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Results for “functional MRI”

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At least 685 records · Page 38Linked to original sources
PubMed1998

Proprioception acts as the main source of input in human S-I activation experiments: a functional MRI study.

During tactile exploration cells in human somatosensory cortex S-I receive input from skin receptors and from proprioceptive feedback. To study the extent to which these sources contribute to cell activation we used functional magnetic resonance imaging (fMRI) in order to visualize the spatial extent and amplitude of activation in S-I during active finger movement and passive stimulation of finger tips. In all subjects (n = 6) we measured activation elicited by unilateral single finger tapping (active task) and mechanical stimulation of the palm of the index finger (passive task). In the finger tapping condition all subjects showed a strict contralateral activation of somatosensory cortex S-I and motor cortex M-I. In the passive stimulation experiment we found activation of the contralateral somatosensory cortex S-I only. Although subjects were trained to perform the finger movement with the same frequency and pressure in comparison to the passive stimulation, the activation within S-I induced by finger movements was always significantly larger than that induced by passive stimulation. This result implies that activation of somatosensory cortex originates to a large extent from proprioception while tactile input plays a minor role in S-I excitation.

Adult↗
PubMed1997

Frequency-dependent changes of regional cerebral blood flow during finger movements: functional MRI compared to PET.

To evaluate the effect of the repetition rate of a simple movement on the magnitude of neuronal recruitment in the primary sensorimotor cortex, we used a blood flow-sensitive, echo planar functional magnetic resonance imaging (fMRI) sequence in six normal volunteers. Three of the volunteers also had [15O]water positron emission tomography (PET) studies using the same paradigm. Previous PET studies had shown an increase in regional CBF (rCBF) with movement frequencies up to 2 Hz and then a plateau of regional cerebral blood flow (rCBF) at faster frequencies. To evaluate the extent of the activation, the correlation coefficient (cc) of the Fourier-transformed time-signal intensity change with the Fourier-transformed reference function was calculated pixel by pixel. The degree of activation was measured as the signal percent change of each region of interest with a cc > 0.5. The left primary sensorimotor cortex was constantly activated at 1, 1.5, 2, and 4 Hz, while there was only inconsistent activation at 0.25 and 0.5 Hz. Percent change in signal intensity linearly increased from 1 to 4 Hz. Area of activation increased up to 2 Hz and showed a tendency to decrease at higher frequencies. Individual analysis of PET data showed activation in the same location as that revealed by fMRI. The combination of progressively increasing signal intensity with an area that increases to 2 Hz and declines at faster frequencies explains the PET finding of plateau of rCBF at the faster frequencies. Functional magnetic resonance imaging shows similar results to PET, but is better able to dissociate area and magnitude of change.

Adult↗
PubMed2003

Characterization of the CNS effects of naftidrofuryl (Praxilène) by quantitative EEG and functional MRI: a study in healthy elderly subjects.

In the present study, we investigated the effects of a single and a repeated (5 days) administration of naftidrofuryl, a serotonin 5-HT2 receptor inhibitor having neuroprotective properties, on functional brain physiology in male healthy elderly subjects, using quantitative electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Twelve subjects aged 60 +/- 3.8 years completed the quantitative EEG study, where the effects of 400 and 600 mg were assessed, and 12 other subjects (aged 56 +/- 4.7 years) completed the fMRI study, where the effect of 400 mg was assessed on the brain activation induced by the continuous performance test (CPT). Naftidrofuryl induced a transient reduction in alpha activity followed by a specific synchronisation of the 9.5- to 11-Hz EEG activity most pronounced after repeated administration. Such regimen also increased the CPT-induced brain activation visualized by way of fMRI. The results of the present study can be interpreted at the functional level that naftidrofuryl induced an improved level of vigilance or an increased capacity of alertness in healthy elderly subjects.

Brain↗
PubMed1997

Combined utility of functional MRI, cortical mapping, and frameless stereotaxy in the resection of lesions in eloquent areas of brain in children.

