Correspondence between functional magnetic resonance imaging somatotopy and individual brain anatomy of the central region: comparison with intraoperative stimulation in patients with brain tumors

OBJECT

The goal of this study was to determine the somatotopical structure-function relationships of the primary motor cortex in individual patients by using functional magnetic resonance (fMR) imaging. This was done to assess whether there is a displacement of functional areas compared with anatomical landmarks in patients harboring brain tumors close to the central region, and to validate these findings with intraoperative cortical stimulation.

METHODS

One hundred twenty hemispheres in 60 patients were studied by obtaining blood oxygen level-dependent fMR images in patients while they performed movements of the foot, hand, and face on both sides. There was a good correspondence between anatomical landmarks in the deep portion of the central sulcus on axial slices and the somatotopical organization of primary motor areas. Pixels activated during hand movements were centered on a small characteristic digitation; those activated during movements in the face and foot areas were located in the lower portion of the central sulcus (lateral to the hand area) and around the termination of the central sulcus, respectively. In diseased hemispheres, signal-intensity changes were still observed in the projection of the expected anatomical area. The fMR imaging data mapped intraoperative electrical stimulation in 92% of positive sites.

CONCLUSIONS

There was a high correspondence between the somatotopical anatomy and function in the central sulcus, which was similar in normal and diseased hemispheres. The fMR imaging and electrical stimulation data were highly concordant. These findings may enable the neurosurgeon to locate primary motor areas more easily during surgery.

[Study of hemispheric lateralization of language regions by functional MRI. Validation with the Wada test]

Language hemispheric dominance can be assessed by the Wada test before temporal lobectomy, in patients with refractory epilepsy of the mesial temporal lobe. Functional MR is a less invasive technique which recently demonstrated the ability to localize some language areas. The aim of this study was to determine if functional MR can be substituted to the Wada test in the determination of language dominance. Seven patients with a refractory epilepsy of the mesial temporal lobe were included. The MR protocol was carried out using a 1.5 T system including 12 axial gradient echo BOLD EPI images. A semantic fluency task, interspersed with rest periods, was used as activation paradigm. An activation of middle and inferior frontal region was found in all patients. It predominated in the left hemisphere in 6 cases and in the right hemisphere in 1 case. The hemispheric and frontal lobe lateralization of the functional MR activation was correlated with the hemispheric language dominance as assessed by the Wada test. In conclusion, functional MR is a promising technique in the determination of language dominance.

Decreased hemispheric water mobility in hemiplegic migraine related to mutation of CACNA1A gene

We report a reversible reduction of water diffusion in the brain during a prolonged attack of hemiplegic migraine. The patient had a sporadic mutation of the CACNA1A gene. The diffusion changes were observed in the contralateral hemisphere 3 and 5 weeks after the onset of hemiplegia. These results suggest the occurrence of hemispheric cytotoxic edema during severe attacks of hemiplegic aura. The mechanisms underlying such ultrastructural modifications are unknown but an abnormal release of excitatory amino acids can be hypothesized.

Human taste cortical areas studied with functional magnetic resonance imaging: evidence of functional lateralization related to handedness

Whole-brain functional magnetic resonance imaging was used to detect local hemodynamic changes reflecting cortical activation in five left handed and five right handed human subjects during bilateral stimulation of the tongue with various tastes. Activation was found bilaterally in the insula and the perisylvian region. These regions correspond to the primary taste cortical areas identified with electrophysiological recordings in monkeys and suggested from former clinical observations in human subjects. Moreover, a unilateral projection was described for the first time in the inferior part of the insula of the dominant hemisphere, according to the subject's handedness.

Clinical severity in CADASIL related to ultrastructural damage in white matter: in vivo study with diffusion tensor MRI

BACKGROUND AND PURPOSE

CADASIL is a newly recognized cause of subcortical ischemic strokes that progressively leads to dementia associated with pseudobulbar palsy and severe motor disability. This deleterious progression and the severity of clinical presentation are widely variable among affected subjects. The exact role played by MRI white-matter abnormalities, a hallmark of the disease, in the severity of the clinical phenotype remains poorly understood.

