, n = 33; right MTL glioma, n = 35) and 40 healthy controls were recruited and scanned with 3D T1 MRI and rest-fMRI. We explored the structure of the contralesional MTL using voxel-based morphometry (VBM) and assessed the memory networks of the contralesional hemisphere using resting-state functional connectivity (rs-FC). The association between FC and cognitive function was assessed with partial correlation analysis.

Results

Compared with healthy controls, both patient groups exhibited (1) a large cluster of voxels with gray matter (GM) volume decrease in the contralesional MTL using region of interest (ROI)-based VBM analysis (cluster level p < 0.05, FDR corrected); and (2) decreased intrahemispheric FC between the posterior hippocampus (pHPC) and posterior cingulate cortex (PCC) (p < 0.01, Bonferroni corrected). Intrahemispheric FC between the pHPC and PCC was positively correlated with cognitive function in both patient groups.

Conclusion

Using multi-modality brain imaging tools, we found structural and functional changes in the contralesional MTL in patients with unilateral MTL glioma. These findings suggest that the contralesional cortex may have decompensation of structure and function in patients with unilateral glioma, except for compensatory structural and functional adaptations. Our study provides additional insight into the neuroanatomical and functional network changes in the contralesional cortex in patients with glioma.

Staged treatment and acceptability guidelines in early psychosis study (STAGES): A randomized placebo controlled trial of intensive psychosocial treatment plus or minus antipsychotic medication for first-episode psychosis with low-risk of self-harm or aggression. Study protocol and baseline characteristics of participants

AIM

It is now necessary to investigate whether recovery in psychosis is possible without the use of antipsychotic medication. This study will determine (1) whether a first-episode psychosis (FEP) group receiving intensive psychosocial interventions alone can achieve symptomatic remission and functional recovery; (2) whether prolonging the duration of untreated psychosis (DUP) in a sub-group according to randomisation will be associated with a poorer outcome and thereby establish whether the relationship between DUP and outcome is causative; and (3) whether neurobiological changes observed in FEP are associated with the psychotic disorder or antipsychotic medication. Baseline characteristics of participants will be presented.

METHODS

This study is a triple-blind randomized placebo-controlled non-inferiority trial. The primary outcome is the level of functioning measured by the Social and Occupational Functioning Assessment Scale at 6 months. This study is being conducted at the Early Psychosis Prevention and Intervention Centre, Melbourne and includes young people aged 15 to 24 years with a DSM-IV psychotic disorder, a DUP less than 6 months and not high risk for suicide or harm to others. Strict discontinuation criteria are being applied. Participants are also undergoing three 3-Tesla-MRI scans.

RESULTS

Ninety participants have been recruited and baseline characteristics are presented.

CONCLUSIONS

Staged treatment and acceptability guidelines in early psychosis will determine whether antipsychotic medications are indicated in all young people with a FEP and whether antipsychotic medication can be safely delayed. Furthermore, the relative contribution of psychotic illness and antipsychotic medication in terms of structural brain changes will also be elucidated. The findings will inform clinical practice guidelines.

Stress hormones and verbal memory in young people over the first 12 weeks of treatment for psychosis

AIMS

Memory impairment in psychosis may be mediated through detrimental effects of hypothalamic-pituitary-adrenal (HPA) axis function. This study prospectively investigated the relationship between cortisol, sulphate dehydroepiandrosterone (DHEA(S) and cortisol: DHEA(S) ratio and memory in 35 first-episode psychosis (FEP) patients during the first 12 weeks of treatment and 23 healthy controls (HC).

METHODS

Morning blood sampling and tests of attention, working memory and verbal memory occurred at baseline and 12-week follow-up.

RESULTS

FEP and HC groups did not significantly differ in levels of cortisol, DHEA(S) or their ratio at baseline or over 12-weeks. The FEP group performed significantly below HC on all cognitive measures at baseline and over 12-weeks. Cortisol levels were unrelated to cognition in both groups. At baseline, DHEA(S) was positively associated with attention in HCs, but negatively associated with attention in FEP participants. Change in DHEA(S) was negatively associated with change in memory over 12-weeks in both groups. At 12-weeks, there was a negative correlation between the cortisol: DHEA(S) ratio and attention in both groups.

CONCLUSIONS

These findings are mostly in contrast to findings in chronic schizophrenia. Investigation at different illness phases and over longer-follow-up periods is required to determine the complex relationship between HPA-axis and memory functioning in psychosis.

