Two years later - Revisiting autobiographical memory representations in vmPFC and hippocampus

A long-standing question in memory neuroscience concerns how and where autobiographical memories of personal experiences are represented in the brain. In a previous high resolution multivoxel pattern analysis fMRI study, we examined two week old (recent) and ten year old (remote) autobiographical memories (Bonnici et al., 2012, J. Neurosci. 32:16982-16991). We found that remote memories were particularly well represented in ventromedial prefrontal cortex (vmPFC) compared to recent memories. Moreover, while both types of memory were represented within anterior and posterior hippocampus, remote memories were more easily distinguished in the posterior portion. These findings suggested that a change of some kind had occurred between two weeks and ten years in terms of where autobiographical memories were represented in the brain. In order to examine this further, here participants from the original study returned two years later and recalled the memories again. We found that there was no difference in the detectability of memory representations within vmPFC for the now 2 year old and 12 year old memories, and this was also the case for the posterior hippocampus. Direct comparison of the two week old memories (original study) with themselves two years later (present study) confirmed that their representation within vmPFC had become more evident. Overall, this within-subjects longitudinal fMRI study extends our understanding of autobiographical memory representations by allowing us to narrow the window within which their consolidation is likely to occur. We conclude that after a memory is initially encoded, its representation within vmPFC has stablised by, at most, two years later

Obesity and insulin resistance are associated with reduced activity in core memory regions of the brain

Increasing research in animals and humans suggests that obesity may be associated with learning and memory deficits, and in particular with reductions in episodic memory. Rodent models have implicated the hippocampus in obesity-related memory impairments, but the neural mechanisms underlying episodic memory deficits in obese humans remain undetermined. In the present study, lean and obese human participants were scanned using fMRI while completing a What-Where-When episodic memory test (the “Treasure-Hunt Task”) that assessed the ability to remember integrated item, spatial, and temporal details of previously encoded complex events. In lean participants, the Treasure-Hunt task elicited significant activity in regions of the brain known to be important for recollecting episodic memories, such as the hippocampus, angular gyrus, and dorsolateral prefrontal cortex. Both obesity and insulin resistance were associated with significantly reduced functional activity throughout the core recollection network. These findings indicate that obesity is associated with reduced functional activity in core brain areas supporting episodic memory and that insulin resistance may be a key player in this association.This study was funded by an MRC Centenary Early Career Award to LGC and a James S. McDonnell Foundation Scholar Award to JSS. LGC was funded by the Sarah Woodhead Research Fellowship at Girton College Cambridge. NSC was funded by an Experimental Psychology Society Mid-Career Award and the ERC

Grey and white matter correlates of recent and remote autobiographical memory retrieval:Insights from the dementias

The capacity to remember self-referential past events relies on the integrity of a distributed neural network. Controversy exists, however, regarding the involvement of specific brain structures for the retrieval of recently experienced versus more distant events. Here, we explored how characteristic patterns of atrophy in neurodegenerative disorders differentially disrupt remote versus recent autobiographical memory. Eleven behavioural-variant frontotemporal dementia, 10 semantic dementia, 15 Alzheimer's disease patients and 14 healthy older Controls completed the Autobiographical Interview. All patient groups displayed significant remote memory impairments relative to Controls. Similarly, recent period retrieval was significantly compromised in behavioural-variant frontotemporal dementia and Alzheimer's disease, yet semantic dementia patients scored in line with Controls. Voxel-based morphometry and diffusion tensor imaging analyses, for all participants combined, were conducted to investigate grey and white matter correlates of remote and recent autobiographical memory retrieval. Neural correlates common to both recent and remote time periods were identified, including the hippocampus, medial prefrontal, and frontopolar cortices, and the forceps minor and left hippocampal portion of the cingulum bundle. Regions exclusively implicated in each time period were also identified. The integrity of the anterior temporal cortices was related to the retrieval of remote memories, whereas the posterior cingulate cortex emerged as a structure significantly associated with recent autobiographical memory retrieval. This study represents the first investigation of the grey and white matter correlates of remote and recent autobiographical memory retrieval in neurodegenerative disorders. Our findings demonstrate the importance of core brain structures, including the medial prefrontal cortex and hippocampus, irrespective of time period, and point towards the contribution of discrete regions in mediating successful retrieval of distant versus recently experienced events

