Arnold Bakker

Pattern Separation in the Human Hippocampal CA3 and Dentate Gyrus

Pattern separation, the process of transforming similar representations or memories into highly dissimilar, nonoverlapping representations, is a key component of many functions ascribed to the hippocampus. Computational models have stressed the role of the hippocampus and, in particular, the dentate gyrus and its projections into the CA3 subregion in pattern separation. We used high-resolution (1.5-millimeter isotropic voxels) functional magnetic resonance imaging to measure brain activity during incidental memory encoding. Although activity consistent with a bias toward pattern completion was observed in CA1, the subiculum, and the entorhinal and parahippocampal cortices, activity consistent with a strong bias toward pattern separation was observed in, and limited to, the CA3/dentate gyrus. These results provide compelling evidence of a key role of the human CA3/dentate gyrus in pattern separation.

High-resolution structural and functional MRI of hippocampal CA3 and dentate gyrus in patients with amnestic mild cognitive impairment

Functional magnetic resonance imaging (fMRI) studies have observed hyperactivity in the hippocampal region in individuals with Mild Cognitive Impairment (MCI). However, the actual source of such hyperactivity is not well understood. Studies of aged rats observed similar hyperactive signals in the CA3 region of the hippocampus that correlated with spatial memory deficits and, in particular, with their ability to represent novel environments as being distinct from familiar ones (pattern separation). In this study, we tested the hypothesis that patients with amnestic MCI (aMCI) have deficits in pattern separation, along with hyperactive fMRI BOLD activity in the CA3 region of the hippocampus. We used high-resolution fMRI during a continuous recognition task designed to emphasize pattern separation. We conducted hippocampal subfield-level region of interest analyses to test for dysfunctional activity in aMCI patients. We found that patients showed impaired performance on trials that taxed their pattern separation abilities. We also observed hyperactive BOLD signals in the CA3/dentate and hypoactive signals in the entorhinal cortex during the separation condition. In a high-resolution morphometric analysis of hippocampal subfields, aMCI patients also had smaller CA3/dentate and CA1 volumes (no difference in the subiculum). The CA3/dentate region bilaterally also exhibited the largest shape deformations in aMCI patients, suggesting that this locus is affected early in the course of the disease. These findings suggest that structural and functional changes in the CA3/dentate region of the hippocampus contribute to the deficits in episodic memory that are observed in patients with aMCI. The functional hyperactivity may be evidence for a dysfunctional encoding mechanism, consistent with the predictions of computational models of hippocampal learning.

Regional white matter change in pre-symptomatic Huntington's disease : A diffusion tensor imaging study

The pathology of Huntingtons disease (HD) is characterized by diffuse brain atrophy, with the most substantial neuronal loss occurring in the caudate and putamen. Recent evidence suggests that there may be more widespread neuronal degeneration with significant involvement of extrastriate structures, including white matter. In this study of pre-symptomatic carriers of the HD genetic mutation, we have used diffusion tensor imaging to examine the integrity and organization of white matter in a group of individuals who previously demonstrated abnormalities in response to a functional magnetic resonance imaging paradigm. Our results indicate that, before the onset of manifest HD, there are regional decreases in fractional anisotropy, indicating early white matter disorganization.

Response of the medial temporal lobe network in amnestic mild cognitive impairment to therapeutic intervention assessed by fMRI and memory task performance

Studies of individuals with amnestic mild cognitive impairment (aMCI) have detected hyperactivity in the hippocampus during task-related functional magnetic resonance imaging (fMRI). Such elevated activation has been localized to the hippocampal dentate gyrus/CA3 (DG/CA3) during performance of a task designed to detect the computational contributions of those hippocampal circuits to episodic memory. The current investigation was conducted to test the hypothesis that greater hippocampal activation in aMCI represents a dysfunctional shift in the normal computational balance of the DG/CA3 regions, augmenting CA3-driven pattern completion at the expense of pattern separation mediated by the dentate gyrus. We tested this hypothesis using an intervention based on animal research demonstrating a beneficial effect on cognition by reducing excess hippocampal neural activity with low doses of the atypical anti-epileptic levetiracetam. In a within-subject design we assessed the effects of levetiracetam in three cohorts of aMCI participants, each receiving a different dose of levetiracetam. Elevated activation in the DG/CA3 region, together with impaired task performance, was detected in each aMCI cohort relative to an aged control group. We observed significant improvement in memory task performance under drug treatment relative to placebo in the aMCI cohorts at the 62.5 and 125 mg BID doses of levetiracetam. Drug treatment in those cohorts increased accuracy dependent on pattern separation processes and reduced errors attributable to an over-riding effect of pattern completion while normalizing fMRI activation in the DG/CA3 and entorhinal cortex. Similar to findings in animal studies, higher dosing at 250 mg BID had no significant benefit on either task performance or fMRI activation. Consistent with predictions based on the computational functions of the DG/CA3 elucidated in basic animal research, these data support a dysfunctional encoding mechanism detected by fMRI in individuals with aMCI and therapeutic intervention using fMRI to detect target engagement in response to treatment.