Material specific lateralization of medial temporal lobe function: An fMRI investigation

The theory of material specific lateralization of memory function posits that left and right MTL regions are asymmetrically involved in mnemonic processing of verbal and nonverbal material respectively. Lesion and functional imaging (fMRI) studies provide robust evidence for a left MTL asymmetry in the verbal memory domain. Evidence for a right MTL/nonverbal asymmetry is not as robust. A handful of fMRI studies have investigated this issue but have generally utilised nonverbal stimuli which are amenable to semantic elaboration. This fMRI study aimed to investigate the neural correlates of recognition memory processing in 20 healthy young adults (mean age = 26 years) for verbal stimuli and nonverbal stimuli that were specifically designed to minimize verbalisation. Analyses revealed that the neural correlates of recognition memory processing for verbal and nonverbal stimuli were differentiable and asymmetrically recruited the left and right MTL respectively. The right perirhinal cortex and hippocampus were preferentially involved in successful recognition memory of items devoid of semantic information. In contrast, the left anterior hippocampus was preferentially involved in successful recognition memory of stimuli which contained semantic meaning. These results suggest that the left MTL is preferentially involved in mnemonic processing of verbal/semantic information. In contrast, the right MTL is preferentially involved in visual/non-semantic mnemonic processing. We propose that during development, the left MTL becomes specialised for verbal mnemonic processing due to its proximity with left lateralised cortical language processing areas while visual/non-semantic mnemonic processing gets 'crowded out' to become predominantly, but not completely, the domain of the right MTL.

Reduced Benefit of Memory Elaboration in Older Adults with Subjective Memory Decline

BACKGROUND

Cognitive interventions for neurodegenerative diseases, such as Alzheimer's disease (AD), are best targeted at the preclinical stages, and subjective memory decline (SMD) without objective memory impairment on standard tests in older adults may represent a very early preclinical stage. Elaborated encoding effectively enhances memory performance for healthy older adults (HOAs), but has not been examined in people with SMD.

OBJECTIVE

To examine elaborated encoding in people with SMD, compared with HOAs.

METHODS

Participants were 32 HOAs and 22 people with SMD, defined using the Memory Complaint Questionnaire. Participants completed a verbal paired associate learning (PAL) task with delayed recall under elaborated and non-elaborated encoding conditions, as well as the California Verbal Learning Test-II.

RESULTS

On the PAL learning trials, with age controlled, a significant interaction of group X encoding condition emerged, F(1, 51) =  6.47, MSE = 6.54, p = 0.014, ηp² = 0.11. Simple main effects revealed no differences between groups in the non-elaborated condition, but in the elaborated condition HOAs recalled more pairs than SMD, although both groups benefited from elaboration. At delayed recall, HOA recalled more pairs than SMD, F(1, 51) =  4.59, p = 0.037, ηp²= 0.08, and both groups benefited from elaboration, F(1, 52) =  19.25, p <  0.001, ηp² = 0.27.

CONCLUSION

People with SMD benefit from elaborated encoding, although not to the same extent as HOAs. This objective difference in complex learning and memory suggests neural changes in SMD that may represent preclinical AD. Elaborated encoding is a promising technique to help maintain memory and decrease anxiety in this at-risk population.

The impact of neuropsychological functioning and coping style on perceived stress in individuals with first-episode psychosis and healthy controls

Stress is implicated in the development and course of psychotic illness, but the factors that influence stress levels are not well understood. The aim of this study was to examine the impact of neuropsychological functioning and coping styles on perceived stress in people with first-episode psychosis (FEP) and healthy controls (HC). Thirty-four minimally treated FEP patients from the Early Psychosis Prevention and Intervention Centre, Melbourne, Australia, and 26 HC participants from a similar demographic area participated in the study. Participants completed a comprehensive neuropsychological test battery as well as the Coping Inventory for Stressful Situations (task-, emotion- and avoidance-focussed coping styles) and Perceived Stress Scale (PSS). Linear regressions were used to determine the contribution of neuropsychological functioning and coping style to perceived stress in the two groups. In the FEP group, higher levels of emotion-focussed and lower levels of task-focussed coping were associated with elevated stress. Higher premorbid IQ and working memory were also associated with higher subjective stress. In the HC group, higher levels of emotion-focussed coping, and contrary to the FEP group, lower premorbid IQ, working memory and executive functioning, were associated with increased stress. Lower intellectual functioning may provide some protection against perceived stress in FEP.

