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

Effects of acute high-altitude exposure on working memory: A functional near-infrared spectroscopy study

INTRODUCTION

Inadequate oxygen availability may lead to impairment of neurocognitive functions. The aim of the present study was to investigate the effect of acute high-altitude exposure on the cerebral hemodynamic response and working memory.

METHODS

The same subjects performed working memory exercises with forward and backward digit span tasks both under normal oxygen conditions and in large simulated hypobaric hypoxia chambers, and a series of physiological parameters were evaluated. Functional near-infrared spectroscopy was used to measure cerebral blood flow changes in the dorsolateral prefrontal cortex (DLPFC) during the tasks.

RESULTS

Compared with normoxic conditions, under hypoxic conditions, the heart rate and blood pressure increased, blood oxygen saturation decreased significantly, and the forward task had similar accuracy and response time, while the backward task had lower accuracy and longer response time. Neuroimaging analysis showed increased activation in the DLPFC during the forward task and deactivation during the backward task under hypobaric hypoxia conditions.

CONCLUSION

Acute high-altitude exposure leads to physiological adaptations. The abnormal hemodynamic responses of the DLPFC to hypoxia at low pressure reveal the disruption of neurocognitive function by acute high-altitude exposure, which compromises complex cognitive functions, and provides a promising application for functional near infrared spectroscopy in the exploration of neural mechanisms in the brain during high-altitude exposure.

Neural mechanisms of language learning from social contexts

Humans learn languages in real-life situations by integrating multiple signals, including linguistic forms, their meanings, and the actions and intentions of speakers. However, little is known about the neural bases underlying the social learning of a second language (L2) in adults. In this study, 36 adults were asked to learn two sets of L2 spoken words through translation versus simulated social interactive videos (social learning). Brain activation during word learning was measured using fMRI. Greater activation was observed in the bilateral superior temporal sulcus, posterior middle temporal gyri, and right inferior parietal lobule during social learning as compared with translation learning. Furthermore, higher activity in the right temporal parietal junction, right hippocampus, and motor areas was observed during the initial stage of social learning, with the more successful performance being at the time of overnight testing. We argue that social learning may strengthen the link from new L2 forms to rich L2 semantic representations wherein memory properties are embodied, multimodal, and richly contextualized.

Structural complexity is negatively associated with brain activity: a novel multimodal test of compensation theories of aging

Fractal dimensionality (FD) measures the complexity within the folds and ridges of cortical and subcortical structures. We tested the degree that FD might provide a new perspective on the atrophy-compensation hypothesis: age or disease-related atrophy causes a compensatory neural response in the form of increased brain activity in the prefrontal cortex to maintain cognition. Brain structural and functional data were collected from 63 middle-aged and older adults and 18 young-adult controls. Two distinct patterns of FD were found that separated cortical from subcortical structures. Subcortical FD was more strongly negatively correlated with age than cortical FD, and cortical FD was negatively associated with brain activity during memory retrieval in medial and lateral parietal cortices uniquely in middle-aged and older adults. Multivariate analyses revealed that the lower FD/higher brain activity pattern was associated with poorer cognition-patterns not present in young adults, consistent with compensation. Bayesian analyses provide further evidence against the modal interpretation of the atrophy-compensation hypothesis in the prefrontal cortex-a key principle found in some neurocognitive theories of aging.

Episodic memory for visual scenes suggests compensatory brain activity in breast cancer patients: a prospective longitudinal fMRI study

It has been hypothesized that breast cancer and its chemotherapy can impart functional neural changes via an overlap with biological mechanisms associated with aging. Here we used fMRI to assess whether changes in neural activity accompanying visual episodic memory encoding and retrieval suggest altered activations according to patterns seen in functional imaging of cognitive aging. In a prospective longitudinal design, breast cancer patients (n = 13) were scanned during memory encoding and retrieval before and after chemotherapy treatment, and compared to healthy-age matched controls (n = 13). Our results indicate that despite equivalent behavioral performance, encoding and retrieval resulted in increased activation of prefrontal regions for the breast cancer group compared to controls for both before and after chemotherapy treatment. This was accompanied by decreased activity in posterior brain regions after chemotherapy, particularly those involved in visual processing, for the breast cancer group compared to controls. These findings are discussed as evidence for a possible anterior shift in neural processing to compensate for deficiencies in posterior brain regions, consistent with an accelerated aging account. Cancer and chemotherapy can impact brain regions underlying episodic memory, leading to additional recruitment of control regions, which may be linked to mechanisms related to aging.