We studied 16 children with lesions in the eloquent brain to determine if the amalgamation of information from functional magnetic resonance imaging (fMRI), frameless stereotaxy, and direct cortical mapping and recording could facilitate the excision of these lesions while minimizing potential neurological deficits. The mean age of the children was 10 years. Fourteen children presented with seizures. All lesions were located in or near eloquent cerebral cortex. fMRI was successful in all patients in delineating the relationship between the lesion and regions of task-activated cortex. The ISG wand was utilized in all cases for scalp and bone flap placement, and for intraoperative localization of the lesion. Direct cortical stimulation or recording of phase reversals with somatosensory evoked potentials helped delineate the central sulcus and language cortex in patients with lesions near the motor or language cortex. Intraoperative electrocorticography (ECoG) was utilized in all patients who presented with seizures to guide the extent of resection of the epileptiform cortex. Ten children had benign cerebral neoplasms, nine of which were totally resected. The other diagnoses included vascular malformations, Sturge-Weber, tuberous sclerosis, Rasmussen's encephalitis, and primitive neuroectodermal tumor. Only 1 patient with a left Rolandic AVM developed a new neurological deficit postoperatively. Thirteen of fourteen patients who presented with seizure disorders were rendered either seizure free or improved in terms of seizure control postoperatively. Follow-up has ranged from 12 to 18 months, with a mean follow-up of 15 months. We conclude that the techniques of fMRI, frameless stereotaxy, direct cortical stimulation and recording can be utilized in sequence to accurately localize intracerebral lesions in eloquent brain, and to reduce the morbidity of resecting these lesions in children.

Adolescent↗
PubMed2003

Serial MRI, functional recovery, and long-term infarct maturation in a non-human primate model of stroke.

We have examined the effects of permanent middle cerebral artery occlusion (pMCAO) in marmoset monkeys over 5 months, using behavioural and magnetic resonance imaging (MRI) techniques. Three marmosets were trained on behavioural tests before pMCAO. Shortly after surgery, these marmosets were scanned with T2-weighted (T2W) and diffusion-weighted (DW) MRI. Three, 10 and 20 weeks after surgery, these marmosets were re-tested on the behavioural tasks and had further MRI sessions to monitor lesion development. This was followed by histological analysis. All these marmosets had a persistent contralesional motor deficit and a spatial neglect which resolved over the 20 weeks of testing. Percentage infarct volume assessed by MRI on the day of surgery and at 20 weeks matched the percentage infarct volume measured histologically at 20 weeks. However, the apparent infarct size at 3 weeks was considerably less than that measured by histological analysis or that measured at the other MRI time points. Additional histological analysis of the brains of two further marmosets removed 3 weeks after pMCAO found considerable infiltration by lipid filled macrophages into the ischaemic zone which may have caused an MRI "fogging" effect leading to an apparent reduction in infarct volume.

Animals↗
PubMed1999

Event-related potentials and functional MRI: a comparison of localization in sensory, perceptual and cognitive tasks.

Combining ERP and fMRI methods to elucidate the time course and anatomical basis of information processing may provide a powerful new tool for understanding human brain function. Attempts to model the time course of fMRI activations by recording surface electromagnetic fields require a better understanding of the relationship between ERPs and fMRI activations. The results presented here show that good correspondence can be obtained between the location of ERP generators and fMRI activations in sensorimotor cortex, and in face perception and target detection tasks. However, difficulties in obtaining somatosensory fMRI activations with stimuli known to evoke robust SEPs, and the lack of fMRI activations within the hippocampus in tasks that elicit large hippocampal field potentials suggest that not all stimuli or tasks that evoke focal ERPs will evoke fMRI activations. This may be related to differences in the sensitivity of the fMRI that will be overcome with better technique and with more sensitive instruments. However, caution must be exercised in developing models that assume a one-to-one correspondence between ERP generators and fMRI activations.

Cognition↗
PubMed2000

Exaggerated amygdala response to masked facial stimuli in posttraumatic stress disorder: a functional MRI study.