The NAB shape learning subtest as a predictor of lateralized seizure onset

This study presents the first empirical evaluation of the predictive value of the Neuropsychological Assessment Battery Shape Learning (NAB-SL) subtest in a sample of patients with unilateral temporal lobe epilepsy. Stimulus characteristics of the NAB-SL may improve predictive ability over other commonly used visual memory tests. Forty-nine patients with unilateral temporal lobe epilepsy were compared on measures of non-verbal and verbal memory (NAB-SL and Wechsler Memory Scale-III subtests). Univariate and forward conditional logistic regressions identified predictive values for each memory test individually and in combination. The NAB-SL delayed memory demonstrated consistently stronger predictive power over visual reproduction at the univariate and multivariate levels. The NAB-SL was a good predictor (80% range) of lateralized seizure onset when combined with a verbal memory measure. These preliminary results provide support for the use of the NAB-SL in preoperative epilepsy evaluations as a predictor of non-dominant temporal lobe dysfunction. Potential benefits of this test are discussed.

The hippocampus reevaluated in unconscious learning and memory: at a tipping point?

Classic findings from the neuropsychological literature invariably indicated that performances on tests of memory that can be accomplished without conscious awareness were largely spared in amnesia, while those that required conscious retrieval (e.g., via recognition or recall) of information learned in the very same sessions was devastatingly impaired. Based on reports of such dissociations, it was proposed that one of the fundamental distinctions between memory systems is whether or not they support conscious access to remembered content. Only recently have we come to realize that the putative systemic division of labor between conscious and unconscious memory is not so clean. A primary goal of this review is to examine recent evidence that has been advanced against the view that the hippocampus is selectively critical for conscious memory. Along the way, consideration is given to criticisms that have been levied against these findings, potential explanations for differences in the reported results are proposed, and methodological pitfalls in investigations of unconscious memory are discussed. Ultimately, it is concluded that a tipping point has been reached, and that while conscious recollection depends critically on hippocampal integrity, the reach of the hippocampus extends to unconscious aspects of memory performance when relational memory processing and representation are required.

Memory for Faces and Paired Associates After Temporal Lobectomy in Childhood

Findings are reported from the first examination of the Beardsworth Memory for Children's Faces Test (BMCFT) (Beardsworth & Zaidel, 1994) tested on a selected sample of 10 left (LATL) and 7 right (RATL) anterior temporal lobectomy patients an average of 7 years post surgery. No differences between LATL and RATL groups were found on immediate or delayed memory trials of the BMCFT. Verbal memory was assessed with a verbal analogue of this test, the Extended Verbal Paired Associates Test (EVPAT) specifically constructed for this study. Differences were obtained on the immediate (F = 5.17; p < .01) and delayed (F = 6.55; p < .01) memory trials of the EVPAT with LATL patients learning and retrieving fewer verbal associations than their right ATL counterparts. No differences were found on the memory decline to delayed recall scores on the EVPAT. The findings suggest that people who have left or right ATL in childhood cannot be distinguished on the basis of their immediate or delayed memory for paired faces an average of 7 years post-surgery. These results are consistent with the immediate memory performance of children 6 months post-surgery as reported by Beardsworth and Zaidel (1994), but are not consonant with the delayed memory performance of their children on this task. This indicates that left-right dissociations found in children on measures of delayed memory for faces 6 months post-surgery may represent a component of memory that is still in a state of recovery. The findings for the immediate memory trials of the EVPAT conformed to expectations and underscore the dependability of simple rote verbal memory tasks in dissociating left from right ATL 7 years post-surgery. The study addresses the issue of the recovery of immediate versus delayed memory and time of assessment post-surgery.

Relational memory in first episode psychosis: implications for progressive hippocampal dysfunction after illness onset

OBJECTIVE

Verbal episodic memory deficits are prominent in schizophrenia and have also been found in first episode psychosis (FEP) and individuals at clinical risk of the disorder. The central role of the hippocampus in verbal memory processing and the consistent findings of hippocampal volume reductions in chronic patients have prompted the suggestion that impaired verbal memory performance may be a biomarker of schizophrenia. However, it is currently unclear as to when, during the early phase of psychosis, verbal memory performance becomes significantly impaired. The current study investigated verbal relational memory in FEP using a novel verbal paired associate task, and tested whether performance was dependent on phase of illness within FEP, where patients with a diagnosis of schizophrenia were considered to be in a more advanced stage than those with schizophreniform disorder.