fNIRS can robustly measure brain activity during memory encoding and retrieval in healthy subjects

Early intervention in Alzheimer’s Disease (AD) requires novel biomarkers that can capture changes in brain activity at an early stage. Current AD biomarkers are expensive and/or invasive and therefore unsuitable for use as screening tools, but a non-invasive, inexpensive, easily accessible screening method could be useful in both clinical and research settings. Prior studies suggest that especially paired-associate learning tasks may be useful in detecting the earliest memory impairment in AD. Here, we investigated the utility of functional Near Infrared Spectroscopy in measuring brain activity from prefrontal, parietal and temporal cortices of healthy adults (n = 19) during memory encoding and retrieval under a face-name paired-associate learning task. Our findings demonstrate that encoding of novel face-name pairs compared to baseline as well as compared to repeated face-name pairs resulted in significant activation in left dorsolateral prefrontal cortex while recalling resulted in activation in dorsolateral prefrontal cortex bilaterally. Moreover, brain response to recalling was significantly higher than encoding in medial, superior and middle frontal cortices for novel faces. Overall, this study shows that fNIRS can reliably measure cortical brain activation during a face-name paired-associate learning task. Future work will include similar measurements in populations with progressing memory deficits.

The effects of changes in object location on object identity detection: A simultaneous EEG-fMRI study

Object identity and location are bound together to form a unique integration that is maintained and processed in visual working memory (VWM). Changes in task-irrelevant object location have been shown to impair the retrieval of memorial representations and the detection of object identity changes. However, the neural correlates of this cognitive process remain largely unknown. In the present study, we aim to investigate the underlying brain activation during object color change detection and the modulatory effects of changes in object location and VWM load. To this end we used simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) recordings, which can reveal the neural activity with both high temporal and high spatial resolution. Subjects responded faster and with greater accuracy in the repeated compared to the changed object location condition, when a higher VWM load was utilized. These results support the spatial congruency advantage theory and suggest that it is more pronounced with higher VWM load. Furthermore, the spatial congruency effect was associated with larger posterior N1 activity, greater activation of the right inferior frontal gyrus (IFG) and less suppression of the right supramarginal gyrus (SMG), when object location was repeated compared to when it was changed. The ERP-fMRI integrative analysis demonstrated that the object location discrimination-related N1 component is generated in the right SMG.

Neural mechanisms underlying the reward-related enhancement of motivation when remembering episodic memories with high difficulty

The motivation to receive rewards enhances episodic memories, and the motivation is modulated by task difficulty. In episodic retrieval, however, functional neuroimaging evidence regarding the motivation that mediates interactions between reward and task difficulty is scarce. The present fMRI study investigated this issue. During encoding performed without fMRI, participants encoded Japanese words using either deep or shallow strategies, which led to variation in difficulty level during subsequent retrieval. During retrieval with fMRI, participants recognized the target words in either high or low monetary reward conditions. In the behavioral results, a reward-related enhancement of memory was found only when the memory retrieval was difficult, and the rewarding effect on subjective motivation was greater in the retrieval of memories with high difficulty than those with low difficulty. The fMRI data showed that reward-related increases in the activation of the substantia nigra/ventral tegmental area (SN/VTA), medial temporal lobe (MTL), dorsomedial prefrontal cortex (dmPFC), and dorsolateral prefrontal cortex (dlPFC) were greater during the retrieval of memories with high difficulty than those with low difficulty. Furthermore, reward-related enhancement of functional connectivity between the SN/VTA and MTL and between the SN/VTA and dmPFC during the retrieval of memories with high difficulty was significantly correlated with reward-related increases of retrieval accuracy and subjective motivation. The reward-related enhancement of episodic retrieval and retrieval-related motivation could be most effective when the level of retrieval difficulty is optimized. Such reward-related enhancement of memory and motivation could be modulated by a network including the reward-related SN/VTA, motivation-related dmPFC, and memory-related MTL. Hum Brain Mapp 38:3428-3443, 2017. © 2017 Wiley Periodicals, Inc.

