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Biomedical subjects

Y Samson

Publications and source records attributed to Y Samson.

At least 55 records · Page 3Linked to original sources
PubMed1999

Dopaminergic dysfunction in midbrain dystonia: anatomoclinical study using 3-dimensional magnetic resonance imaging and fluorodopa F 18 positron emission tomography.

OBJECTIVE: To determine the role of damage to neuronal systems, especially the dopaminergic system, in patients with symptomatic dystonia and mesencephalic lesions. DESIGN: Stereotaxic magnetic resonance imaging analysis and positron emission tomography after the administration of fluorodopa F 18. PATIENTS: Of a group of 48 patients with unilateral dystonia following a stroke, 7 patients with a well-defined midbrain lesion were selected. RESULTS: All patients had unilateral dystonic posture of an upper extremity and cerebellar dysmetria or hypotonia. Cerebellar tremor was present in 1 patient. Two patients had resting and postural tremor, which showed a marked improvement with treatment with levodopa. In patients with dystonia only, dopaminergic lesions were mostly confined to the ventromesial mesencephalon and red nucleus area, including the substantia nigra and nigrostriatal and cerebellothalamic fibers. Dystonia was severe and did not resolve with time in patients with lesions involving the nigrostriatal pathway, and the degree of dopaminergic denervation revealed by positron emission tomography was correlated with the severity of dystonia. In patients with resting and postural tremor, lesions of the dopaminergic structures were larger and located more laterally and dorsally in the pars compacta, the perirubral and retrorubral areas, and extending to the central tegmental tract. CONCLUSIONS: Dopaminergic dysfunction plays a role in the occurrence and severity of midbrain dystonia, and additional lesions to dopaminergic neurons in the perirubral and retrorubral areas result in tremor that responds to levodopa treatment.

Adolescent↗
PubMed1999

Cortical control of optokinetic nystagmus in humans: a positron emission tomography study.

Positron emission tomography (PET) was used to address the issue of physiological changes in the cerebral cortex associated to optokinetic nystagmus (OKN) in humans. We studied regional cerebral blood flow in eight volunteers during reflexive induction of OKN by a pattern of dots moving unidirectionally (toward the left side). We used two control conditions, with subjects passively viewing either stationary or incoherently moving dots. This paradigm was designed in order to differentiate the OKN-related activations from blood flow changes related to visual motion. When compared with the stationary condition, OKN activated a set of occipital areas known to be sensitive to visual motion. Bilateral activation was found in the striate cortex (V1) and the parieto-occipital fissure, while area V5, the intraparietal sulcus, and the pulvinar were activated only in the left hemisphere. When compared with incoherent motion, OKN activated the V1 and the parieto-occipital fissure bilaterally and the right lingual gyrus, while a signal decrease was observed in the V5 region in both hemispheres. No significant signal changes were found in areas implicated in saccades or in processing vestibular information. These results indicate that processing of OKN-related information is associated with neural activity in a specific set of visual motion areas and suggest that this network can be asymmetrically activated by a strictly unidirectional stimulation. Results are also discussed in terms of the specific kinds of OKN-related information processing subserved by each area in this network.

Adult↗
PubMed1999

Primary somatosensory cortex activation is not altered in patients with ventroposterior thalamic lesions: a PET study.

BACKGROUND AND PURPOSE: We know remarkably little about the mechanisms underlying cortical activation. Such mechanisms might be better understood by studying the effect of well-localized lesions on the cortical activations in simple paradigms. METHODS: We used H(2)(15)O and positron emission tomography to measure regional cerebral blood flow (rCBF) at rest and during hand vibration in 7 patients with unilateral thalamic lesion involving the ventroposterior (VP) somatosensory thalamic relay nuclei. We compared the results with those obtained in 6 patients with thalamic lesions sparing the VP nuclei and 6 healthy controls. RESULTS: The patients with VP lesions had a selective hypoperfusion at rest in the ipsilesional primary sensorimotor cortex (SM1). This hypoperfusion was significantly correlated with the degree of contralateral somatosensory deficit. This abnormality may reflect the deafferentation of SM1 from its somatosensory thalamic input. Despite this deafferentation, the ipsilesional SM1 was normally activated by the vibration of the hypoesthetic hand. CONCLUSIONS: The fact that a lesion of the somatosensory thalamic relay nuclei alters the rCBF at rest in SM1 but not its activation by hand vibration indicates that the mechanism of cortical activation is complex, even in the case of simple sensory stimulation. In addition, a dissociation may occur between obvious neurological deficits and apparently normal activation patterns, which suggests that activation studies should be interpreted cautiously in patients with focal brain lesions.

