Functional MRI with reduced susceptibility artifact: high-resolution mapping of episodic memory encoding

Hippocampal activation and connectivity in the aging brain

The hippocampus and underlying cortices are highly susceptible to pathologic change with increasing age. Using an associative face-scene (Face-Place) encoding task designed to target these regions, we investigated activation and connectivity patterns in cognitively normal older adults. Functional MRI scans were collected in 210 older participants (mean age = 76.4 yrs) in the Baltimore Longitudinal Study of Aging (BLSA). Brain activation patterns were examined during encoding of novel Face-Place pairs. Functional connectivity of the hippocampus was also examined during encoding, with seed regions placed along the longitudinal axis in the head, body and tail of the structure. In the temporal lobe, task activation patterns included coverage of the hippocampus and underlying ventral temporal cortices. Extensive activation was also seen in frontal, parietal and occipital lobes of the brain. Functional connectivity analyses during overall encoding showed that the head of the hippocampus was connected to frontal and anterior/middle temporal regions, the body with frontal, widespread temporal and occipital regions, and the tail with posterior temporal and occipital cortical regions. Connectivity limited to encoding of subsequently remembered stimuli showed a similar pattern for the hippocampal body, but differing patterns for the head and tail regions. These results show that the Face-Place task produces activation along the occipitotemporal visual pathway including medial temporal areas. The connectivity results also show that patterns of functional connectivity vary throughout the anterior-posterior extent of the hippocampus during memory encoding. As these patterns include regions vulnerable to pathologic change in early stages of Alzheimer's disease, continued longitudinal assessment of these individuals can provide valuable information regarding changes in brain-behavior relationships that may occur with advancing age and the onset of cognitive decline.

Role of the Wada test and functional magnetic resonance imaging in preoperative mapping of language and memory: two atypical cases

The Wada test is an invasive procedure used to determine cerebral memory and language dominance as well as risk of cognitive deficits following neurosurgery. However, the potential risks of Wada testing have led some to consider foregoing Wada testing in candidates for resective epilepsy surgery with right hemispheric seizure onset. We present two atypical cases in which the Wada test showed unexpected memory and language lateralization. These cases underscore the importance of functional magnetic resonance in which imaging and Wada examination in right-handed individuals even when the lesion would not suggest atypical language representation.

Preoperative prediction of verbal episodic memory outcome using FMRI

This article focuses on an important neurosurgical problem for which functional imaging may have a role. Temporal lobe epilepsy surgery typically involves removal of much of the anterior medial temporal lobe, which is critical for encoding and retrieval of long-term episodic memories. Verbal episodic memory decline after left anterior temporal lobe resection occurs in 30% to 60% of such patients. Recent studies show that preoperative fMRI can predict the degree of verbal memory change that will occur, and that fMRI improves prediction accuracy when combined with other routine tests. The predictive power of fMRI appears to be at least as good as the Wada memory test, making fMRI a viable noninvasive alternative to the Wada for preoperative assessment.

Functional MRI is a valid noninvasive alternative to Wada testing

Partial removal of the anterior temporal lobe (ATL) is a highly effective surgical treatment for intractable temporal lobe epilepsy, yet roughly half of patients who undergo left ATL resection show a decline in language or verbal memory function postoperatively. Two recent studies demonstrate that preoperative fMRI can predict postoperative naming and verbal memory changes in such patients. Most importantly, fMRI significantly improves the accuracy of prediction relative to other noninvasive measures used alone. Addition of language and memory lateralization data from the intracarotid amobarbital (Wada) test did not improve prediction accuracy in these studies. Thus, fMRI provides patients and practitioners with a safe, noninvasive, and well-validated tool for making better-informed decisions regarding elective surgery based on a quantitative assessment of cognitive risk.

A comparison of two fMRI methods for predicting verbal memory decline after left temporal lobectomy: language lateralization versus hippocampal activation asymmetry

PURPOSE

Language lateralization measured by preoperative functional magnetic resonance imaging (fMRI) was shown recently to be predictive of verbal memory outcome in patients undergoing left anterior temporal lobe (L-ATL) resection. The aim of this study was to determine whether language lateralization or functional lateralization in the hippocampus is a better predictor of outcome in this setting.