Prefrontal contributions to relational encoding in amnestic mild cognitive impairment

Relational memory declines are well documented as an early marker for amnestic mild cognitive impairment (aMCI). Episodic memory formation relies on relational processing supported by two mnemonic mechanisms, generation and binding. Neuroimaging studies using functional magnetic resonance imaging (fMRI) have primarily focused on binding deficits which are thought to be mediated by medial temporal lobe dysfunction. In this study, prefrontal contributions to relational encoding were also investigated using fMRI by parametrically manipulating generation demands during the encoding of word triads. Participants diagnosed with aMCI and healthy control subjects encoded word triads consisting of a category word with either, zero, one, or two semantically related exemplars. As the need to generate increased (i.e., two- to one- to zero-link triads), both groups recruited a core set of regions associated with the encoding of word triads including the parahippocampal gyrus, superior temporal gyrus, and superior parietal lobule. Participants diagnosed with aMCI also parametrically recruited several frontal regions including the inferior frontal gyrus and middle frontal gyrus as the need to generate increased, whereas the control participants did not show this modulation. While there is some functional overlap in regions recruited by generation demands between the groups, the recruitment of frontal regions in the aMCI participants coincides with worse memory performance, likely representing a form of neural inefficiency associated with Alzheimer's disease.

•

This study investigated prefrontal contributions to relational encoding in aMCI.

•

Subjects encoded word triads that were varied by semantic relatedness.

•

aMCI participants modulated activity in frontal regions during encoding.

•

Healthy controls showed no such modulation in frontal regions.

•

Modulation in the aMCI group correlated with worse memory performance.

Situation awareness in obsessive-compulsive disorder

Past studies have suggested that OCD patients suffer memory impairment on tasks using complex stimuli that require memory for combined elements to be maintained, but not for more simplistic memory tests. We tested this with 42 OCD patients and 42 healthy controls performed a computerized situation awareness task. In addition, participants completed the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and Maudsley Obsessive-Compulsive Inventory (MOCI). The OCD patients had poorer accuracy in integration/comprehension and perception levels than controls. There were significant correlations between situational awareness scores (i.e., visuo-spatial monitoring and processing) and Y-BOCS obsession-compulsion and slowness and doubt scores of MOCI in OCD patients. In addition, there were also significant correlations between situational awareness and controlling, cleaning, slowness, rumination and total scores of MOCI in control group. Results indicated that (I) OCD patients have problems of perception, integration, and comprehension of complex visual perceptions; (II) situation awareness deficits associated with severity and prevalence of obsessions and compulsions.

Neural repetition effects in the medial temporal lobe complex are modulated by previous encoding experience

It remains an intriguing question why the medial temporal lobe (MTL) can display either attenuation or enhancement of neural activity following repetition of previously studied items. To isolate the role of encoding experience itself, we assessed neural repetition effects in the absence of any ongoing task demand or intentional orientation to retrieve. Experiment 1 showed that the hippocampus and surrounding MTL regions displayed neural repetition suppression (RS) upon repetition of past items that were merely attended during an earlier study phase but this was not the case following re-occurrence of items that had been encoded into working memory (WM). In this latter case a trend toward neural repetition enhancement (RE) was observed, though this was highly variable across individuals. Interestingly, participants with a higher degree of neural RE in the MTL complex displayed higher memory sensitivity in a later, surprise recognition test. Experiment 2 showed that massive exposure at encoding effected a change in the neural architecture supporting incidental repetition effects, with regions of the posterior parietal and ventral-frontal cortex in addition to the hippocampus displaying neural RE, while no neural RS was observed. The nature of encoding experience therefore modulates the expression of neural repetition effects in the MTL and the neocortex in the absence of memory goals.

