Bridging the gap: integrating cellular and functional magnetic resonance imaging studies of the hippocampus

Cerebral serotonin transporter asymmetry in females, males and male-to-female transsexuals measured by PET in vivo

The serotonergic system modulates brain functions that are considered to underlie affective states, emotion and cognition. Several lines of evidence point towards a strong lateralization of these mental processes, which indicates similar asymmetries in associated neurotransmitter systems. Here, our aim was to investigate a potential asymmetry of the serotonin transporter distribution using positron emission tomography and the radioligand [(11)C]DASB in vivo. As brain asymmetries may differ between sexes, we further aimed to compare serotonin transporter asymmetry between females, males and male-to-female (MtF) transsexuals whose brains are considered to be partly feminized. Voxel-wise analysis of serotonin transporter binding in all groups showed both strong left and rightward asymmetries in several cortical and subcortical structures including temporal and frontal cortices, anterior cingulate, hippocampus, caudate and thalamus. Further, male controls showed a rightward asymmetry in the midcingulate cortex, which was absent in females and MtF transsexuals. The present data support the notion of a lateralized serotonergic system, which is in line with previous findings of asymmetric serotonin-1A receptor distributions, extracellular serotonin concentrations, serotonin turnover and uptake. The absence of serotonin transporter asymmetry in the midcingulate in MtF transsexuals may be attributed to an absence of brain masculinization in this region.

Neural correlates of temporal-order judgments versus those of spatial-location: Deactivation of hippocampus may facilitate spatial performance

The retrieval of temporal-order versus spatial-location information was investigated using fMRI. The primary finding in the hippocampus proper, seen in region of interest analyses, was an increase in BOLD signal intensity for temporal retrieval, and a decrease in signal intensity for spatial retrieval, relative to baseline. The negative BOLD signal change with spatial memory processing, while unexpected, is consistent with the recent fMRI literature indicating decreased BOLD can be associated with neuronal activation, and it is argued that the deactivation observed here may facilitate spatial performance. Spatial-location judgments also yielded a stronger (positive) response in the right midfrontal gyrus, while temporal-order judgments (autobiographic condition only) showed greater activity in the left superior temporal gyrus, suggesting greater working memory demands and greater semantization for each judgment type, respectively. Finally, all conditions activated the left midfrontal gyrus, although autobiographic memories showed additional activity in the medial frontal gyrus.

fMRI Hippocampal Activity During a VirtualRadial Arm Maze

Numerous studies have shown that the hippocampus is critical for spatial memory. Within nonhuman research, a task often used to assess spatial memory is the radial arm maze. Because of the spatial nature of this task, this maze is often used to assess the function of the hippocampus. Our goal was to extrapolate this task to humans and examine whether healthy undergraduates utilize their hippocampus while performing a virtual reality version of the radial arm maze task. Thirteen undergraduates performed a virtual radial arm maze during functional magnetic resonance imaging. The brain maps of activity reveal bilateral hippocampal BOLD signal changes during the performance of this task. However, paradoxically, this BOLD signal change decreases during the spatial memory component of the task. Additionally, we note frontal cortex activity reflective of working memory circuits. These data reveal that, as predicted by the rodent literature, the hippocampus is involved in performing the virtual radial arm maze in humans. Hence, this virtual reality version may be used to assess the integrity of hippocampus so as to predict risk or severity in a variety of psychiatric disorders.

Selective neurotoxic damage to the hippocampal formation impairs performance of the transverse patterning and location memory tasks in rhesus macaques

Monkeys with neurotoxic (ibotenic acid) damage to the hippocampal formation and unoperated controls were trained on two sets of transverse patterning problems (A+/B-, B+/C-, C+/A-, and D+/E-, E+/F-, F+/D-) and a delayed nonmatching-to-location paradigm (DNML) with delays of 10s, 30s, 120s, and 600s. Hippocampal lesions produced a size- and area-dependent impairment on transverse patterning. Damage largely limited to the right hippocampus in one subject had no effect on performance on the task. Of the remaining four subjects, two with hippocampal damage greater than 40% bilaterally were unable to solve the two transverse patterning sets, but could solve the linear set of discriminations (A+/B-, B+/C-, C+/X-). The two remaining operated animals were impaired in acquisition of both sets, but were eventually able to solve one of the two transverse patterning discrimination sets. All five operated monkeys were impaired relative to normal controls on DNML, but not on the standard delayed nonmatching-to-sample (DNMS) version with trial-unique objects. The results confirm our previous findings (Alvarado et al., Hippocampus 12:421-433, 2002) using aspiration lesions of the hippocampal formation and strengthen the view that the hippocampal formation is critical for object and spatial relational memory.