METHODS

To address this issue, we used diffusion tensor imaging (DTI), a new MRI technique highly sensitive to white-matter microstructural changes, in 16 symptomatic patients and 10 age-matched controls. Mean diffusivity and anisotropy of diffusion were measured within hyperintensities identified on T2-weighted images (T2WI) and outside these lesions on 4 slices at the level of centrum semiovale.

RESULTS

We found a 60% increase of water mean diffusivity and a parallel loss of diffusion anisotropy in hyperintensities identified on T2WI. The same pattern of diffusion changes, but of lesser intensity, was found in the normal-appearing white matter on T2WI. Mean diffusivity in regions with increased signal on T2WI was higher in patients with severe clinical disability compared with those with no or mild deficit (1.33+/-0.11 versus 1.13+/-0.11 10(-3) mm(2)/s, P<0.01). Furthermore, diffusion measured within T2 hyperintensities correlated with both the Mini-Mental State Examination and Rankin scale scores. In patients with a severe clinical status, the increase of water diffusion in these regions exceeded 70% in comparison with values obtained in the normal white matter in control subjects.

CONCLUSIONS

These results indicate that DTI is able to detect important ultrastructural changes in regions with increased signal on T2WI and within the normal-appearing white matter in CADASIL. The diffusion changes might be related to both neuronal loss and demyelination. The degree of the underlying ultrastructural alterations is related to the severity of the clinical status with a possible threshold level of white-matter damage above which severe neurological impairment may occur in this disease. DTI appears to be a promising technique for monitoring disease progression in CADASIL.

[What to expect from MRi in the investigation of the central nervous system?]

Functional Magnetic Resonance Imaging (fMRI) has appeared as a new tool which is very powerful for cognitive neuroscience, offering the potential to look at the dynamics of cerebral processes underlying cognition, noninvasively and on an individual basis. Work remains to be done to optimize the technique and to better understand its basic mechanisms, but one may expect to build in a few years a functional list of the brain cortical networks of regions implicated in sensory-motor or cognitive processes. Still, the real understanding of brain function requires direct access to the functional unit made of the neuron, so that one may look at the transient temporal relationships that exist between largely distributed groups of hundreds or thousands of neurons. Furthermore, communication pathways between networks must be identified to establish connectivity maps at the individual scale, taking into account individual variability resulting from genetic factors and cerebral plasticity. In this respect, MRI of molecular diffusion is very sensitive to water molecular motion and, thus, to tissue dynamic microstructure, such as cell geometry. Preliminary data suggest that diffusion MRI visualizes dynamic tissue changes associated with large neuronal activation and space orientation of large bundles of myelinated axons in the white matter.

Cortical areas activated by bilateral galvanic vestibular stimulation

The brain areas activated by bilateral galvanic vestibular stimulation (GVS) were studied using functional magnetic resonance imaging. In six human volunteers, GVS led to activation in the region of the temporoparietal junction, the central sulcus, and the anterior interior intraparietal sulcus, which may correspond to macaque areas PIVC, 3aV, and 2v, respectively. In addition, activation was found in premotor regions of the frontal lobe, presumably analogous to areas 6pa and 8a in the monkey. Since these areas were not detected in previous studies using caloric vestibular stimulation, they could be related to the modulation of otolith afferent activity by GVS. However, the simple paradigm used did not allow separation of the otolithic and semicircular canal effects of GVS. Further studies must be performed to clarify the question of cortical representation of the otolithic information in the human and monkey brain.

Event-related fMRI analysis of the cerebral circuit for number comparison

Cerebral activity during number comparison was studied with functional magnetic resonance imaging using an event-related design. We identified an extended network of task-related areas that showed a phasic activation following each trial, including anterior cingulate, bilateral sensorimotor areas, inferior occipito-temporal cortices, posterior parietal cortices, inferior and dorsolateral prefrontal cortices, and thalami. We then tested which of these areas were affected by number notation, numerical distance and response side, three variables that specifically target processes of visual identification, quantity manipulation and motor response in a serial-stage model of the number comparison task. Our results confirm the role of the right fusiform gyrus in digit identification processes, and of the inferior parietal lobule in the internal manipulation of numerical quantities.