METHODS

Thirty L-ATL patients underwent preoperative language fMRI, preoperative hippocampal fMRI using a scene encoding task, and pre- and postoperative neuropsychological testing. A group of 37 right ATL (R-ATL) surgery patients was included for comparison.

RESULTS

Verbal memory decline occurred in roughly half of the L-ATL patients. Preoperative language lateralization was correlated with postoperative verbal memory change. Hippocampal activation asymmetry was strongly related to side of seizure focus and to Wada memory asymmetry but was unrelated to verbal memory outcome.

DISCUSSION

Preoperative hippocampal activation asymmetry elicited by a scene encoding task is not predictive of verbal memory outcome. Risk of verbal memory decline is likely to be related to lateralization of material-specific verbal memory networks, which are more closely correlated with language lateralization than with overall asymmetry of episodic memory processes.

Imaging mesial temporal lobe activation during scene encoding: comparison of fMRI using BOLD and arterial spin labeling

Memory encoding is a critical brain function subserved by the hippocampus (HP) and mesial temporal lobe (mTL) structures. Visualization of mTL memory activation with BOLD fMRI is complicated by the presence of static susceptibility gradients in this region. Arterial spin labeled (ASL) perfusion fMRI offers an alternative approach not dependent on susceptibility contrast that instead suffers from lower intrinsic signal-to-noise ratio. An improved ASL perfusion fMRI approach combining pseudo-continuous ASL and a T(2)*-insensitive sequence (GRASE) with background suppression was compared to BOLD fMRI at 3 T during a scene encoding task known to activate the HP. Overall, an approximate sixfold sensitivity increase of ASL fMRI was achieved, with improved coverage in the anterior mTL, while suppression of the static tissue enhanced the stability of the ASL series by a factor of 2.4. Perfusion fMRI using this approach with 4 mm isotropic resolution yielded better localized and stronger group activation maps than BOLD fMRI at a standard resolution of 3 mm isotropic voxels. Increasing the resolution for BOLD to 2.5 mm isotropic produced stronger mTL and hippocampal activation in the group and individual subjects than the ASL technique, due to superior temporal resolution and reduced partial volume effects. Future improvements in ASL spatial and temporal resolution would allow the benefits of both approaches to be combined to further enhance the sensitivity for detecting mTL activation during memory encoding.

Spatio-temporal Modeling of Evoked Brain Activity During Memory Encoding and Target Comparison in Visual Tasks

In this paper, the temporal pattern of activity and approximate locations of brain areas related to selective attention and visual working memory processes were studied with event related potential (ERP) recordings in healthy humans. Three experimental series included pairs of the following conditions: Face comparison (familiar faces), Pattern comparison (abstract dot patterns), and Passive viewing. Participants compared pairs of consecutive targets presented in composite images on a computer screen. Spatio-temporal multiple dipole models were developed for 128-channel ERPs. Keeping dipole locations and orientations constant, we compared the source activities for ERPs recorded (1) in different tasks for task-specificity of activations, (2) after the first and second stimuli in the pair, i.e. on the encoding and comparison stages of the task, and (3) after the first stimulus in different series to compare encoding in different conditions. Sources located in the inferotemporal brain areas, especially in the left hemisphere, showed increased activity after 200 ms from the first stimulus onset that may indicate encoding into visual working memory. The anterior sources, located near midline and showing activity around or after 300 ms, presumably reflect non-specific memory processes and attentional control. Major task-specific differences were observed in the temporo-parieto-occipital region in 250-500 ms.

Anterior medial temporal lobe activation during encoding of words: FMRI methods to optimize sensitivity

The existence of a rostrocaudal gradient of medial temporal lobe (MTL) activation during memory encoding has historically received support from positron emission tomography studies, but less so from functional MRI (FMRI) studies. More recently, FMRI studies have demonstrated that characteristics of the stimuli can affect the location of activation seen in the MTL when those stimuli are encoded. The current study tested the hypothesis that MTL activation during memory encoding is related to the modality of stimulus presentation. Subjects encoded auditorily or visually presented words in an FMRI novelty paradigm. Imaging and analysis parameters were optimized to minimize susceptibility artifact in the anterior MTL. Greater activation was observed in the anterior than posterior MTL for both modalities of stimulus presentation. The results indicate that anterior MTL activation occurred during encoding, independent of stimulus modality and provide support for the hypothesis that verbal-semantic memory processing occurs in anterior MTL. The authors suggest that technical factors are critical for observing the rostrocaudal gradient in MTL memory activation.