Adult↗
PubMed1999

Somatosensory cortical activations are suppressed in patients with tactile extinction: a PET study.

OBJECTIVE: To investigate whether tactile extinction alters the cortical somatosensory activations induced by hand vibration. BACKGROUND: Tactile extinction occurs mainly after right-brain lesions and consists of the inability to perceive a contralesional cutaneous stimulation when a similar stimulus is applied to the mirror region of the ipsilesional hemibody. The pathophysiology of tactile extinction is poorly understood, but it is considered to be a deficit of selective attention of somatosensory stimuli. Although other theories have been proposed, our understanding of the pathophysiology of tactile extinction may benefit from functional imaging studies. METHODS: We selected three patients with pure tactile extinction and a mainly subcortical right-brain lesion that spared the primary sensorimotor cortex (SM1). We used PET to investigate the responses to unilateral and bilateral hand vibration in SM1 and the secondary somatosensory cortical area (SII). RESULTS: During bilateral hand vibration, activation was normal in the left SM1, suppressed in the right SM1, and markedly decreased in both SII, which was consistent with the extinction of the left-hand stimulus. During unilateral left-hand vibration, the activation of the right SM1 was still markedly impaired, but the activation of both SII was normal. CONCLUSIONS: We found marked changes in the activation of cortical somatosensory areas induced by hand vibration in patients with tactile extinction. The role of selective attention in cortical activation is also examined.

Aged↗
PubMed1999

Early and delayed MR and PET changes after selective temporomesial radiosurgery in mesial temporal lobe epilepsy.

We report a patient with medically refractory mesial temporal lobe epilepsy treated by gamma knife radiosurgery. In lieu of a microsurgical procedure, an entorhinoamygdalohippocampectomy was performed with a gamma knife and low marginal doses (25 Gy). The clinical and imaging studies, including CT, MR imaging, 18F-fluorodeoxyglucose positron emission tomography (FDG-PET), and long-term follow-up MR examinations, are reported. The patient has been seizure-free since the day of treatment, with no clinical complications. MR studies accurately depicted the effect on the target structures and the transient secondary changes around them. FDG-PET scans showed decreased metabolism after gamma knife surgery throughout the anteromesial part of the epileptogenic temporal lobe. This metabolic decrease was reversible in the lateral temporal cortex. Our case suggests that gamma knife surgery is a promising tool for use as a minimally invasive approach to the treatment of epilepsy.

Adult↗
PubMed1999

In vivo imaging of muscarinic cholinergic receptors in temporal lobe epilepsy with a new PET tracer: [76Br]4-bromodexetimide.

UNLABELLED: Muscarinic acetyl cholinergic receptors (mAChRs) may be involved in the pathophysiology of partial epilepsy. Previous experimental and imaging studies have reported medial temporal abnormalities of mAChR in patients with medial temporal lobe epilepsy (MTLE). Suitable radiotracers for mAChR are required to evaluate these disturbances in vivo using PET. Dexetimide is a specific mAChR antagonist that has been labeled recently with 76Br. This first study in humans focused on regional distribution and binding kinetics of [76Br]4-bromodexetimide (BDEX) in patients with MTLE. METHODS: Ten patients with well-lateralized MTLE had combined MRI, 18F-fluorodeoxyglucose (FDG) PET and 76Br-BDEX PET studies. Time-activity curves were generated in PET-defined regions of interest, including the medial, polar and lateral regions of the temporal lobe; the basal ganglia; the external and medial occipital cortex; and the white matter. RESULTS: The highest radioactivity concentration was observed in the basal ganglia and in the cortical regions, whereas radioactivity was lower in the white matter. On late images of PET studies, 76Br-BDEX uptake was statistically significantly decreased only in the medial temporal region ipsilateral to the seizure focus (1.37 +/-0.28, P < 0.01) as determined by FDG PET imaging, anatomic MRI and electroencephalogram correlation, compared with the contralateral medial temporal region (1.46 +/- 0.31). CONCLUSION: 76Br-BDEX concentration is reduced in the temporal lobe ipsilateral to the seizure focus in patients with MTLE. This preliminary study suggests that 76Br-BDEX is a suitable radiotracer for studies of mAChR in humans. Further studies are required to investigate the potential value of 76Br-BDEX PET in other neurological disorders with muscarinic disturbances.

Adult↗
PubMed1999

[Mechanisms of aphasia recovery and brain imaging].