[Functional MRI: brain plasticity, brain disease and functional recovery]

Brain plasticity may be defined as long-term alteration in behavior related activity of distributed neural systems. Functional imaging, and particularly functional MRI, allows to investigate the mechanisms underlying brain plasticity. Two aspects may be distinguished: one is learning in healthy subjects; the other is functional reorganization following or associated with acute or chronic brain injury. Because functional MRI is totally non invasive, it appears well suited to follow such reorganization over time. This theme will be developed in the following text.

[Practical aspects of realization of a functional MRI]

Functional MRI (fMRI) allows cognitive processes to be studied on an individual basis. fMRI is at the crossing of many fields, such as physiology, MRI physics, cognitive neurosciences, data analysis,.... Hence, it requires the collaboration of inter-disciplinary teams, in which neuroradiologists should play an important role. This article provides an overview of the general principles necessary to conduct fMRI studies on a practical matter (choice of magnetic field, patient set-up and preparation, paradigm design, data acquisition and analysis).

A fronto-parietal system for computing the egocentric spatial frame of reference in humans

Spatial orientation is based on coordinates referring to the subject's body. A fundamental principle is the mid-sagittal plane, which divides the body and space into the left and right sides. Its neural bases were investigated by functional magnetic resonance imaging (fMRI). Seven normal subjects pressed a button when a vertical bar, moving horizontally, crossed the subjective mid-sagittal plane. In the control condition, the subjects' task was to press a button when the direction of the bar movement changed, at the end of each leftward or rightward movement. The task involving the computation of the mid-sagittal plane yielded increased signal in posterior parietal and lateral frontal premotor regions, with a more extensive activation in the right cerebral hemisphere. This direct evidence in normal human subjects that a bilateral, mainly right hemisphere-based, cortical network is active during the computation of the egocentric reference is consistent with neuropsychological studies in patients with unilateral cerebral lesions. Damage to the right hemisphere, more frequently to the posterior-inferior parietal region, may bring about a neglect syndrome of the contralesional, left side of space, including a major rightward displacement of the subjective mid-sagittal plane. The existence of a posterior parietal-lateral premotor frontal network concerned with egocentric spatial reference frames is also in line with neurophysiological studies in the monkey.

fMRI study of taste cortical areas in humans

We used 3 Tesla functional magnetic resonance imaging (fMRI) with echo planar imaging to map taste projections in the human. Activations were found surrounding and buried in the sylvian fissure; the upper part of insula, the frontal operculum, and the foot of the pre- and postcentral gyri were usually activated. Moreover, we could describe, for the first time, a lateralized associative projection located in the lower part of the dominant hemisphere of the subject (n = 10). We also observed activations in the anterior cingulate gyrus, the centromedial thalamus, and other areas related to emotional or cognitive processes. Thirty subjects were submitted to a familiarization experiment sampling neophobic and nonneophobic stimuli. Measurements of isointense concentrations, magnitude estimates and hedonic values were assessed repetitively for 10 weeks. Five subjects performed 3 fMRI experiments, before, during and after familiarization. Psychophysical data showed a relationship between the evolution of the hedonic assessment and the intensity of the perception, and fMRI results showed a relationship between the evolution of the hedonic assessment and the evolution of the percent of activated pixels in taste cortical area.