A comparison of two FMRI protocols for eliciting hippocampal activation

PURPOSE

Previous research suggests that the hippocampus is modulated both by stimulus novelty and by the extent to which relational processing (formation of associations) occurs during episodic encoding. The aim of this study was to compare hippocampal activation patterns measured by functional magnetic resonance imaging (fMRI) during encoding protocols emphasizing either novelty or relational processing.

METHODS

fMRI was performed on 32 healthy volunteers while they encoded complex visual scenes or unrecognizable scrambled versions of the same scenes. In the Novelty contrast, encoding of novel scenes was compared with encoding of a repeated pair of scenes. In the Relational Processing contrast, semantic encoding of novel scenes was compared with structural encoding of scrambled scenes.

RESULTS

Both protocols elicited bilateral hippocampal activation. Overall mean activation values were similar for the two protocols, but the Relational Processing protocol resulted in a larger volume of hippocampal activation. The pattern of activation along the longitudinal hippocampal axis differed for the two protocols. The Novelty contrast produced stronger activation in the posterior hippocampus, whereas the Relational Processing contrast produced stronger activation in the anterior hippocampus.

CONCLUSIONS

Hippocampal activation is determined by both stimulus novelty and degree of relational processing during encoding. Given the importance of anterior hippocampal pathology in temporal lobe epilepsy, an approach emphasizing modulation of relational processing may be preferable for clinical fMRI of the medial temporal lobes.

The reliability of fMRI activations in the medial temporal lobes in a verbal episodic memory task

The test-retest reliability of activation patterns elicited by encoding and recognition of word-pair associates within the whole brain and a predefined medial temporal region of interest (ROI) was investigated. Twenty healthy right-handed subjects were studied within two sessions, either on the same day or 210-308 days later. Three quantitative measures of reliability were calculated for the contrasts encoding and recognition versus a control condition within the ROI and also for the whole brain: A group correlational analysis between the lateralization indices of the first and second session, correlations of the individual SPM(t) maps of the first and the second run, and overlap ratios between both sessions. For the ROI, correlational analysis of lateralization indices during both encoding trials was significant. Eighty percent of the individual positive correlation coefficients of SPM(t) maps during encoding, and 75% during recognition reached significance. The mean percentage of overlapping voxels was 18% during encoding and 19% during recognition. The reproducibility measures evaluated for the whole brain demonstrated significantly higher values compared to the ROI. For the group that stayed inside the scanner, better whole brain test-retest reliability was observed, and no influence of the memory process (encoding or recognition) on reproducibility was found.

Sensitivity-encoded (SENSE) echo planar fMRI at 3T in the medial temporal lobe

Parallel imaging techniques are useful for fMRI studies in light of the increasing susceptibility effects at high magnetic field strength. Yet, spatially varying noise amplification constitutes a challenge for the application of these techniques. The medial temporal lobe is particularly vulnerable to susceptibility effect with increasingly strong signal reduction. We present two fMRI studies comparing SENSE single-shot (ssh) echo planar imaging (EPI) at acceleration factors of 2.0, 2.4, 2.7, and 3.0 with conventional sshEPI at TE of 22 and 35 ms. Data were acquired during a learning task which activates the medial temporal lobe bilaterally. Susceptibility related image distortion was markedly reduced with increasing SENSE acceleration. Moreover, in the group results, statistical power increased in the whole brain with SENSE compared to conventional imaging and with a TE of 35 ms compared to 22 ms. Higher SENSE acceleration factors further improved image quality and increased statistical power in the occipital lobe and fusiform gyrus, but not in the medial temporal lobe. We therefore conclude that an sshEPI acquisition protocol with a moderate SENSE acceleration factor of R = 2.0 and TE 35 ms is suitable for the detection of medial temporal activation at 3T.