METHOD

Forty-seven currently psychotic FEP patients and 36 healthy non-psychiatric controls, aged 15-25 years old, completed a test comprising four trials of learning and cued recall of word pairs (denoted AB pairs), an interference phase comprising two trials with new second words (AC pairs), and finally cued recall for the original AB pairings.

RESULTS

FEP patients performed similarly to controls on the relational memory task. There was no difference in performance between FEP patients who had a diagnosis of schizophrenia and those with a diagnosis of schizophreniform disorder.

CONCLUSIONS

Verbal relational memory appears to be intact in FEP. This finding, along with chronic patient literature, suggests that decline in hippocampal and medial temporal lobe functioning occurs during later illness stages. Further research is needed to aid in the development of intervention strategies that may prevent decline in such cognitive domains at this crucial early stage of the illness.

Visual paired-associate learning: in search of material-specific effects in adult patients who have undergone temporal lobectomy

The mesial temporal lobes are important for learning arbitrary associations. It has previously been demonstrated that left mesial temporal structures are involved in learning word pairs, but it is not yet known whether comparable lesions in the right temporal lobe impair visually mediated associative learning. Patients who had undergone left (n=16) or right (n=18) temporal lobectomy for relief of intractable epilepsy and healthy controls (n=13) were administered two paired-associate learning tasks assessing their learning and memory of pairs of abstract designs or pairs of symbols in unique locations. Both patient groups had deficits in learning the designs, but only the right temporal group was impaired in recognition. For the symbol location task, differences were not found in learning, but again a recognition deficit was found for the right temporal group. The findings implicate the mesial temporal structures in relational learning. They support a material-specific effect for recognition but not for learning and recall of arbitrary visual and visual-spatial associative information.

The use of self-generation procedures facilitates verbal memory in individuals with seizure disorders

The efficacy of a self-generation encoding procedure in facilitating the encoding and retrieval of verbal memories was compared with the didactic presentation of information in individuals with seizure disorders. Through a within-subject design, 87 patients (25 left temporal seizure onset, 29 right temporal, 8 frontal, and 25 psychogenic nonepileptic seizures) received a self-generation learning condition and a didactic learning condition and were subsequently tested for verbal paired associate free recall, cued recall, and recognition memory. All patient groups benefited from the use of the self-generation condition relative to the didactic condition. Better performance occurred with the self-generation procedure for cued recall and recognition memory test performance, but not free recall. Individuals with a left temporal seizure onset (patients with the poorest memory performance on the didactic condition) benefited the most from the self-generation condition. A memory encoding strategy that actively involves patient participation enhances memory performance.

Recall and recognition of verbal paired associates in early Alzheimer's disease

The primary impairment in early Alzheimer's disease (AD) is encoding/consolidation, resulting from medial temporal lobe (MTL) pathology. AD patients perform poorly on cued-recall paired associate learning (PAL) tasks, which assess the ability of the MTLs to encode relational memory. Since encoding and retrieval processes are confounded within performance indexes on cued-recall PAL, its specificity for AD is limited. Recognition paradigms tend to show good specificity for AD, and are well tolerated, but are typically less sensitive than recall tasks. Associate-recognition is a novel PAL task requiring a combination of recall and recognition processes. We administered a verbal associate-recognition test and cued-recall analogue to 22 early AD patients and 55 elderly controls to compare their ability to discriminate these groups. Both paradigms used eight arbitrarily related word pairs (e.g., pool-teeth) with varying degrees of imageability. Associate-recognition was equally effective as the cued-recall analogue in discriminating the groups, and logistic regression demonstrated classification rates by both tasks were equivalent. These preliminary findings provide support for the clinical value of this recognition tool. Conceptually it has potential for greater specificity in informing neuropsychological diagnosis of AD in clinical samples but this requires further empirical support.