Theta power during encoding predicts subsequent-memory performance and default mode network deactivation

The subsequent memory paradigm, according to which cerebral activity for later remembered (LR) and later forgotten (LF) items is contrasted, can be used to characterize the processes necessary for successful memory encoding. Previous simultaneous electroencephalography/functional magnetic resonance imaging (EEG/fMRI) memory studies suggest an inverse relationship between frontal theta band power and the blood oxygenation level dependent (BOLD) signal in the default mode network (DMN). The principal aim of this EEG/fMRI study was to test the hypothesis that this putative theta-DMN relationship is less evident in LF compared with LR trials. Fourteen healthy participants performed an episodic memory task in which pictorial stimuli were presented during encoding, and categorized (as LR or LF) by subsequent memory performance. For each encoding trial, the mean of the Hilbert envelope of the theta signal from 400 to 800 ms after stimulus presentation was calculated. To integrate the EEG and fMRI data, general linear models (GLMs) were used to assess the extent to which these single-trial theta values (as modulators of the main effect of stimulus) predicted DMN BOLD signal change, using: (i) whole-head univariate GLMs and (ii) GLMs in which the outcome variable was the time-course of a DMN component derived from spatial independent component analysis of the fMRI data. Theta was significantly greater for LR than LF stimuli. Furthermore, the inverse relationship between theta and BOLD in the DMN was consistently stronger for LR than LF pictures. These findings imply that theta oscillations are key to attenuating processes which may otherwise impair memory encoding.

Task-selective memory effects for successfully implemented encoding strategies

Previous behavioral evidence suggests that instructed strategy use benefits associative memory formation in paired associate tasks. Two such effective encoding strategies–visual imagery and sentence generation–facilitate memory through the production of different types of mediators (e.g., mental images and sentences). Neuroimaging evidence suggests that regions of the brain support memory reflecting the mental operations engaged at the time of study. That work, however, has not taken into account self-reported encoding task success (i.e., whether participants successfully generated a mediator). It is unknown, therefore, whether task-selective memory effects specific to each strategy might be found when encoding strategies are successfully implemented. In this experiment, participants studied pairs of abstract nouns under either visual imagery or sentence generation encoding instructions. At the time of study, participants reported their success at generating a mediator. Outside of the scanner, participants further reported the quality of the generated mediator (e.g., images, sentences) for each word pair. We observed task-selective memory effects for visual imagery in the left middle occipital gyrus, the left precuneus, and the lingual gyrus. No such task-selective effects were observed for sentence generation. Intriguingly, activity at the time of study in the left precuneus was modulated by the self-reported quality (vividness) of the generated mental images with greater activity for trials given higher ratings of quality. These data suggest that regions of the brain support memory in accord with the encoding operations engaged at the time of study.

Statistical epistasis and functional brain imaging support a role of voltage-gated potassium channels in human memory

Despite the current progress in high-throughput, dense genome scans, a major portion of complex traits' heritability still remains unexplained, a phenomenon commonly termed “missing heritability.” The negligence of analytical approaches accounting for gene-gene interaction effects, such as statistical epistasis, is probably central to this phenomenon. Here we performed a comprehensive two-way SNP interaction analysis of human episodic memory, which is a heritable complex trait, and focused on 120 genes known to show differential, memory-related expression patterns in rat hippocampus. Functional magnetic resonance imaging was also used to capture genotype-dependent differences in memory-related brain activity. A significant, episodic memory-related interaction between two markers located in potassium channel genes (KCNB2 and KCNH5) was observed (P_{nominal combined} = 0.000001). The epistatic interaction was robust, as it was significant in a screening (P_nominal = 0.0000012) and in a replication sample (P_nominal = 0.01). Finally, we found genotype-dependent activity differences in the parahippocampal gyrus (P_nominal = 0.001) supporting the behavioral genetics finding. Our results demonstrate the importance of analytical approaches that go beyond single marker statistics of complex traits.

Structural and metabolic correlates of episodic memory in relation to the depth of encoding in normal aging

This study set out to establish the relationship between changes in episodic memory retrieval in normal aging on the one hand and gray matter volume and (18)FDG uptake on the other. Structural MRI, resting-state (18)FDG-PET, and an episodic memory task manipulating the depth of encoding and the retention interval were administered to 46 healthy subjects divided into three groups according to their age (young, middle-aged, and elderly adults). Memory decline was found not to be linear in the course of normal aging: Whatever the retention interval, the retrieval of shallowly encoded words was impaired in both the middle-aged and the elderly, whereas the retrieval of deeply encoded words only declined in the elderly. In middle-aged and elderly subjects, the reduced performance in the shallow encoding condition was mainly related to posterior mediotemporal volume and metabolism. By contrast, the impaired retrieval of deeply encoded words in the elderly group was mainly related to frontal and parietal regions, suggesting the adoption of inefficient strategic processes.