Experimental design for functional MRI of scene memory encoding

The use of functional imaging to identify encoding-related areas in the medial temporal lobe has previously been explored for presurgical evaluation in patients with temporal lobe epilepsy. Optimizing sensitivity in such paradigms is critical for the reliable detection of regions most closely engaged in memory encoding. A variety of experimental designs have been used to detect encoding-related activity, including blocked, sparse event-related, and rapid event-related designs. Although blocked designs are generally more sensitive than event-related designs, design and analysis advantages could potentially overcome this difference. In the present study, we directly contrast different experimental designs in terms of the intensity, extent, and lateralization of activation detected in healthy subjects. Our results suggest that although improved design augments the sensitivity of event-related designs, these benefits are not sufficient to overcome the sensitivity advantages of traditional blocked designs.

fMRI: Applications in Epilepsy

Functional MRI (fMRI) methods use currently standard MRI scanning hardware to detect changes in regional blood flow and metabolism that accompany regional brain activation. Major applications of fMRI in epilepsy include the localization of task-correlated language and memory function, and the localization of ictal and paroxysmal phenomena. Language lateralization by fMRI provides comparable results to intracarotid amobarbital testing, and memory lateralization by fMRI also shows promise. The recent development of methodologies to allow interpretable electroencephalographic data to be recorded during MRI scanning has opened up new opportunities for combining the spatial resolution of imaging with the temporal resolution of electrophysiology in seizure localization.

Attentional systems in target and distractor processing: a combined ERP and fMRI study

The interplay of "top-down" and "bottom-up" regulated mechanisms is of particular relevance for the rapid (re-)focusing of attention to environmental changes. The purpose of the study was to explore the differential contributions of frontoparietal attentional networks involved in top-down and stimulus-driven processing to the detection of "target" and "distractor" events in a visual three-stimulus oddball paradigm. Thirteen healthy subjects underwent separate event-related potential (ERP) and whole-brain functional magnetic resonance imaging (fMRI) measurements during the oddball task. The targets, which were difficult to detect, elicited a classical posterior P3b whereas the distractor stimuli were followed by a centro-frontal P3a ERP. The fMRI data showed activation of the temporoparietal junction (TPJ) bilaterally and right prefrontal cortex associated with both the target and distractor conditions. This network has previously been described as an attentional system that is predominantly stimulus driven and that responds to rare events. Furthermore, target processing produced bilateral perisylvian activity, which has been related to the "retrieval mode". Processing of the distractors activated the frontal eye fields (FEFs) and bilateral superior parietal cortex, areas engaged in attention switching and voluntary allocation of attention. Additional left prefrontal activation suggested an involvement of the cortical system for working memory encoding. Our results thus demonstrate that distractor and target processing engage a common neuronal system for the detection of rare events, but also task-specific subsystems related to attention and memory processes.

Functional neuroimaging: a new generation of human brain studies in obesity research

Obesity is predominantly caused by overeating, an abnormal behaviour for which there is no unequivocal neurophysiological explanation. Functional neuroimaging techniques, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have recently emerged as new tools to search for regions of the brain that are involved in the regulation of eating behaviours and those that are involved in the pathophysiology of obesity. Using these techniques, a limited number of studies have provided the first in vivo images of the human hypothalamic response to nutritional stimuli and revealed the complexity of the human brain response to hunger, taste, and satiation. Selective differences have been reported in the functional architecture of the brain of obese and lean individuals. We discuss current use and possible future developments of functional neuroimaging applied to obesity research. We conclude that functional neuroimaging provides an increasingly important tool for investigating how different regions of the brain work in concert to orchestrate normal eating behaviours and how they conspire to produce obesity and other eating disorders.

Exploring the neural bases of episodic and semantic memory: the role of structural and functional neuroimaging

Exploration of the neural bases of episodic and semantic memory is best pursued through the combined examination of the effects of identified lesions on memory and functional neuroimaging of both normal people and patients when they engage in memory processing of various kinds. Both structural and functional neuroimaging acquisition and analysis techniques have developed rapidly and will continue to do so. This review briefly outlines the history of neuroimaging as it impacts on memory research. Next, what has been learned so far from lesion-based research is outlined with emphasis on areas of disagreement as well as agreement. What has been learned from functional neuroimaging, particularly emission tomography and functional magnetic resonance imaging, is then discussed, and some stress is placed on topics where the interpretation of imaging studies has so far been unclear. Finally, how functional and structural imaging techniques can be optimally used to help resolve three areas of disagreement in the lesion literature will be discussed. These disagreements concern what the hippocampus and perirhinal cortex contribute to memory; whether any form of priming depends on the medial temporal lobes; and whether remote episodic as well as semantic memories cease to depend on the medial temporal lobes. Although the discussion will show the value of imaging techniques, it will also emphasize some of the limitations of current neuroimaging studies.