BACKGROUND: Converging lines of evidence have implicated the amygdala in the pathophysiology of posttraumatic stress disorder (PTSD). We previously developed a method for measuring automatic amygdala responses to general threat-related stimuli; in conjunction with functional magnetic resonance imaging, we used a passive viewing task involving masked presentations of human facial stimuli. METHODS: We applied this method to study veterans with PTSD and a comparison cohort of combat-exposed veterans without PTSD. RESULTS: The findings indicate that patients with PTSD exhibit exaggerated amygdala responses to masked-fearful versus masked-happy faces. CONCLUSIONS: Although some previous neuroimaging studies of PTSD have demonstrated amygdala recruitment in response to reminders of traumatic events, this represents the first evidence for exaggerated amygdala responses to general negative stimuli in PTSD. Furthermore, by using a probe that emphasizes automaticity, we provide initial evidence of amygdala hyperresponsivity dissociated from the "top-down" influences of medial frontal cortex.

Amygdala↗
PubMed2002

Brain potential and functional MRI evidence for how to handle two languages with one brain.

Bilingual individuals need effective mechanisms to prevent interference from one language while processing material in the other. Here we show, using event-related brain potentials and functional magnetic resonance imaging (fMRI), that words from the non-target language are rejected at an early stage before semantic analysis in bilinguals. Bilingual Spanish/Catalan and monolingual Spanish subjects were instructed to press a button when presented with words in one language, while ignoring words in the other language and pseudowords. The brain potentials of bilingual subjects in response to words of the non-target language were not sensitive to word frequency, indicating that the meaning of non-target words was not accessed in bilinguals. The fMRI activation patterns of bilinguals included a number of areas previously implicated in phonological and pseudoword processing, suggesting that bilinguals use an indirect phonological access route to the lexicon of the target language to avoid interference.

Adult↗
PubMed2006

Feasibility of velocity selective arterial spin labeling in functional MRI.

Arterial spin labeling (ASL) magnetically inverts or saturates the spins in arterial blood and uses them as endogenous tracers. Conventionally, the tagging band is upstream or nonselective to the target slices. In the brain, ASL-based functional magnetic resonance imaging (fMRI) has been shown to detect activation better localized in gray matter than blood oxygenation level dependent contrast. More recently, velocity selective-ASL (VS-ASL) was proposed to tag spins according to their flow velocity. One desirable characteristic of VS-ASL is its capability to generate tags sufficiently close to the target slices and thereby circumvent the complication of non-zero transit delay. In this study, we investigate the feasibility of VS-ASL in fMRI by comparing it with a conventional ASL method (PICORE). The results from the visual cortex of healthy volunteers show that VS-ASL and PICORE have comparable spatial specificity in detecting the flow change induced by neuronal activity. Velocity selective-arterial spin labeling can further distinguish the contribution from different flow directions but spurious elevation of fractional signal change may occur when the VS tagging is applied off the direction of blood supply. The flow reaches the vicinity of perfusion at a cutoff velocity (Vc) of 2 cm/sec whereas the activation exclusively detected by Vc=4 cm/sec implies the arteriolar response to the neuronal activity and a respondent vessel diameter up to 240 microm. Velocity selective imaging can remove intravascular signal from the vessels where the flow velocity is above Vc.

Adult↗
PubMed2001

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

Visual episodic memory encoding was investigated using echoplanar magnetic resonance imaging at 2.0 x 2.0 mm2 resolution and 1.0 mm section thickness, which allows for functional mapping of hippocampal, parahippocampal, and ventral occipital regions with reduced magnetic susceptibility artifact. The memory task was based on 54 image pairs each consisting of a complex visual scene and the face of one of six different photographers. A second group of subjects viewed the same set of images without memory instruction as well as a reversing checkerboard. Apart from visual activation in occipital cortical areas, episodic memory encoding revealed consistent activation in the parahippocampal gyrus but not in the hippocampus proper. This finding was most prominently evidenced in sagittal maps covering the right hippocampal formation. Mean activated volumes were 432 +/- 293 microl and 259 +/- 179 microl for intentional memory encoding and non-instructed viewing, respectively. In contrast, the checkerboard paradigm elicited pure visual activation without parahippocampal involvement.