Cognitive Mappers to Creatures of Habit: Differential Engagement of Place and Response Learning Mechanisms Predicts Human Navigational Behavior

Learning to navigate plays an integral role in the survival of humans and other animals. Research on human navigation has largely focused on how we deliberately map out our world. However, many of us also have experiences of navigating on “autopilot” or out of habit. Animal models have identified this cognitive mapping versus habit learning as two dissociable systems for learning a space—a hippocampal place-learning system and a striatal response-learning system. Here, we use this dichotomy in humans to understand variability in navigational style by demonstrating that brain activation during spatial encoding can predict where a persons behavior falls on a continuum from a more flexible cognitive map-like strategy to a more rigid creature-of-habit approach. These findings bridge the wealth of knowledge gained from animal models and the study of human behavior, opening the door to a more comprehensive understanding of variability in human spatial learning and navigation.

Differential brain activation in anorexia nervosa to Fat and Thin words during a Stroop task.

We measured brain activation in six anorexia nervosa patients and six healthy controls performing a novel emotional Stroop task using Fat, Thin, and Neutral words, and words made of XXXXs. Reaction times increased in the patient group in Thin and Fat conditions. In the Thin–XXXX contrast, patients showed greater activation than controls at the junction of left insula, frontal and temporal lobes and in left middle and medial frontal gyri. In the Fat–XXXX contrast, controls showed greater activation in left dorsolateral prefrontal cortex and right parietal areas. Mechanisms underlying attentional bias in anorexia nervosa likely differ under conditions of positive and negative valence. This paradigm is a promising tool to examine neural mediation of emotional response in anorexia nervosa.

Depressive symptoms in prodromal Huntington's Disease correlate with Stroop-interference related functional connectivity in the ventromedial prefrontal cortex

Huntingtons Disease (HD) is a neurodegenerative disorder caused by a cytosine-adenine-guanine (CAG) triplet repeat-expansion in the Huntingtin (HTT) gene. Diagnosis of HD is classically defined by the presence of motor symptoms; however, cognitive and depressive symptoms frequently precede motor manifestations, and may occur early in the prodromal phase. There are sparse data so far on functional brain correlates of depressive symptoms in prodromal HD. A Stroop color-naming test was administered to 32 subjects in the prodromal phase of HD and 52 expansion-negative controls while performing functional magnetic resonance imaging at 3Tesla. Networks of functional connectivity were identified using group independent component analysis, followed by an analysis of functional network interactions. A contrast of temporal regression-based beta-weights was calculated as a reflection of Stroop-interference related activity and correlated with Center for Epidemiologic Studies Depression (CES-D) scores. For secondary analysis, patients were stratified into two subgroups by median split of CAG repeat-length. Stroop performance was independent of HTT mutation-carrier status and CES-D score. Stroop-interference-related activity of the ventromedial prefrontal cortex-node of the default-mode network, calculated by temporal-regression beta-weights, was more highly correlated with depressive symptoms in subjects in the prodromal phase of HD than in controls, differing significantly. The strength of this correlation and its difference from controls increased when a subgroup of patients with longer CAG repeat lengths was analyzed. These findings suggest that depressive symptoms in prodromal HD subjects may reflect altered functional brain network activity in the context of early HD-related brain alterations.

Bridging neurocognitive aging and disease modification: targeting functional mechanisms of memory impairment.

Risk for Alzheimers disease escalates dramatically with increasing age in the later decades of life. It is widely recognized that a preclinical condition in which memory loss is greater than would be expected for a persons age, referred to as amnestic mild cognitive impairment, may offer the best opportunity for intervention to treat symptoms and modify disease progression. Here we discuss a basis for age-related memory impairment, first discovered in animal models and recently isolated in the medial temporal lobe system of man, that offers a novel entry point for restoring memory function with the possible benefit in slowing progression to Alzheimers disease.

Testing prospective memory: does the value of a borrowed item help people remember to get it back?

The relationship between the value of an item borrowed from a patient for a prospective memory task and the number of cues required by the patient for requesting its return was analyzed in 63 consecutive patients referred for clinical neuropsychological evaluation. The correlation was negligible (? = -.001). Number of cues needed was better predicted by score on the Mini-Mental State Exam (r = -.541, p < .001) than by subjects age, sex, or education, or the value of the item taken.

Increased hippocampal activation in ApoE-4 carriers and non-carriers with amnestic mild cognitive impairment

Increased fMRI activation in the hippocampus is recognized as a signature characteristic of the amnestic mild cognitive impairment (aMCI) stage of Alzheimers disease (AD). Previous work has localized this increased activation to the dentate gyrus/CA3 subregion of the hippocampus and showed a correlation with memory impairments in those patients. Increased hippocampal activation has also been reported in carriers of the ApoE-4 allelic variation independently of mild cognitive impairment although these findings were not localized to a hippocampal subregion. To assess the ApoE-4 contribution to increased hippocampal fMRI activation, patients with aMCI genotyped for ApoE-4 status and healthy age-matched control participants completed a high-resolution fMRI scan while performing a memory task designed to tax hippocampal subregion specific functions. Consistent with previous reports, patients with aMCI showed increased hippocampal activation in the left dentate gyrus/CA3 region of the hippocampus as well as memory task errors attributable to this subregion. However, this increased fMRI activation in the hippocampus did not differ between ApoE-4 carriers and ApoE-4 non-carriers and the proportion of memory errors attributable to dentate gyrus/CA3 function did not differ between ApoE-4 carriers and ApoE-4 non-carriers. These results indicate that increased fMRI activation of the hippocampus observed in patients with aMCI is independent of ApoE-4 status and that ApoE-4 does not contribute to the dysfunctional hippocampal activation or the memory errors attributable to this subregion in these patients.