Aphasia recovery may depend on right hemisphere or non-lesioned left hemisphere structures, pre-morbid brain language organization, and de novo learning of language. Here we review the brain imaging evidence supporting these different hypotheses. CT-scan studies have investigated the prognosis value of size and site of left hemisphere lesions. The size of the lesion is a global but not an individual predictor of the initial severity and subsequent recovery of aphasia. Studies on the site of the lesion have given different results for verbal expression and comprehension. There is no consensus on a single critical site for recovery of verbal expression in non-fluent aphasia, which may depend on sub-cortical more than cortical extend of the lesion. Conversely the extend of the lesion in the superior temporal gyrus emerges as a critical negative factor for comprehension recovery. Rest measurements of brain metabolism have consistently shown that aphasia severity depends much more on the degree of dysfunction of language-related areas in the left hemisphere than on the site of the lesion it-self. This suggests that aphasia recovery may depend on metabolic dysfunction recovery in peri-lesional structures. More recently, activation studies have shown consistent right hemisphere activation during language tasks in aphasic subjects, but their role in recovery remains debated. It is likely limited, and may depend on atypical pre-morbid language lateralization. Left hemisphere activations are also found in aphasic patients. They are often relocalized in peri-lesional areas, and emerge in most studies as the main factor of aphasia recovery.

Aphasia↗
PubMed1999

[Diffusion MRI and cerebral ischemia. When to calculate the coefficient of diffusion?].

INTRODUCTION: There are two types of diffusion images: so-called "diffusion-weighted" images (DWI) and apparent diffusion coefficient (ADC) images. For certain authors, ADC mapping is crucial for interpreting diffusion images while for others the ADC map adds no further sensitivity or specificity compared with diffusion weighted images. The objective of this work was to determine those situations where ADC mapping modifies image interpretation. MATERIAL AND METHODS: T2-weighted and diffusion-weighted (DIF) MRI sequences were acquired in 197 patients with suspected cerebral ischemia (< or = 48 hr). For each lesion (239 lesions in the 197 patients), we analyzed MRI interpretation with and without ADC mapping and compared the interpretations with the final diagnosis established on clinical data and complementary explorations. RESULTS: We observed 3 groups. In group A (36% of the lesions), ADC mapping did not change image interpretation. This group was subdivided into 3 subgroups. A1: T2 and DIF weighted images are normal: no lesions and normal ACD map (n = 38 patients); A2: High intensity signal on T2 and low intensity signal on DIF: ischemic sequelae, ADC always increased (n = 32 lesions); A3: T2 normal and high intensity signal on DIF: hyper acute ischemia and ADC always decreased (n = 16 patients) In group B (high intensity signal on T2 and DIF, 54.5% of the lesions), ADC mapping changed the MRI interpretation: there was acute ischemia if the ADC was decreased (n = 113) and "pseudo-ischemic" lesions if the ADC was normal or increased (n = 17 patients). Group C was comprised of 23 lesions with a false negative ADC (9.5%). These lesions were always small recent ischemic lesions (< or = 5 mm) with a high intensity signal on DIF and a strictly normal ADC map. CONCLUSION: ADC mapping was found to be useful in 54.5% of the lesions and should not be considered as solely a research tool but also as a useful tool for routine clinical practice.

Adult↗
PubMed1998

The functional anatomy of sound intensity discrimination.

The human neuroanatomical substrate of sound intensity discrimination was investigated by combining psychoacoustics and functional neuroimaging. Seven normal subjects were trained to detect deviant sounds presented with a slightly higher intensity than a standard harmonic sound, using a Go/No Go paradigm. Individual psychometric curves were carefully assessed using a three-step psychoacoustic procedure. Subjects were scanned while passively listening to the standard sound and while discriminating changes in sound intensity at four different performance levels (d' = 1.5, 2.5, 3.5, and 4.5). Analysis of regional cerebral blood flow data outlined activation, during the discrimination conditions, of a right hemispheric frontoparietal network already reported in other studies of selective or sustained attention to sensory input, and in which activity appeared inversely proportional to intensity discriminability. Conversely, a right posterior temporal region included in secondary auditory cortex was activated during discrimination of sound intensity independently of performance level. These findings suggest that discrimination of sound intensity involves two different cortical networks: a supramodal right frontoparietal network responsible for allocation of sensory attentional resources, and a region of secondary auditory cortex specifically involved in sensory computation of sound intensity differences.

Adult↗
PubMed1998

PET study and neuropsychological assessment of a long-lasting post-encephalitic parkinsonism.