Adult↗
PubMed2005

Functional MRI BOLD response to Tower of London performance of first-episode schizophrenia patients using cortical pattern matching.

Due to its three-dimensional folding pattern, the human neocortex poses a challenge for accurate co-registration of grouped functional brain imaging data. The present study addressed this problem by employing three-dimensional continuum-mechanical image-warping techniques to derive average anatomical representations for co-registration of functional magnetic resonance brain imaging data obtained from 10 male first-episode schizophrenia patients and 10 age-matched male healthy volunteers while they performed a version of the Tower of London task. This novel technique produced an equivalent representation of blood oxygenation level dependent (BOLD) response across hemispheres, cortical regions, and groups, respectively, when compared to intensity average co-registration, using a deformable Brodmann area atlas as anatomical reference. Somewhat closer association of Brodmann area boundaries with primary visual and auditory areas was evident using the gyral pattern average model. Statistically-thresholded BOLD cluster data confirmed predominantly bilateral prefrontal and parietal, right frontal and dorsolateral prefrontal, and left occipital activation in healthy subjects, while patients' hemispheric dominance pattern was diminished or reversed, particularly decreasing cortical BOLD response with increasing task difficulty in the right superior temporal gyrus. Reduced regional gray matter thickness correlated with reduced left-hemispheric prefrontal/frontal and bilateral parietal BOLD activation in patients. This is the first study demonstrating that reduction of regional gray matter in first-episode schizophrenia patients is associated with impaired brain function when performing the Tower of London task, and supports previous findings of impaired executive attention and working memory in schizophrenia.

Adolescent↗
PubMed2005

A functional MRI study of automatic movements in patients with Parkinson's disease.

Patients with Parkinson's disease have great difficulty performing learned movements automatically. The neural contribution to the problem has not been identified. In the current study, we used functional magnetic resonance imaging (fMRI) to investigate the underlying neural mechanisms of movement automaticity in Parkinson's disease patients. Fifteen patients with Parkinson's disease were recruited. Three patients were finally excluded because they could not achieve automaticity. The remaining 12 patients were aged from 52 to 67 years, with a mean age of 61.2 years. Controls included 14 age-matched normal subjects. The subjects were asked to practise four tasks, including two self-initiated, self-paced sequences of finger movements with different complexity until they could perform the tasks automatically. Two dual tasks were used to evaluate automaticity. For dual tasks, subjects performed a visual letter-counting task simultaneously with the sequential movements. Twelve normal subjects performed all sequences automatically. All patients performed sequences correctly; 12 patients could perform the simpler sequence automatically; and only 3 patients could perform the more complex sequence automatically. fMRI results showed that for both groups, sequential movements activated similar brain regions before and after automaticity was achieved. No additional activity was observed in the automatic condition. In normal subjects, many areas had reduced activity at the automatic stage, whereas in patients, only the bilateral superior parietal lobes and left insular cortex were less activated. Patients had greater activity in the cerebellum, premotor area, parietal cortex, precuneus and prefrontal cortex compared with normal subjects while performing automatic movements. We conclude that Parkinson's disease patients can achieve automaticity after proper training, but with more difficulty. Our study is the first to demonstrate that patients with Parkinson's disease require more brain activity to compensate for basal ganglia dysfunction in order to perform automatic movements.

Aged↗
PubMed1997

Functional MRI mapping of occipital and frontal cortical activity during voluntary and imagined saccades.

