Sexual dimorphism in the parietal substrate associated with visuospatial cognition independent of general intelligence.

Sex differences in visuospatial cognition (VSC) with male advantage are frequently reported in the literature. There is evidence for sexual dimorphisms in the human brain, one of which postulates more gray matter (GM) in females and more white matter (WM) in males relative to total intracranial volume. We investigated the neuroanatomy of VSC independent of general intelligence (g) in sex-separated populations, homogenous in age, education, memory performance, a memory- and brain morphology-related gene, and g. VSC and g were assessed with the Wechsler adult intelligence scale. The influence of g on VSC was removed using a hierarchical factor analysis and the Schmid-Leiman solution. Structural high-resolution magnetic resonance images were acquired and analyzed with voxel-based morphometry. As hypothesized, the clusters of positive correlations between local volumes and VSC performance independent of g were found mainly in parietal areas, but also in pre- and postcentral regions, predominantly in the WM in males, whereas in females these correlations were located in parietal and superior temporal areas, predominantly in the GM. Our results suggest that VSC depends more strongly on parietal WM structures in males and on parietal GM structures in females. This sex difference might have to do with the increased axonal and decreased somatodendritic tissue in males relative to females. Whether such sex-specific implementations of the VSC network can be explained genetically as suggested in investigations into the Turner syndrome or as a result of structural neural plasticity upon different experience and usage remains to be shown.

Cerebellum and source memory.

We report the case of a 40-year-old right-handed German-speaking man who presented with ischemic stroke in the territories of the right superior cerebellar artery and posterior inferior cerebellar artery. The objective of the present study was to investigate the consequences of this cerebellar damage with regard to higher cognitive functions. On admission to the stroke unit, the patient presented with dysarthria, right-sided appendicular ataxia, gait ataxia, and right-sided horizontal nystagmus (National Institutes of Health Stroke Scale, NIHSS, score 4). When examined 10 days after his stroke using a set of neuropsychological tests, he showed a marked deficit in the ability to remember when and in which context he had previously encountered verbal material. This aspect of memory, so-called 'source memory', is known to be mediated mainly by frontal and medial temporal structures. The present case suggests the existence of a strong functional connectivity between cerebellum and cortical regions underlying specific memory processes.

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.

Prion protein M129V polymorphism affects retrieval-related brain activity.

The prion protein Met129Val polymorphism has recently been related to human long-term memory with carriers of either the 129MM or the 129MV genotype recalling 17% more words than 129(VV) carriers at 24h following learning. Here, we sampled genotype differences in retrieval-related brain activity at 30min and 24h following learning. Furthermore, genotype groups were compared regarding grey matter concentrations and cognitive profiles. We used event-related functional magnetic resonance imaging (fMRI) during a word recognition task on 12 Met/Met carriers, 12 Val/Met carriers, and 12 Val/Val carriers. These groups were matched for retrieval performance, gender, age, education, and other memory-related genetic polymorphisms. Although retrieval performance was matched, Val carriers exhibited enhanced retrieval-related brain activity at 30min and 24h following learning. At both time lags, correlations between retrieval-related brain activity and retrieval success were negative for Val homozygotes (the more activity, the worse retrieval success), while correlations showed no significance or were positive for Met homozygotes and heterozygotes. These results suggest a less economic use of retrieval-related neural resources in Val relative to Met carriers. Furthermore, Val carriers exhibited higher neocortical grey matter concentrations compared to Met carriers. When controlling for grey matter concentration, genotype effects in retrieval-related brain activity remained significant. Val and Met carriers yielded comparable brain activations for correct rejections of non-studied words and for working memory, which speaks to the specificity of the genotype effect. Findings suggest that the prion protein Met129Val polymorphism affects neural plasticity following learning at a time-scale of minutes to hours.

Enhanced brain activity may precede the diagnosis of Alzheimer's disease by 30 years.

Presenilin 1 (PSEN1) mutations cause autosomal dominant familial Alzheimer's disease (FAD). PSEN1 mutation carriers undergo the course of cognitive deterioration, which is typical for sporadic Alzheimer's disease but disease onset is earlier and disease progression is faster. Here, we sought to detect signs of FAD in presymptomatic carriers of the PSEN1 mutation (C410Y) by use of a neuropsychological examination, functional MRI during learning and memory tasks and MRI volumetry. We examined five non-demented members of a FAD family and 21 non-related controls. Two of the five family members were carrying the mutation; one was 20 years old and the other 45 years old. The age of clinical manifestation of FAD in the family studied here is approximately 48 years. Neuropsychological assessments suggested subtle problems with episodic memory in the 20-year-old mutation carrier. The middle-aged mutation carrier fulfilled criteria for amnestic mild cognitive impairment. The 20-year-old mutation carrier exhibited increased, while the middle-aged mutation carrier exhibited decreased brain activity compared to controls within memory-related neural networks during episodic learning and retrieval, but not during a working-memory task. The increased memory-related brain activity in the young mutation carrier might reflect a compensatory effort to overcome preclinical neural dysfunction caused by first pathological changes. The activity reductions in the middle-aged mutation carrier might reflect gross neural dysfunction in a more advanced stage of neuropathology. These data suggest that functional neuroimaging along with tasks that challenge specifically those brain areas which are initial targets of Alzheimer's disease pathology may reveal activity alterations on a single-subject level decades before the clinical manifestation of Alzheimer's disease.

