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PubMed2005

Attentional integration between anatomically distinct stimulus representations in early visual cortex.

Vision often requires attending to, and integrating information from, distant parts of the visual field. However, the neural basis for such long-range integration is not clearly understood. Here, we demonstrate a specific neural signature of attentional integration between stimuli in different parts of the visual field. Using functional MRI, we found that a task requiring the integration of information between two attended but spatially separated stimuli actively modulated the degree of functional integration (in terms of effective connectivity) between their retinotopic representations in visual cortical areas V1, V2, and V4. Spatial attention enhanced long-distance coupling between distinct neuronal populations that represented the attended visual stimuli, even at the earliest stages of cortical processing. In contrast, unattended stimulus representations were decoupled both from attended representations and particularly strongly from each other. Furthermore, enhanced functional integration between cortical representations was associated with enhanced behavioral performance. Attention may thus serve to "bind" together cortical loci at multiple levels of the visual hierarchy that are commonly involved in processing attended stimuli, promoting integration between otherwise functionally isolated cortical loci.

Adult↗

PubMed2006

Functional connectivity of the fusiform gyrus during a face-matching task in subjects with mild cognitive impairment.

Cognitive function requires a high level of functional interaction between regions of a network supporting cognition. Assuming that brain activation changes denote an advanced state of disease progression, changes in functional connectivity may precede changes in brain activation. The objective of this study was to investigate changes in functional connectivity of the right middle fusiform gyrus (FG) in subjects with mild cognitive impairment (MCI) during performance of a face-matching task. The right middle FG is a key area for processing face stimuli. Brain activity was measured using functional MRI. There were 16 MCI subjects and 19 age-matched healthy controls. The linear correlation coefficient was utilized as a measure of functional connectivity between the right middle FG and all other voxels in the brain. There were no statistical differences found in task performance or activation between groups. The right middle FG of the healthy control and MCI groups showed strong bilateral positive linear correlation with the visual cortex, inferior and superior parietal lobules, dorsolateral prefrontal cortex (DLPFC) and anterior cingulate. The healthy controls showed higher positive linear correlation of the right middle FG to the visual cortex, parietal lobes and right DLPFC than the MCI group, whereas the latter had higher positive linear correlation in the left cuneus. In the healthy controls, the right middle FG had negative linear correlation with right medial frontal gyrus and superior temporal gyrus and with left inferior parietal lobule (IPL), angular gyrus, superior frontal gyrus and anterior cingulate gyrus, but the MCI group had negative linear correlation with the left IPL, angular gyrus, precuneus, anterior cingulate, and to right middle temporal gyrus and posterior cingulate gyrus. In the negatively linearly correlated regions, the MCI group had reduced functional connectivity to the frontal areas, right superior temporal gyrus and left IPL. Different regions of the cuneus and IPL had increased functional connectivity in either group. The putative presence of Alzheimer's disease neuropathology in MCI affects functional connectivity from the right middle FG to the visual areas and medial frontal areas. In addition, higher linear correlation in the MCI group in the parietal lobe may indicate the initial appearance of compensatory processes. The results demonstrate that functional connectivity can be an effective marker for the detection of changes in brain function in MCI subjects.

Aged↗

PubMed1998

Directions of aetiologic research on attention deficit hyperactivity disorder.

OBJECTIVE: The aim of this paper is to review and integrate recent literature on aetiological factors that have been postulated for attention deficit hyperactivity disorder (ADHD). METHOD: Recent studies relating to perinatal brain damage, intra-uterine toxic effects, neurochemical, brain imaging and genetic studies are reviewed, and those considered most significant are discussed. Where possible, recent findings are integrated and directions of future research are suggested. Clinical implications are briefly discussed. RESULTS: Perinatal studies indicate that children with a birth weight under 750 g may be disadvantaged for attentional skills. Magnetic resonance imaging (MRI) and steady state visually evoked potential studies show differences in prefrontal, caudate and parietal areas in ADHD children, suggesting right hemispheric dysfunction. Functional MRI studies hold promise in further elucidating attentional systems in the central nervous system that are involved in ADHD. Genetic studies suggest genes related to dopaminergic systems may be important. CONCLUSIONS: Recent research on ADHD has made considerable advances, particularly in the areas of brain imaging and genetic studies. Genetic studies should provide further aetiological understandings of ADHD, leading to more targeted treatments.