A 76-year old woman was affected by lethargic encephalitis in 1918, at the age of 3 months. Long-term clinical follow-up with late neuropsychological evaluation revealed post-encephalitic parkinsonism, which worsened very slowly and was improved by levodopa. Obsessive and compulsive disorders (OCD) were associated to nosophobia. Neuropsychological evaluation showed mild visuocontructional memory deficit, which was isolated. 18 Fluoro-Dopa PET demonstrated a severe bilateral and symmetrical reduction in fluoro-dopa uptake, which was more marked in the putamen than in the caudate. Thus, the pattern of dopaminergic denervation was similar to the one observed in idiopathic Parkinson's disease.

Aged↗
PubMed1998

Cortical networks implicated in semantic and episodic memory: common or unique?

While previous functional neuroimaging studies have shown that semantic and episodic memory tasks activate different cortical regions, they never compared regional cerebral blood flow (rCBF) patterns associated with semantic and episodic memory within the same experimental design. In this study, we used H2(15)O PET to study subjects in the course of semantic and episodic memory tasks. rCBF was measured in 9 normal volunteers during a resting baseline condition and two cognitive tasks. In the semantic categorisation task subjects heard a list of concrete words and had to respond to words belonging to the "animals" or "food" category. In the episodic recognition task subjects heard a list of concrete words, half "old", i.e. belonging to the list of the semantic categorisation task, and half "new", i.e. presented for the first time. Subjects had to respond to the "old" words. Both tasks were compared to a resting condition. Statistical analysis was performed with Statistical Parametric Mapping (SPM). Compared to the resting condition, the semantic tasks, activated the superior temporal gyri bilaterally, the left frontal cortex, and right premotor cortex. The episodic tasks activated the left superior temporal gyrus, the frontal cortex bilaterally, and the right inferior parietal cortex. Compared to the episodic memory tasks, the semantic memory tasks activated the superior temporal/insular cortex bilaterally and the right premotor cortex. Compared to the semantic memory tasks, the episodic memory tasks activated the right frontal cortex. These results suggest that cortical networks implicated in semantic and episodic memory show both common and unique regions, with the right prefrontal cortex being the neural correlate specific of episodic remembering.

Adult↗
PubMed1998

Frontal dysfunction in neurologically normal chronic alcoholic subjects: metabolic and neuropsychological findings.

BACKGROUND: Neuropsychological and imaging studies suggest that frontal dysfunction may occur in apparently normal chronic alcoholic subjects. METHODS: To investigate this issue further, we performed neuropsychological and fluorodeoxy-glucose-PET studies in 17 chronic alcoholics without patent neurological and psychiatric complications. RESULTS: Metabolic abnormalities were found in the mediofrontal and in the left dorsolateral prefrontal cortex, but not in the orbitofrontal cortex. Neuropsychological testing revealed significantly reduced verbal fluency and impaired performance on the Stroop test. The mediofrontal hypometabolism correlated with the reduction in verbal fluency and the time necessary to perform the interference condition of the Stroop test. The left dorsolateral prefrontal hypometabolism correlated with the number of errors on the Stroop test. CONCLUSION: These data indicate that circumscribed frontal dysfunctions may occur in chronic alcoholic subjects before clinically obvious neurological complications, and may account for some of the alcohol-related neuropsychological and behavioural impairments.

Adult↗
PubMed1998

Diffusion-weighted magnetic resonance imaging in a case of cerebral venous thrombosis.

BACKGROUND: Diffusion-weighted imaging (DWI) is the most sensitive MR sequence in acute arterial ischemic stroke but has not yet been evaluated in venous cerebral ischemia. We describe a patient with DWI performed at the acute phase of a venous ischemic stroke. CASE DESCRIPTION: A rapid cerebral MRI including DWI and fast fluid-attenuated inversion recovery (FLAIR) sequences was performed at the acute phase of a venous stroke confirmed by conventional angiography. DWI showed a slight decrease in apparent diffusion coefficient values 3 hours after onset (0.53+/-0.07x10(-3) mm2/s) and was normal 48 hours later (0.064+/-0.15x10(-3) mm2/s). Fast FLAIR sequences showed large left frontoparietal hyperintensities. The lack of a clear decrease in apparent diffusion coefficient values associated with marked FLAIR abnormalities may suggest prominent or early associated vasogenic edema. Physiopathological differences between arterial and venous ischemia may explain the different type of DWI FLAIR abnormalities during the acute phase as well as the better recovery of neurological deficit in venous stroke than in arterial ischemic stroke. CONCLUSIONS: In the context of an acute stroke, the contrast between marked FLAIR and subtle DWI abnormalities on MRI may reflect the venous mechanism of cerebral ischemia.

Brain↗
PubMed1998

Lateralization of speech and auditory temporal processing.