Early detection of memory impairment in Alzheimer's disease: a neurocognitive perspective on assessment

We propose that the earliest neuropsychological detection of Alzheimer's disease (AD) can be informed by current views about the neuropathogenesis of AD and cognitive models of memory and its neurobiological substrates. The primary impairment in early AD is encoding/consolidation, resulting from medial temporal lobe (MTL) pathology. On theoretical and empirical grounds, paired associate learning (PAL) appears to be the ideal paradigm for detecting MTL dysfunction in early AD. It has not been embraced as a test of choice, however, and this critical review discusses why the paradigm may have not fulfilled its potential. We suggest that a new PAL variant, 'associate-recognition', may prove to be clinically efficacious.

Temporal lobe epilepsy as a model to understand human memory: the distinction between explicit and implicit memory

Decades of research have provided substantial evidence of memory impairments in patients with temporal lobe epilepsy (TLE), including deficits in the encoding, storage, and retrieval of new information. These findings are not surprising, given the associated underlying neuroanatomy, including the hippocampus and surrounding medial temporal lobe structures. Because of its associated anatomic and cognitive characteristics, TLE has provided an excellent model by which to examine specific aspects of human memory functioning, including classic distinctions such as that between explicit and implicit memory. Various clinical and experimental research studies have supported the idea that both conscious and unconscious processes support memory functioning, but the role of relevant brain structures has been the subject of debate. This review is concerned with a discussion of the current status of this research and, importantly, how TLE can inform future studies of memory distinctions.

Implicit Associative Learning Engages the Hippocampus and Interacts with Explicit Associative Learning

The hippocampus is crucial for conscious, explicit memory, but whether it is also involved in nonconscious, implicit memory is uncertain. We investigated with functional magnetic resonance imaging whether implicit learning engages the hippocampus and interacts with subsequent explicit learning. The presentation of subliminal faces-written profession pairs for implicit learning was followed by the explicit learning of supraliminal pairs composed of the same faces combined with written professions semantically incongruous to those presented subliminally (experiment 1), semantically congruous professions (experiment 2), or identical professions (experiment 3). We found that implicit face-profession learning interacted with explicit face-profession learning in all experiments, impairing the explicit retrieval of the associations. Hippocampal activity increased during the subliminal presentation of face-profession pairs versus face-nonword pairs and correlated with the later impairment of explicit retrieval. These findings suggest that implicit semantic associative learning engages the hippocampus and influences explicit memory.

Levels of processing with free and cued recall and unilateral temporal lobe epilepsy

This study investigates the role of the temporal lobes in levels-of-processing tasks (phonetic and semantic encoding) according to the nature of recall tasks (free and cued recall). These tasks were administered to 48 patients with unilateral temporal epilepsy (right "RTLE"=24; left "LTLE"=24) and a normal group (n=24). The results indicated that LTLE patients were impaired for semantic processing (free and cued recall) and for phonetic processing (free and cued recall), while for RTLE patients deficits appeared in free recall with semantic processing. It is suggested that the left temporal lobe is involved in all aspects of verbal memory, and that the right temporal lobe is specialized in semantic processing. Moreover, our data seem to indicate that RTLE patients present a retrieval processing impairment (semantic condition), whereas the LTLE group is characterized by encoding difficulties in the phonetic and semantic condition.

Nonconscious formation and reactivation of semantic associations by way of the medial temporal lobe

A successful strategy to memorize unrelated items is to associate them semantically. This learning method is typical for declarative memory and depends on the medial temporal lobe (MTL). Yet, only a small fraction of perceived items emerge into conscious awareness and receive the status of representations in declarative memory. This functional magnetic resonance imaging (fMRI) study tackled the mnemonic fate of unrelated item pairs processed without conscious awareness. Stimuli consisted of a face and a written profession (experimental condition) or of a face (control condition) exposed very briefly between pattern masks. Although the participants were unaware of the stimuli, activity in the hippocampus and perirhinal cortex was changed in the experimental versus the control condition; perirhinal activity changes correlated with the reaction time measure of the later nonconscious retrieval. For retrieval, the previously presented faces were shown again, this time for conscious inspection. The task was to guess the professional category of each face. This task was to induce a nonconscious retrieval of previously formed face-profession associations. Remarkably, activity in the hippocampus and perirhinal cortex was enhanced when subjects were confronted with faces from the experimental versus the control condition. The degree of hippocampal and perirhinal activation changes correlated with the reaction time measure of nonconscious retrieval. Together, our findings suggest that new semantic associations can be formed and retrieved by way of the medial temporal lobe without awareness of the associations or its components at encoding or any awareness that one is remembering at retrieval.

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.