Attention Deficit Disorder with Hyperactivity↗

PubMed1994

[Diagnosis, analysis and evaluation of functional disorders of the upper cervical spine within the scope of whiplash injuries with nuclear magnetic resonance tomography].

We tested the diagnosis of malfunctions in the upper cervical spine after whiplash injury using MRI. In the spinal segments C 0-1 and C 1-2 MRI functional diagnosis showed clear advantages compared to X-ray diagnosis. Apart from data about the normal course of movement in these joints it was possible to classify and reproduce malfunctions exactly. The malfunctions diagnosed can be found in normal volunteers as well, so that in an evaluation no causal link between the very existence of a malfunction and the complaints can be established.

Adult↗

PubMed2005

Brain motor system function after chronic, complete spinal cord injury.

Most therapies under development to restore motor function after spinal cord injury (SCI) assume intact brain motor functions. To examine this assumption, 12 patients with chronic, complete SCI and 12 controls underwent functional MRI during attempted, and during imagined, right foot movement, each at two force levels. In patients with SCI, many features of normal motor system function were preserved, however, several departures from normal were apparent: (i) volume of activation was generally much reduced, e.g. 4-8% of normal in primary sensorimotor cortex, in the setting of twice normal variance in signal change; (ii) abnormal activation patterns were present, e.g. increased pallido-thalamocortical loop activity during attempted movement and abnormal processing in primary sensorimotor cortex during imagined movement; and (iii) modulation of function with change in task or in force level did not conform to patterns seen in controls, e.g. in controls, attempted movement activated more than imagined movement did within left primary sensorimotor cortex and right dorsal cerebellum, while imagined movement activated more than attempted movement did in dorsolateral prefrontal cortex and right precentral gyrus. These modulations were absent in patients with SCI. Many features of brain motor system function during foot movement persist after chronic complete SCI. However, substantial derangements of brain activation, poor modulation of function with change in task demands and emergence of pathological brain events were present in patients. Because brain function is central to voluntary movement, interventions that aim to improve motor function after chronic SCI likely also need to attend to these abnormalities of brain function.

Adult↗

PubMed2000

Intraoperative magnetic resonance imaging.

Intraoperative magnetic resonance imaging (MRI) allows neurosurgeons to perform surgery interactively using magnetic resonance (MR) guidance. Low-field and high-field strength MRI has been developed and implemented for multiple neurosurgical procedures, including brain biopsies, craniotomies for resection of mass lesions, cyst drainages, laminectomies, thermal ablations, functional neurosurgery, and a variety of miscellaneous cases. Both technologies have the advantage over frameless neuronavigational systems of being able to perform near real-time imaging, which allows the surgeon to compensate for intraoperative brain shift. Intraoperative functional techniques such as MR spectroscopy, functional MRI, MR angiography and venography, and diffusion-weighted imaging, which have become routine at some high-field MR units, can significantly influence surgical decision making. The potential complications associated with intraoperative MR-guided neurosurgery are similar in incidence to those seen in the conventional neurosurgical operating room. However, the immediate recognition of such intraoperative complications with MRI should lead to improved outcomes and decreased medical costs. Untoward events associated with performing surgery in an MR environment are uncommon. Intraoperative MR-guided neurosurgery represents a natural progression from framed and frameless stereotactic techniques. Intraoperative MRI is still in its infancy, and the full capabilities of this technology have yet to be determined or implemented.

Humans↗

PubMed1998

New multimeric magnetic resonance imaging agents.