Hippocampal dampening during a relational memory task

The hippocampus (HPC) has an essential role in relational memory. One task used to test relational memory is the transverse patterning (TP) problem (A+ B-, B+ C-, and C+ A-), which is sensitive to HPC damage across species. Using functional magnetic resonance imaging in humans, the authors observed activation maps and time course data indicating that the HPC is involved in this task, but it is paradoxically less active during the hippocampal-dependent relational memory phase relative to both a hippocampal-independent control memory phase and to a fixation control phase. This suggests that traditional assumptions suggesting that brain regions critical for a task must produce an increased blood oxygen level-dependent response during performance of that task are probably inaccurate and alternative explanations should be entertained.

Regional analysis of hippocampal activation during memory encoding and retrieval: fMRI study

Investigators have recently begun to examine the differential role of subregions of the hippocampus in episodic memory. Two distinct models have gained prominence in the field. One model, outlined by Moser and Moser (Hippocampus 1998;8:608-619), based mainly on animal studies, has proposed that episodic memory is subserved by the posterior two-thirds of the hippocampus alone. A second model, derived by Lepage et al. (Hippocampus 1998;8:313-322) from their review of 52 PET studies, has suggested that the anterior hippocampus is activated by memory encoding while the posterior hippocampus is activated by memory retrieval. Functional magnetic resonance imaging (fMRI) studies have tended to show limited activation in the anteriormost regions of the hippocampus, providing support for the Moser and Moser model. A potential confounding factor in these fMRI studies, however, is that susceptibility artifact may differentially reduce signal in the anterior versus the posterior hippocampus. In the present study, we examined activation differences between hippocampal subregions during encoding and retrieval of words and interpreted our findings within the context of these two models. We also examined the extent to which susceptibility artifact affects the analysis and interpretation of hippocampal activation by demonstrating its differential effect on the anterior versus the posterior hippocampus. Both voxel-by-voxel and region-of-interest analyses were conducted, allowing us to quantify differences between the anterior and posterior aspects of the hippocampus. We detected significant hippocampal activation in both the encoding and retrieval conditions. Our data do not provide evidence for regional anatomic differences in activation between encoding and retrieval. The data do suggest that, even after accounting for susceptibility artifact, both encoding and retrieval of verbal stimuli activate the middle and posterior hippocampus more strongly than the anterior hippocampus. Finally, this study is the first to quantify the effects of susceptibility-induced signal loss on hippocampal activation and suggests that this artifact has significantly biased the interpretation of earlier fMRI studies.

Brain ageing in the new millennium

OBJECTIVE

This paper examines the current literature pertaining to brain ageing. The objective of this review is to provide an overview of the effects of ageing on brain structure and function and to examine possible mediators of these changes.

METHODS

A MEDLINE search was conducted for each area of interest. A selective review was undertaken of relevant articles.

RESULTS

Although fundamental changes in fluid intellectual abilities occur with age, global cognitive decline is not a hallmark of the ageing process. Decline in fluid intellectual ability is paralleled by regionally specific age related changes apparent from both structural and functional neuroimaging studies. The histopathological mediators of these changes do not appear to be reduction in neuronal number, which, with the exception of selected hippocampal regions, remain relatively stable across age. At the molecular level, several mechanisms of age related change have been postulated. Such theoretical models await refinement and may eventually provide a basis for therapy designed to reduce effects of the ageing process. The role of possible protective factors such as 'brain reserve', neuroprotective agents and hormonal factors in modifying individual vulnerability to the ageing process has been the focus of a limited number of studies.