Prefrontal cortex gyrification index in twins: an MRI study

Cortical development and folding seems to be under environmental as well as genetic control. The aim of our study was to estimate the genetic influence on gyrification and cortical volumes, comparing prefrontal gyrification index (GI) in monozygotic (MZ) and dizygotic (DZ) twin pairs, and unrelated pairs. Twenty-four subjects (6 pairs of MZ and 6 pairs of DZ twins) were included in this study. Prefrontal cortical folding (gyrification) was measured by an automated and manual version of the gyrification index (A-GI, M-GI) according to previously published protocols. MR-imaging was performed and 3 representative slices were selected from coronar MR-imaging scans. The volumes of the total brain, temporal lobes, prefrontal lobes, and cerebellum were analyzed, too. To evaluate similarity in GI, absolute differences in GI, and brain volumes as well as intraclass correlations of twin pairs were compared with regard to twin status. Finally, a control group of unrelated pairs was assembled from the first two study groups and analyzed. Compared to unrelated pairs, twin pairs exhibited more similarity concerning different brain volumes and a trend to more similarity concerning A-GI. MZ twins did not present more similarity concerning GI (automatically and manually measured) and volume measurements compared to DZ twins. Different factors, like intrauterine factors, postnatal development conditions, and especially environmental factors might account for the differences between related and unrelated pairs. The nonexistence of a pronounced similarity in MZ twins compared to DZ twins concerning prefrontal GI raises questions about the extent of genetic influence on GI

Reduced prefrontal gyrification in obsessive–compulsive disorder

Structural magnetic resonance imaging (MRI) studies reveal evidence for brain abnormalities in obsessive–compulsive disorder (OCD), for instance, reduction of gray matter volume in the prefrontal cortex. Disturbances of gyrification in the prefrontal cortex have been described several times in schizophrenia pointing to a neurodevelopmental etiology, while gyrification has not been studied so far in OCD patients. In 26 OCD patients and 38 healthy control subjects MR-imaging was performed. Prefrontal cortical folding (gyrification) was measured bilaterally by an automated version of the automated-gyrification index (A-GI), a ratio reflecting the extent of folding, from the slice containing the inner genu of the corpus callosum up to the frontal pole. Analysis of covariance (ANCOVA, independent factor diagnosis, covariates age, duration of education) demonstrated that compared with control subjects, patients with OCD displayed a significantly reduced A-GI in the left hemisphere (p = 0.021) and a trend for a decreased A-GI in the right hemisphere (p = 0.076). Significant correlations between prefrontal lobe volume and A-GI were only observed in controls, but not in OCD patients. In conclusion, prefrontal hypogyrification in OCD patients may be a structural correlate of the impairment in executive function of this patient group and may point to a neurodevelopmental origin of this disease

Left dorsolateral prefrontal cortex supports context-dependent prioritisation of off-task thought

When environments lack compelling goals, humans often let their minds wander to thoughts with greater personal relevance; however, we currently do not understand how this context-dependent prioritisation process operates. Dorsolateral prefrontal cortex (DLPFC) maintains goal representations in a context-dependent manner. Here, we show this region is involved in prioritising off-task thought in an analogous way. In a whole brain analysis we established that neural activity in DLPFC is high both when ‘on-task’ under demanding conditions and ‘off-task’ in a non-demanding task. Furthermore, individuals who increase off-task thought when external demands decrease, show lower correlation between neural signals linked to external tasks and lateral regions of the DMN within DLPFC, as well as less cortical grey matter in regions sensitive to these external task relevant signals. We conclude humans prioritise daydreaming when environmental demands decrease by aligning cognition with their personal goals using DLPFC