RATIONALE AND OBJECTIVES: The authors investigated the effect of multimerization on the relaxivity of macrocyclic gadolinium (Gd) chelates. The objective was to develop more sensitive magnetic resonance imaging (MRI) contrast agents to study biochemical processes. METHODS: Covalently linked nonionic, macrocyclic, multimeric lanthanide chelates that belong to the classes of dimers, trimers, tetramers, hexamer, and octamer, in the molecular weight range approximately 1 to 5 KDa, were synthesized. The chemical linkage was based on either the amide bond or the 2-hydroxypropylidene bond. Relaxivity values, 20r1, on Gd3+ chelates and hydration numbers, Q, on Tb3+ chelates were determined. RESULTS: Relaxivity values increased with molecular weight and Q values were not affected, the increase in r1 in attributable to the expected increase in the overall rotational correlation time, tau r with an increase in molecular weight. The rigidity of the linkers, which is expected to affect the intrachelate rotational correlation time tau r* that makes a contribution to the overall correlation time, tau r, exerted a noticeable effect. The hydroxyl-based chelates generally had lower r1 values than the amide-based chelates. This is rationalized as arising from the longer and thereby rate-limiting effect of the tau m value for the hydroxyl chelates compared with that reported of the amide-based chelates. This rate limiting effect of tau m becomes a dominant factor controlling attainable enhanced relaxivity when multimers based on traditional chelate designs are used for MRI applications. CONCLUSIONS: Approaches aimed at enhancing relaxivity by modulating the water relaxation time, tau m, will be important for the future development of functional MRI contrast agents for the imaging of biochemical processes.

Contrast Media↗

PubMed1999

[New methods for evaluation of memory in temporal lobe epilepsy: a functional imaging approach].

Temporal lobe epilepsy (TLE) is usually associated with a specific impairment of memory functions. A reliable assessment of memory function seems therefore a crucial element of the presurgical evaluation for surgery epilepsy, especially for temporal lobectomy. To date, intracarotid amobarbital testing remains the gold standard to evaluate memory before planning surgery in patients with refractory TLE. Functional imaging, either positron emission tomography or functional MRI, holds great promise as a powerful tool in memory evaluation. Preliminary studies, essentially in healthy volunteers, have shown promising results. In the future, functional imaging, combined with other techniques, will certainly contribute to the clinical presurgical evaluation of patients with epilepsy.

Epilepsy, Temporal Lobe↗

PubMed2003

[Rehabilitation after stroke. Imaging techniques show how the cortical reorganization is affected by training].

It is widely accepted that reorganisation of the brain occurs after a focal brain lesion such as stroke. Neuroimaging methods are used to study such reorganisation in vivo. Improvements in arm and hand motor function during recovery post-stroke have been related to reorganisation in primary and secondary sensorimotor areas by indirect measure of synaptic activity with functional MRI. Reorganisation occurs in both the affected and the unaffected hemispheres. Preliminary training studies post-stroke have shown correlations between improvements in motor function and brain activity changes. Recent research findings are reviewed herein. Further understanding of the neurobiological mechanisms of post-stroke recovery will lead to development of optimal treatment strategies during rehabilitation of stroke survivors.

Brain↗

PubMed2004

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.

Brain↗

PubMed1995

[The objective and perspective of recording electrical activity from the central nervous system].

In 1929 Hans Bergr successfully recorded electrical activity from the human brain using surface scalp electrodes. Since, the EEG has brought significant contributions to the fields of clinical neurology and neurophysiology. As a clinical diagnostic tool, the EEG has provided information for functional as well anatomical (structural) brain disturbances. As a functional diagnosis, the EEG is far superior to other functional tests such as PET, SPECT or functional MRI because of its excellent temporal resolution representing moment to moment changes in the level of consciousness. However, the progress has been hampered due to difficulty in quantifying EEG data because of its extreme dynamics and variability, which perhaps reflects yet unknown brain functions. This difficulty will be overcome by improved quantification methods and statistical measures using various computer applications including topographic mapping, power spectrum analysis, covariance, correlation or coherence function. Because of its poor spacial resolution, the EEG has relinquished its place to CT and MRI scans as an anatomical diagnostic tool. The surface recorded electrical activity is greatly attenuated and distorted by the time it reaches to the scalp. This is due to the inhomogeneous tissues (CSF, dura, skull, scalp) that intervene between the recording electrodes and cortical surface. Because the human body or head consists of electroconductive media of various geometries, current distortion, far-field potentials or paradoxical lateralizations further complicate the interpretation of EEG in postulating the location of anatomical generator sources from surface recorded EEG. Advanced computer technology is in progress to quantify the EEG data by taking into account the factors of conductivity, geometry shape and individual head size. This will improve the accuracy of localizing current generator sources in relationship to brain anatomy. By solving these problems in a methodical fashion, EEG will become the revelation for understanding the functional anatomy of brain and will step closer to Berger's dream, "EEG is a window of human mental activity".