To investigate the role of temporal processing in language lateralization, we monitored asymmetry of cerebral activation in human volunteers using positron emission tomography (PET). Subjects were scanned during passive auditory stimulation with nonverbal sounds containing rapid (40 msec) or extended (200 msec) frequency transitions. Bilateral symmetric activation was observed in the auditory cortex for slow frequency transitions. In contrast, left-biased asymmetry was observed in response to rapid frequency transitions due to reduced response of the right auditory cortex. These results provide direct evidence that auditory processing of rapid acoustic transitions is lateralized in the human brain. Such functional asymmetry in temporal processing is likely to contribute to language lateralization from the lowest levels of cortical processing.

Acoustic Stimulation↗
PubMed1998

Clinicometabolic dissociation of cognitive functions and social behavior in frontal lobe lesions.

OBJECTIVE/BACKGROUND: Case studies suggest a dissociation between cognitive functions that have been impaired after damage to the dorsolateral prefrontal cortex and social skills disturbed when the ventromedial prefrontal areas are affected. Because this dissociation had not been confirmed in a clinical setting, clinicometabolic correlations were sought in 13 patients with various lesions of the prefrontal cortex. DESIGN/METHODS: The clinical assessment included extensive testing of executive functions and evaluation of behavioral abnormalities based on an informant questionnaire. Regional cerebral glucose metabolism (rCMRGlu) was measured with [l8F] fluorodeoxyglucose ([18F] FDG) and 31-slide high-resolution PET. RESULTS: Executive-function test performance was significantly correlated with rCMRGlu in the dorsolateral prefrontal cortex (Brodmann's areas 8, 9, 45, 46, and 47) and anterior cingulate cortex (Brodmann's areas 24 and 32). Behavioral scores were significantly correlated with rCMRGlu in the frontopolar (Brodmann's area 10) and orbitofrontal cortex (Brodmann's areas 11, 12, 13, and 14). CONCLUSION: These results show that impaired executive functions and serial skill deficits are associated with distinct metabolic patterns in patients with frontal lobe pathology. In agreement with activation studies in normal subjects, our data suggest the existence of a modular organization of the frontal cortex in humans, as previously reported in nonhuman primates.

Adult↗
PubMed1998

The underlying pathophysiology of ictal dystonia in temporal lobe epilepsy: an FDG-PET study.

BACKGROUND: Unilateral dystonic posturing of limbs occurs frequently in medial temporal lobe epilepsy (MTLE) but its underlying anatomic basis is still unknown. OBJECTIVE: To investigate the pathophysiology of dystonic posturing, we examined the relation between the occurrence of dystonia and interictal cerebral metabolic abnormalities using fluorodeoxyglucose (FDG)-PET in MTLE patients. METHODS: Video recordings of 30 patients with documented MTLE were reviewed to assess the presence of ictal dystonic posturing. Interictal FDG-PET was performed in all patients. RESULTS: Eighteen patients exhibited dystonic posturing--contralateral to the seizure focus in 16 cases, bilateral in one, and ipsilateral in one. Dystonia was statistically associated with more severe hypometabolism in the striatal and in the orbitofrontal regions ipsilateral to the seizure focus. CONCLUSION: Hypometabolism observed in the striatal region of patients with ictal dystonic posturing suggests that the basal ganglia are involved in the generation of ictal dystonic posturing in MTLE.

Adult↗
PubMed1997

Parametric images of benzodiazepine receptor concentration using a partial-saturation injection.

The in vivo quantification of the benzodiazepine receptor concentration in human brain using positron emission tomography (PET) and 11C-flumazenil (11C-FMZ), is usually based on a three-compartment model and on PET curves measured in a small number of large regions of interest; however, it should be interesting to estimate the receptor concentration for each pixel and to build quantified images of the receptor concentration. The main advantage is to allow screening of the receptor site localization and visual observation of the possible abnormalities. Up to now, all the methods described include complex experimental protocols, difficult to use in routine examinations. In this paper, we propose the partial-saturation approach to obtain parametric images of benzodiazepine receptor concentration and FMZ affinity. It consists of a single FMZ injection with a low specific activity, followed by Scatchard analysis. Like other parametric imaging methods, this partial-saturation approach can lead to a small percentage (< 1%) of unrealistic values in receptor-poor regions; however, it is the only method that allows receptor concentration and affinity images to be obtained from a single-injection 40-min experiment without blood sampling. We also propose a second method in which the receptor concentration map is directly deduced from the PET image acquired 5 to 10 min after a partial-saturation injection. This method assumes a known and constant FMZ affinity value but requires only very simple corrections of this PET image. It is robust (negative values are never found) and quite simple to use in routine examination of patients (no blood sampling, single injection, only 10-min experiment).

Brain Chemistry↗