Hippocampal activation in patients with mild cognitive impairment is necessary for successful memory encoding

BACKGROUND

Episodic memory enables us to consciously recollect personally experienced past events. Memory performance is reduced in patients with mild cognitive impairment (MCI), an at-risk condition for Alzheimer's disease (AD).

PATIENTS AND METHODS

We used functional MRI (fMRI) to compare brain activity during memory encoding in 29 healthy elderly subjects (mean age 67.7 (SD 5.4) years) and 21 patients with MCI (mean age 69.7 (SD 7.0) years). Subjects remembered a list of words while fMRI data were acquired. Later, they had to recognise these words among a list of distractor words. The use of an event related paradigm made it possible to selectively analyse successfully encoded items in each individual. We compared activation for successfully encoded words between healthy elderly subjects and patients with MCI.

RESULTS

The main intergroup difference was found in the left hippocampus and surrounding medial temporal lobe (MTL) regions for the patients with MCI compared with healthy subjects during successful encoding.

CONCLUSION

These results suggest that in patients with MCI, an increase in MTL activation is necessary for successful memory encoding. Hippocampal activation may help to link newly learned information to items already stored in memory. Increased activation in MTL regions in MCI may reflect a compensatory response to the beginning of AD pathology.

Magnetic resonance approaches to brain aging and Alzheimer disease-associated neuropathology

The noninvasive, nonradioactive, quantitative nature of magnetic resonance techniques has propelled them to the forefront of neuroscience and neuropsychiatric research. In particular, recent advances have confirmed their enormous potential in patients with Alzheimer disease (AD). Structural and functional magnetic resonance (MR) imaging have demonstrated significant correlation with clinical outcomes and underlying pathology and are used increasingly in the AD clinic. This review will highlight the role of high-resolution structural MR imaging and functional magnetic resonance imaging in the identification of atrophic and hemodynamic changes in AD and their potential as diagnostic biomarkers and surrogates of therapeutic response. Advanced MR techniques based on diffusion, perfusion, and neurochemical abnormalities in the aging brain will be presented briefly. These newer techniques continue to expand our understanding of neuropathology in the aging brain and are likely to play an important clinical role in the future.

Prefrontal and hippocampal contributions to the generation and binding of semantic associations during successful encoding

The ability to form and bind associations between items is an important aspect of successful memory formation. We hypothesize that, during encoding, the left inferior frontal gyrus (IFG) supports generation of associations between items and the hippocampus then binds these associations. This study examined the parametric responses of these regions to varying amounts of generative and relational processing during successful encoding (i.e., for subsequently recognized items). Encoding involved presentation of word triads varying in the number of semantic associations among them (none, one or all); participants judged how many associations were present in each triad. Thus, triads with fewer associations had higher generative load while triads with more associations had higher relational load. Participants later completed a forced-choice recognition test for encoding triads. Successful encoding relative to a control task resulted in activation of bilateral IFG and left hippocampus, and the hippocampus also exhibited a significant subsequent memory effect (hits>misses). Linear parametric analyses revealed that generative load modulated activity in bilateral IFG while relational load correlated with activity in left hippocampus. Although univariate analyses distinguished IFG and hippocampal contributions to the generative and relational stages of encoding, respectively, effective connectivity between these regions did not differ according to condition. Furthermore, this analysis revealed that the left IFG played a pivotal role in coordinating associative encoding processes. Our findings illustrate that modulation of components in a memory network can be independent of patterns of mutual connectivity among those components in mediating successful encoding.

Prefrontal engagement during source memory retrieval depends on the prior encoding task

The prefrontal cortex is strongly engaged by some, but not all, episodic memory tests. Prior work has shown that source recognition tests--those that require memory for conjunctions of studied attributes--yield deficient performance in patients with prefrontal damage and greater prefrontal activity in healthy subjects, as compared to simple recognition tests. Here, we tested the hypothesis that there is no intrinsic relationship between the prefrontal cortex and source memory, but that the prefrontal cortex is engaged by the demand to retrieve weakly encoded relationships. Subjects attempted to remember object/color conjunctions after an encoding task that focused on object identity alone, and an integrative encoding task that encouraged attention to object/color relationships. After the integrative encoding task, the late prefrontal brain electrical activity that typically occurs in source memory tests was eliminated. Earlier brain electrical activity related to successful recognition of the objects was unaffected by the nature of prior encoding.