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.

A CYP46 T/C SNP modulates parahippocampal and hippocampal morphology in young subjects.

There is evidence that brain cholesterol metabolism modulates the vulnerability for Alzheimer's disease (AD). Previous data showed that brain ß-amyloid load in elderly subjects with the CYP46 (cholesterol 24S-hydroxylase) TT-positive genotype was higher than in CYP46 TT-negative elderly subjects. We investigated effects of the CYP46 T/C polymorphism on parahippocampal and hippocampal grey matter (GM) morphology in 81 young subjects using structural magnetic resonance imaging based morphometry. We found that young TT-homozygotes exhibited smallest and CC-homozygotes largest parahippocampal and hippocampal GM volumes with the volumes of the CT-heterozygotes ranging in between. Parahippocampal and hippocampal volumes were positively correlated with delayed memory performance in C-carriers and negatively with immediate memory performance in TT-homozygotes. It has been shown that the brain cholesterol metabolism in general modulates dendrite outgrowth, synaptogenesis, and neuron survival, and it was suggested that CYP46 indirectly influences ß-amyloid metabolism. CYP46 C-carriers are privileged both in terms of ß-amyloid metabolism and in terms of brain reserve due to their larger parahippocampal and hippocampal structures. The exact cellular mechanisms that translate the CYP46 allelic variation into volumetric brain differences in the parahippocampal gyrus and hippocampus are still unknown and need to be further investigated.

Better memory and neural efficiency in young apolipoprotein E epsilon4 carriers.

The apolipoprotein E (APOE) epsilon4 allele is the major genetic risk factor for Alzheimer's disease, but an APOE effect on memory performance and memory-related neurophysiology in young, healthy subjects is unknown. We found an association of APOE epsilon4 with better episodic memory compared with APOE epsilon2 and epsilon3 in 340 young, healthy persons. Neuroimaging was performed in a subset of 34 memory-matched individuals to study genetic effects on memory-related brain activity independently of differential performance. E4 carriers decreased brain activity over 3 learning runs, whereas epsilon2 and epsilon3 carriers increased activity. This smaller neural investment of epsilon4 carriers into learning reappeared during retrieval: epsilon4 carriers exhibited reduced retrieval-related activity with equal retrieval performance. APOE isoforms had no differential effects on cognitive measures other than memory, brain volumes, and brain activity related to working memory. We suggest that APOE epsilon4 is associated with good episodic memory and an economic use of memory-related neural resources in young, healthy humans.

Calmodulin-binding transcription activator 1 (CAMTA1) alleles predispose human episodic memory performance.

Little is known about the genes and proteins involved in the process of human memory. To identify genetic factors related to human episodic memory performance, we conducted an ultra-high-density genome-wide screen at > 500 000 single nucleotide polymorphisms (SNPs) in a sample of normal young adults stratified for performance on an episodic recall memory test. Analysis of this data identified SNPs within the calmodulin-binding transcription activator 1 (CAMTA1) gene that were significantly associated with memory performance. A follow up study, focused on the CAMTA1 locus in an independent cohort consisting of cognitively normal young adults, singled out SNP rs4908449 with a P-value of 0.0002 as the most significant associated SNP in the region. These validated genetic findings were further supported by the identification of CAMTA1 transcript enrichment in memory-related human brain regions and through a functional magnetic resonance imaging experiment on individuals matched for memory performance that identified CAMTA1 allele-specific upregulation of medial temporal lobe brain activity in those individuals harboring the 'at-risk' allele for poorer memory performance. The CAMTA1 locus encodes a purported transcription factor that interfaces with the calcium-calmodulin system of the cell to alter gene expression patterns. Our validated genomic and functional biological findings described herein suggest a role for CAMTA1 in human episodic memory.

Common Kibra alleles are associated with human memory performance.

Human memory is a polygenic trait. We performed a genome-wide screen to identify memory-related gene variants. A genomic locus encoding the brain protein KIBRA was significantly associated with memory performance in three independent, cognitively normal cohorts from Switzerland and the United States. Gene expression studies showed that KIBRA was expressed in memory-related brain structures. Functional magnetic resonance imaging detected KIBRA allele-dependent differences in hippocampal activations during memory retrieval. Evidence from these experiments suggests a role for KIBRA in human memory.