Functional MRI of galvanic vestibular stimulation

The cortical processing of vestibular information is not hierarchically organized as the processing of signals in the visual and auditory modalities. Anatomic and electrophysiological studies in the monkey revealed the existence of multiple interconnected areas in which vestibular signals converge with visual and/or somatosensory inputs. Although recent functional imaging studies using caloric vestibular stimulation (CVS) suggest that vestibular signals in the human cerebral cortex may be similarly distributed, some areas that apparently form essential constituents of the monkey cortical vestibular system have not yet been identified in humans. Galvanic vestibular stimulation (GVS) has been used for almost 200 years for the exploration of the vestibular system. By contrast with CVS, which mediates its effects mainly via the semicircular canals (SCC), GVS has been shown to act equally on SCC and otolith afferents. Because galvanic stimuli can be controlled precisely, GVS is suited ideally for the investigation of the vestibular cortex by means of functional imaging techniques. We studied the brain areas activated by sinusoidal GVS using functional magnetic resonance imaging (fMRI). An adapted set-up including LC filters tuned for resonance at the Larmor frequency protected the volunteers against burns through radio-frequency pickup by the stimulation electrodes. Control experiments ensured that potentially harmful effects or degradation of the functional images did not occur. Six male, right-handed volunteers participated in the study. In all of them, GVS induced clear perceptions of body movement and moderate cutaneous sensations at the electrode sites. Comparison with anatomic data on the primate cortical vestibular system and with imaging studies using somatosensory stimulation indicated that most activation foci could be related to the vestibular component of the stimulus. Activation appeared in the region of the temporo-parietal junction, the central sulcus, and the intraparietal sulcus. These areas may be analogous to areas PIVC, 3aV, and 2v, respectively, which form in the monkey brain, the "inner vestibular circle". Activation also occurred in premotor regions of the frontal lobe. Although undetected in previous imaging-studies using CVS, involvement of these areas could be predicted from anatomic data showing projections from the anterior ventral part of area 6 to the inner vestibular circle and the vestibular nuclei. Using a simple paradigm, we showed that GVS can be implemented safely in the fMRI environment. Manipulating stimulus waveforms and thus the GVS-induced subjective vestibular sensations in future imaging studies may yield further insights into the cortical processing of vestibular signals.

Imaging unconscious semantic priming

Visual words that are masked and presented so briefly that they cannot be seen may nevertheless facilitate the subsequent processing of related words, a phenomenon called masked priming. It has been debated whether masked primes can activate cognitive processes without gaining access to consciousness. Here we use a combination of behavioural and brain-imaging techniques to estimate the depth of processing of masked numerical primes. Our results indicate that masked stimuli have a measurable influence on electrical and haemodynamic measures of brain activity. When subjects engage in an overt semantic comparison task with a clearly visible target numeral, measures of covert motor activity indicate that they also unconsciously apply the task instructions to an unseen masked numeral. A stream of perceptual, semantic and motor processes can therefore occur without awareness.

Somatotopical organization of striatal activation during finger and toe movement: a 3-T functional magnetic resonance imaging study

The present study aimed at determining the distribution and somatotopical organization of striatal activation during performance of simple motor tasks. Ten right-handed healthy volunteers were studied by using a 3-T whole-body magnetic resonance unit and echo planar imaging. The tasks consisted of self-paced flexion/extension of the right fingers or toes. Motor activation was found mainly in the putamen posterior to the anterior commissure (10 of 10 subjects) and the globus pallidus (6 subjects), whereas the caudate nucleus was activated in only 3 subjects, and in a smaller area. Thus, performance of a simple motor task activated the sensorimotor territory of the basal ganglia. Within the putamen, there was a somatotopical organization of the foot and hand areas similar to that observed in nonhuman primates. These data suggest that functional magnetic resonance imaging can be used to study normal function of the basal ganglia and should therefore also allow investigation of patients with movement disorders.

Anatomical variability in the cortical representation of first and second language

Functional magnetic resonance imaging was used to assess inter-subject variability in the cortical representation of language comprehension processes. Moderately fluent French-English bilinguals were scanned while they listened to stories in their first language (L1 = French) or in a second language (L2 = English) acquired at school after the age of seven. In all subjects, listening to L1 always activated a similar set of areas in the left temporal lobe, clustered along the left superior temporal sulcus. Listening to L2, however, activated a highly variable network of left and right temporal and frontal areas, sometimes restricted only to right-hemispheric regions. These results support the hypothesis that first language acquisition relies on a dedicated left-hemispheric cerebral network, while late second language acquisition is not necessarily associated with a reproducible biological substrate. The postulated contribution of the right hemisphere to L2 comprehension is found to hold only on average, individual subjects varying from complete right lateralization to standard left lateralization for L2.