Left mediotemporal structures mediate the retrieval of episodic autobiographical mental images

The aim of this functional magnetic resonance imaging (fMRI) study was to investigate the neuroanatomical substrates associated with the process of mental generation of specific (i.e., exemplar) and episodic autobiographical (i.e., an image of a unique life episode connected with an object) images. The fMRI paradigm in this experiment included a non-image generation baseline and two activation conditions requiring the generation of either specific or episodic autobiographical images. Image generation times and brain activation were recorded. Behavioral results showed that generating specific mental images took significantly less than generating episodic autobiographical images. Individuals generated specific images that were well distinct from the episodic autobiographical ones, semantic in nature without an episodic reference. Episodic autobiographical images did not show a significant bias towards preferential retrieval from any particular life period but were retrieved from across the entire life span. Conjunction analysis of the fMRI data showed that the two image generation conditions significantly activated a common set of neural structures, including mediofrontal areas. This shared pattern of activation might be the result of an underlying similar format and characteristics (e.g., richness in details) between the two types of images and might reflect the involvement of similar cognitive processes. Distinct patterns of significant activation were also present. Activation in the right parietal regions, cuneus, precuneus and left temporal regions was associated solely with the generation of specific images. Regions more specifically devoted to episodic memory retrieval and imagery, such as the left parahippocampal gyrus and precuneus, and the posterior cingulate cortex bilaterally, were significantly activated exclusively by the generation of episodic autobiographical images.

Healthy and pathological processes in adult development: new evidence from neuroimaging of the aging brain

PURPOSE OF REVIEW

Recent research has revealed that the population of older adults is composed not only of individuals who are either healthy or have an age-related disease, most commonly Alzheimer's disease, but also individuals with mild cognitive impairment who are at-risk for or already in the prodromal stage of Alzheimer's disease. These variations in cognitive aging can be related to their neural bases via structural and functional neuroimaging methods.

RECENT FINDINGS

Healthy aging appears to primarily affect a frontal-striatal system that undergirds executive control of cognition, while minimally affecting medial temporal lobe structures. Functional imaging studies suggest that enhanced prefrontal engagement may offer compensatory plasticity that minimizes age-related cognitive losses. Mild cognitive impairment appears to affect the entorhinal cortex in particular, with functional consequences in other brain regions. Alzheimer's disease is characterized by severe hippocampal injury, although early-stage Alzheimer's disease may relatively spare some cortical regions.

SUMMARY

Advances in in-vivo imaging methods are providing the tools for identifying different trajectories of neurocognitive aging, and knowledge about these brain changes may promote opportunities for treatment.

Memory lateralization with 2 functional MR imaging tasks in patients with lesions in the temporal lobe

BACKGROUND

Previous studies to determine memory lateralization with functional MR imaging (fMRI) have used encoding or recall tasks. The convergence between the results of both tasks, however, is unknown.

OBJECTIVE

The objective of this study was to investigate hemispheric asymmetries of temporal lobe activity (parahippocampus and fusiform gyri) in patients with temporal lesions by using both kinds of fMRI tasks.

METHODS

By using blood oxygenation level-dependent fMRI, hemispheric asymmetries of 25 consecutive patients admitted for presurgical evaluation of memory and 12 healthy control participants were studied. Activation was induced by using the picture-encoding task (processing of complex scenes) and the hometown-walking task (requiring mental navigation through one's hometown by using landmarks given by participants themselves).

RESULTS

Results in the control group showed that both tasks activated the parahippocampus similarly. The picture-encoding task, however, yielded greater posterior activations in the parahippocampus than did the hometown-walking task. As observed in other studies, more than half the patients showed contralesional representation of memory in each task. It is important to note that estimated memory lateralization from each task was different in 30% of patients, and several cases showed clear discrepancies between both tasks.

CONCLUSION

Although previous studies showed that both tasks were useful for evaluating memory lateralization, the present study suggested that the administration of both tasks is necessary for presurgical evaluation of memory lateralization in patients with lesions in the temporal lobe. Therefore, both encoding and recall processes should at least be considered in the evaluation of memory.