Action Potentials↗

PubMed2004

[Imaging of dementia with magnetic resonance].

The diagnostic approach of dementia has clearly improved with the progress in medical imaging, notably magnetic resonance imaging. The conventional T1 and T2 sequences or morphological imaging have demonstrated their interest in the positive and differential diagnosis of dementia, together with the more precise description of normal ageing of the brain. The ANAES (French medicines agency) proposes systematic brain imaging, notably by magnetic resonance imaging (MRI) in their practical guidelines for the diagnosis of Alzheimer's disease (http://www.anaes.fr). THE INTEREST OF CERTAIN IMAGING TECHNIQUES: The therapeutic progress in treatment of dementia implies that the different affections be recognised as early as possible. With this in mind, the functional MRI is capable of describing the damage in cases when morphological imaging is not sufficiently specific. Recent studies have reported the interest of diffusion and perfusion imaging, activation MRI and proton spectroscopy. FROM A THERAPEUTIC POINT OF VIEW: The combination of morphological and functional approaches will provide a better definition of the groups at risk in order to target current treatments and those to come.

Aged↗

PubMed2003

Using patient-specific hemodynamic response functions in combined EEG-fMRI studies in epilepsy.

Combining electroencephalogram (EEG) and functional MRI (fMRI) allows localization of brain regions activated as a result of epileptic spikes. The statistical analysis of fMRI data usually includes a standard model of the hemodynamic response function (HRF) but it is not known how well this fits the actual HRF of epileptic spikes. The objective of this exploratory study was to compare the activated areas and t-statistical scores obtained with a standard HRF to those obtained with a patient-specific HRF. Eight patients with focal epilepsy were studied. We obtained an estimate of the patient-specific HRFs for each patient at the local maximum of activation in the standard HRF analysis. The activated areas obtained with the patient-specific HRFs were larger or similar to the originally activated areas. Additional activated areas were seen in five patients, and most were compatible with the EEG and anatomical MRI localization of epileptogenic and lesional regions. Using patient-specific HRFs brings increased sensitivity to the analysis of epileptic spikes by EEG-fMRI.

Cerebrovascular Circulation↗

PubMed2006

Task-modulation of functional synchrony between spontaneous low-frequency oscillations in the human brain detected by fMRI.

Recent developments in functional MRI (fMRI) technology with high spatial and temporal resolution have made it possible to noninvasively detect spontaneous low-frequency oscillations (SLOs) and quantify their functional synchrony in the human brain. In the present fMRI study the dynamic characteristics of the functional synchrony between SLOs were quantitatively determined by the phase shift index (PSI). With the use of an fMRI-guided voxel-selection method, the SLOs and their functional synchrony were found to be modulated by different memory tasks. The results demonstrate that SLOs in episodic memory-related circuitry have significantly higher synchrony during the performance of declarative memory-encoding activities compared to nondeclarative memory-encoding activities. It is suggested that the dynamic property of SLOs and the quantitative assessment of their functional synchrony could be utilized as a biomarker to noninvasively characterize localized pathophysiological functions in the human brain.

Adult↗

PubMed2006

Surgical treatment of polymicrogyria with advanced radiologic and neurophysiologic techniques.

INTRODUCTION: Coexistence of multiple seizure types, inclusion of the motor cortex in the epileptogenic zone, and poor delimitation of the abnormal cortex make most patients with polymicrogyria (PMG) unlikely candidates for epilepsy surgery (Guerrini R et al., Epilepsy and malformations of the cerebral cortex in Epileptic syndromes in infancy, childhood and adolescence, 2005). CASE REPORT: We present here a child with frontal PMG and intractable epilepsy evaluated with advanced magnetic resonance imaging (MRI) and neurophysiologic techniques. Diffusion tensor imaging and fiber tractography showed severe involvement of neighboring white matter tracts besides the cortex. The evaluation also included functional MRI, chronic subdural electroencephalogram monitoring, and intra-operative motor mapping. The patient had a decrease in seizure frequency and an increase in his developmental skills after the surgery. CONCLUSION: Advanced neuroradiologic and neurophysiologic techniques are required to provide an effective and safe resection of the epileptogenic cortex in cortical dysplasias.