CONCLUSION

Our understanding of the functional and structural changes associated with both healthy and pathological ageing is rapidly gaining in sophistication and complexity. An awareness of the fundamental biological substrates underpinning the ageing process will allow improved insights into vulnerability to neuropsychiatric disease associated with advancing age.

Medial temporal lobe activity associated with active maintenance of novel information

Using event-related functional magnetic resonance imaging, we investigated the role of medial temporal regions during active maintenance of information over short delays or working memory. In experiment 1, we observed sustained bilateral hippocampal activation during maintenance of novel faces across a short delay period but not during face encoding or recognition. In contrast, we observed transient right parahippocampal activation during encoding and recognition but not during maintenance. We replicated these findings in experiment 2 and further determined that anterior hippocampal activation was greater during maintenance of novel than familiar faces. Our results reveal the importance of medial temporal lobe regions for the active maintenance of novel information in the absence of perceptual stimulation.

Imaging physiologic dysfunction of individual hippocampal subregions in humans and genetically modified mice

We have developed a variant of functional magnetic resonance imaging (fMRI) designed to be sensitive to static neuronal function. This method is based on resting instead of dynamic changes in oxygen-dependent signal and therefore allows for a spatial resolution that can detect signal from different hippocampal subregions in human subjects as well as in mice. We found that hippocampal signal was significantly diminished in elderly subjects with memory decline compared to age-matched controls, and different subjects showed dysfunction in different subregions. Among healthy elders, signal intensity from the subiculum was correlated selectively with memory performance. This method does not require an activation task; it can be used in anesthetized normal and in genetically modified and cognitively impaired mice. In mice the signal was found to be sufficiently sensitive to detect functional changes in the absence of underlying anatomical changes.

Evaluating the function of hippocampal subregions with high-resolution MRI in Alzheimer's disease and aging

Memory ability declines in older age groups. There is a growing list of physiological processes that target the hippocampal formation in an age-related fashion, and some might underlie the hippocampal component of memory decline. The hippocampal formation is comprised of separate subregions, and physiological processes differentially target these subregions. The ability to evaluate the functional integrity of individual subregions-performing subregional analysis-is a major clinical goal since it can aid in the diagnosis of memory decline, as well as in elucidating mechanisms of disease and testing potential interventions. Because of its superior spatial resolution, magnetic resonance imaging (MRI) is best suited to accomplish this goal. Despite limited success, most functional MRI (fMRI) protocols have difficulty in performing complete subregional analysis of the hippocampal formation. Here we address sources of difficulty by (1) generating T2* -weighted maps of the hippocampal formation with sub-millimeter resolution; and (2) by adapting an approach used by animal investigators to identify the hippocampal subregions using anatomical landmarks. The protocol is tested in patients with Alzheimer's disease and in healthy controls, in an effort to determine whether it can detect neuronal dysfunction. Results showed diminished signal in the hippocampal formation of patients with Alzheimer's disease (AD) compared to controls, and multivariate analysis showed that this difference was most prominent in the entorhinal cortex. The protocol can be used to perform subregional analysis of the hippocampal formation. Testing the protocol in other clinical populations is needed to demonstrate its efficacy in evaluating the neuronal integrity of all hippocampal subregions.

Application of cortical unfolding techniques to functional MRI of the human hippocampal region

We describe a new application of cortical unfolding to high-resolution functional magnetic resonance imaging (fMRI) of the human hippocampal region. This procedure includes techniques to segment and unfold the hippocampus, allowing the fusiform, parahippocampal, perirhinal, entorhinal, subicular, and CA fields to be viewed and compared across subjects. Transformation parameters derived from unfolding high-resolution structural images are applied to coplanar, functional images, yielding two-dimensional "unfolded" activation maps of hippocampi. The application of these unfolding techniques greatly enhances the ability of fMRI to localize and characterize signal changes within the medial temporal lobe. Use of this method on a novelty-encoding paradigm reveals a temporal dissociation between activation along the collateral sulcus and activation in the hippocampus proper.

The hippocampus: The shock of the new

Recent successes in functional brain imaging have suggested that the hippocampus is part of a novelty-detection network; but consideration of the available evidence and of the cognitive demands of novelty processing suggests that things are not so simple.