Increased hippocampal activation in mild cognitive impairment compared to normal aging and AD

OBJECTIVE

To use fMRI to investigate whether hippocampal and entorhinal activation during learning is altered in the earliest phase of mild cognitive impairment (MCI).

METHODS

Three groups of older individuals were studied: 10 cognitively intact controls, 9 individuals at the mild end of the spectrum of MCI, and 10 patients with probable Alzheimer disease (AD). Subjects performed a face-name associative encoding task during fMRI scanning, and were tested for recognition of stimuli afterward. Data were analyzed using a functional-anatomic method in which medial temporal lobe (MTL) regions of interest were identified from each individual's structural MRI, and fMRI activation was quantified within each region.

RESULTS

Significantly greater hippocampal activation was present in the MCI group compared to controls; there were no differences between these two groups in hippocampal or entorhinal volumes. In contrast, the AD group showed hippocampal and entorhinal hypoactivation and atrophy in comparison to controls. The subjects with MCI performed similarly to controls on the fMRI recognition memory task; patients with AD exhibited poorer performance. Across all 29 subjects, greater mean entorhinal activation was found in the subgroup of 13 carriers of the APOE epsilon4 allele than in the 16 noncarriers.

CONCLUSIONS

The authors hypothesize that there is a phase of increased medial temporal lobe activation early in the course of prodromal Alzheimer disease followed by a subsequent decrease as the disease progresses.

Neuroanatomic correlates of CVLT-C performance following pediatric traumatic brain injury

Traumatic brain injury (TBI) frequently results in memory problems, and the degree of memory impairment is related to injury severity and is commonly associated with lesions in frontal and temporal brain areas. This study examined the relationship among injury severity, brain lesions, and memory in children with moderate to severe TBI using Donders' (1999) 5-factor model of performance on the California Verbal Learning Test-Children's Version (CVLT-C). Seventy-six children underwent magnetic resonance imaging (MRI) scans 3 months post-TBI and testing 1 year post-TBI. Results showed injury severity (Glasgow Coma Scale) was not predictive of performance on 4 of the 5 factors. Volume of frontal and/or temporal brain lesions was significantly predictive of performance on 3 of the 5 factors. Unexpectedly, lesion volume outside these areas (extra-frontotemporal) was predictive of performance on all 5 factors. In contrast, Verbal IQ at 1 year was most strongly associated with preinjury factors (socioeconomic status and special education involvement), although extra-frontotemporal lesions also contributed to the variability in this measure. Results suggest that in children with moderate to severe TBI, extra-frontal/temporal lesions are predictive of memory outcome 1 year postinjury above and beyond initial severity or frontal/temporal contusions. This finding may relate to widespread diffuse axonal injury, which potentially disconnects brain circuits mediating memory following moderate to severe TBI.

The effect of encoding strategies on medial temporal lobe activations during the recognition of words: an event-related fMRI study

It is known that manipulation of the encoding strategy affects behavioral and activation data during later retrieval. In the present fMRI study, we examined brain activity during the recognition of words encoded using three different strategies formed by the combination of two factors of relational and self-performed processes. The first encoding strategy involved subjects learning words using both relational and self-performed processes (R+S+). In the second, subjects learned words using only a relational process (R+S-). In the third, subjects learned words without using either process (R-S-). During fMRI after encoding, subjects were randomly presented with words encoded previously and with new words (New) and were required to judge whether or not the word presented had been previously encoded. The fMRI experiment was performed with the event-related design. Compared to New, activation of the left medial temporal lobe (MTL) occurred during the recognition of words encoded using R+S+ and R+S-, whereas right MTL activations only occurred with the R+S+ strategy. ROI analysis for the bilateral hippocampus and parahippocampal gyrus showed a linear increase in left MTL activity (hippocampus and parahippocampal gyrus) during the recognition of words encoded with the R-S-, R+S-, to R+S+, whereas right MTL activity (parahippocampal gyrus) was only increased with the R+S+ strategy. The findings suggest that the left and right MTL structures may contribute differentially to the processes involved in the recognition of stimuli and that these differential activities may depend on the encoding strategies formed by the two factors of relational and self-performed processes.