We investigated the activation of frontal and occipital cortical areas in 14 normal volunteers during voluntary saccades in light or dark and during imagined saccades using functional magnetic resonance imaging (FMRI) with electro-oculogram monitoring. Voluntary saccades in light or dark and imagined saccades led to a significant activation (p < 0.005) of the precentral and posterior medial frontal gyrus (frontal eye field). The medial part of the superior frontal gyrus (supplementary eye field) also showed significant activity during voluntary saccades in all subjects, but only in four subjects during imagined saccades. In addition to frontal activity we found an activated primary visual cortex during voluntary saccades, both in light and in dark. In contrast to executed saccades, imagined eye movements revealed to occipital response under either condition. Our FMRI study supports the concept of frontal eye fields during voluntary saccades and demonstrates that occipital areas are associated with the generation of voluntary eye movements. However, the primary visual cortex is not active when eye movement is only imagined.

Adult↗
PubMed2001

Sensitivity of prefrontal cortex to changes in target probability: a functional MRI study.

Electrophysiological studies suggest sensitivity of the prefrontal cortex to changes in the probability of an event. The purpose of this study was to determine if subregions of the prefrontal cortex respond differentially to changes in target probabilities using functional magnetic resonance imaging (fMRI). Ten right-handed adults were scanned using a gradient-echo, echo planar imaging sequence during performance of an oddball paradigm. Subjects were instructed to respond to any letter but "X". The frequency of targets (i.e., any letter but X) varied across trials. The results showed that dorsal prefrontal regions were active during infrequent events and ventral prefrontal regions were active during frequent events. Further, we observed an inverse relation between the dorsal and ventral prefrontal regions such that when activity in dorsal prefrontal regions increased, activity in ventral prefrontal regions decreased, and vice versa. This finding may index competing cognitive processes or capacity limitations. Most importantly, these findings taken as a whole suggest that any simple theory of prefrontal cortex function must take into account the sensitivity of this region to changes in target probability.

Adolescent↗
PubMed2002

Neural basis of temporal context memory: a functional MRI study.

Temporal context information is crucial to understanding human episodic memory. Human lesion and neuroimaging data indicate that prefrontal regions are important for retrieving temporal context memory, although the exact nature of their involvement is still unclear. We employed functional magnetic resonance imaging (fMRI) to elucidate the neural basis of two kinds of temporal context memory: the temporal order of items between lists and within a list. On the day of the fMRI experiment, subjects memorized a list of 30 pictures in the morning and another list of 30 pictures in the afternoon. During the scanning session, the subjects performed three tasks. In a between-lists task, they were asked to judge the temporal order between two items that had been presented in different lists. In a within-list task, they were asked to judge the temporal order between two items that had been presented in a single list. We found bilateral prefrontal activities during these two temporal context memory tasks compared with a simple item-recognition task. Furthermore, in direct comparison between these two tasks, we found differential prefrontal activities. Thus, right prefrontal activity was associated with temporal order judgment of items between lists, whereas left prefrontal activity was related to temporal order judgment of items within a list. These results indicate that retrieval processes of two kinds of temporal context memory are supported by different, but overlapping, sets of cerebral regions. We speculate that this reflects different cognitive processes for retrieving temporal context memory between separate episodes and within a single episode.

Adult↗
PubMed2003

Coactivation pattern in human quadriceps during isokinetic knee-extension by muscle functional MRI.

We tested activation patterns of individual muscles and neuromuscular compartments (NMCs) of the quadriceps femoris (QF) during knee-extension exercises (KEE). Eight healthy men performed repetitive isokinetic KEE at 120 degrees s(-1). Before and after KEE, muscle functional magnetic resonance images of the thigh were taken to calculate transverse relaxation times (T2) for the individual muscles of the QF. In terms of exercised T2, a moderate correlation was observed between the rectus femoris (RF) and vastus medialis (VM) ( r=0.493, P<0.001), and between the vastus lateralis (VL) and VM ( r=0.347, P<0.01). Furthermore, a strong correlation was also observed between the vastus intermedius (VI) and VM ( r=0.719, P<0.0001), and between the VL and VI ( r=0.783, P<0.0001). There was a significant difference in T2 between the proximal and distal NMCs of the RF. We conclude that at least two strategies are applied for force production in the human QF: (1) coactivation of specific pairs of individual muscles and (2) regional-specific activation in NMCs in the RF.

Adult↗