Brain Diseases↗

PubMed2005

Reductions in interhemispheric motor cortex functional connectivity after muscle fatigue.

Muscle fatigue has been known to differentially affect the activation level of the primary motor cortices (MIs) of the brain's two hemispheres. Whether this fatigue-related decoupling influence on the motor cortical signals extends beyond the motor action to the after-fatigue-task resting state is unknown. This question can be addressed by analyzing functional connectivity (FC) of low-frequency oscillations of resting-state functional MRI (fMRI) signals of the MIs. Low-frequency oscillations (<0.08 Hz) have been detected in many fMRI studies and appear to be synchronized between functionally related areas. These patterns of FC have been shown to differ between normal and various pathological states. The purpose of this study was to examine muscle fatigue-induced resting-state interhemispheric motor cortex FC changes in healthy subjects. We hypothesized that muscle fatigue would create a temporary "disrupted state" in the brain, and would decrease resting state interhemispheric motor cortical FC. Ten healthy subjects performed repetitive unilateral handgrip contractions that induced significant muscle fatigue, with resting state fMRI data collected before and after the task. After excluding two subjects due to gross head motion, interhemispheric motor cortex FC was assessed by cross-correlating the MI fMRI signal time courses. We found that the number of significant interhemispheric correlations in the MI fMRI signals decreased significantly after the performance of the fatigue task. These results suggest that resting state interhemispheric motor cortex FC may be used as an index of recovery from fatigue.

Adult↗

PubMed2004

Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain.

This paper presents a computationally efficient source estimation algorithm that localizes cortical oscillations and their phase relationships. The proposed method employs wavelet-transformed magnetoencephalography (MEG) data and uses anatomical MRI to constrain the current locations to the cortical mantle. In addition, the locations of the sources can be further confined with the help of functional MRI (fMRI) data. As a result, we obtain spatiotemporal maps of spectral power and phase relationships. As an example, we show how the phase locking value (PLV), that is, the trial-by-trial phase relationship between the stimulus and response, can be imaged on the cortex. We apply the method to spontaneous, evoked, and driven cortical oscillations measured with MEG. We test the method of combining MEG, structural MRI, and fMRI using simulated cortical oscillations along Heschl's gyrus (HG). We also analyze sustained auditory gamma-band neuromagnetic fields from MEG and fMRI measurements. Our results show that combining the MEG recording with fMRI improves source localization for the non-noise-normalized wavelet power. In contrast, noise-normalized spectral power or PLV localization may not benefit from the fMRI constraint. We show that if the thresholds are not properly chosen, noise-normalized spectral power or PLV estimates may contain false (phantom) sources, independent of the inclusion of the fMRI prior information. The proposed algorithm can be used for evoked MEG/EEG and block-designed or event-related fMRI paradigms, or for spontaneous MEG data sets. Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain can provide further understanding of large-scale neural activity and communication between different brain regions.

Acoustic Stimulation↗

PubMed2006

Beyond a relay nucleus: neuroimaging views on the human LGN.

The lateral geniculate nucleus (LGN) is the thalamic station in the retinocortical projection and has traditionally been viewed as the gateway for sensory information to enter the cortex. Here, we review recent studies of the human LGN that have investigated the retinotopic organization, physiologic response properties, and modulation of neural activity by selective attention and by visual awareness in a binocular rivalry paradigm. In the retinotopy studies, we found that the contralateral visual field was represented with the lower field in the medial-superior portion and the upper field in the lateral-inferior portion of each LGN. The fovea was represented in posterior and superior portions, with increasing eccentricities represented more anteriorly. Functional MRI responses increased monotonically with stimulus contrast in the LGN and in visual cortical areas. In the LGN, the dynamic response range of the contrast function was larger and contrast gain was lower than in the cortex. In our attention studies, we found that directed attention to a spatial location modulated neural activity in the LGN in several ways: it enhanced neural responses to attended stimuli, attenuated responses to ignored stimuli, and increased baseline activity in the absence of visual stimulation. Furthermore, we showed in a binocular rivalry paradigm that neural activity in the LGN correlated strongly with the subjects' reported percepts. The overall view that emerges from these studies is that the human LGN plays a role in perception and cognition far beyond that of a relay nucleus and, rather, needs to be considered as an early gatekeeper in the control of visual attention and awareness.

Attention↗