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Biomedical subjects

A Roucoux

Publications and source records attributed to A Roucoux.

At least 19 recordsLinked to original sources
PubMed2000

Stabilized rhodium(0) nanoparticles: a reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system.

A colloidal system based on an aqueous suspension of rhodium(o) nanoparticles proved to be an efficient catalyst for the hydrogenation of arene derivatives under biphasic conditions. The rhodium nanoparticles (2-2.5 nm) were synthesized by the reduction of RhCl3 x 3H2O with sodium borohydride and were stabilized by highly water-soluble N-alkyl-N-(2-hydroxyethyl)ammonium salts (HEA-Cn). These surfactant molecules were characterized by measurements of the surface tension and the aqueous dispersions with rhodium were observed by transmission electron cryomicroscopy. The catalytic system is efficient under ultramild conditions, namely room temperature and 1 atm H2 pressure. The aqueous phase which contains the protected rhodium(0) colloids can be reused without significant loss of activity. The microheterogeneous behavior of this catalytic system was confirmed on a mercury poisoning experiment.

Catalysis↗
PubMed1999

PET study of the human foveal fixation system.

Positron emission tomography (PET) was used to investigate the functional anatomy of the foveal fixation system in 10 subjects scanned under three different conditions: at rest (REST), during the fixation of a central point (FIX), and while fixating the same foveal target during the presentation of peripheral visual distractors (DIS). Compared with the REST condition, both FIX and DIS tasks activated a common set of cortical areas. First, in addition to the involvement of the occipital visual cortex, both the frontal eye field (FEF) and the intraparietal sulcus (IPS) were bilaterally activated. Right frontal activation was also found in the dorsolateral prefrontal cortex, the inferior part of the precentral gyrus, and the inferior frontal gyrus. These results suggest that both FEF and IPS may constitute the main cortical regions subserving bilaterally the foveal fixation system in humans. The remaining right frontal activations may be considered as part of the anterior attentional network, supporting a role for the right frontal lobe in the allocation of the attentional mechanisms. Compared with the FIX condition, the DIS task also revealed the perceptual and cognitive processes related to the presence of peripheral visual distractors during foveal fixation. In addition to a bilateral activation of the V5/MT motion-sensitive area, a right FEF-IPS network was activated which may correspond to the engagement of the visuospatial attention. Finally, normalized regional cerebral blood flow (NrCBF) decreases were also observed during both DIS and FIX condition performance. Such NrCBF decreases were centered in the superior and middle temporal gyri, the prefrontal cortex, and the precuneus and the posterior retrosplenial part of the cingulate gyrus.

Adult↗
PubMed1999

Neuronal mechanisms of perceptual learning: changes in human brain activity with training in orientation discrimination.

Using 15O-water 3D positron emission tomography, regional cerebral blood flow was measured twice in six human subjects: before and after extensive training in orientation discrimination. In each session subjects performed two orientation discrimination tasks, during which they discriminated the orientation of a grating at either the trained or untrained reference orientation, and a control task, during which they detected a randomly textured pattern. By comparing the discrimination to the detection tasks, we observed a main effect of task bilaterally in the posterior occipital cortex, extending into the left posterior fusiform gyrus and the right inferior occipital gyrus, bilaterally in the intraparietal sulcus, as well as in the cerebellum, thalamus, and brainstem. When we compared the activation pattern before and after the training period, all the changes observed were activity decreases. The nonspecific changes, which were not related to the orientation used during the training, were situated in the cerebellum and bilaterally in the extrastriate visual cortex. The orientation-specific changes, on the other hand, were restricted to the striate and extrastriate visual cortex, more precisely the right calcarine sulcus, the left lingual gyrus, the left middle occipital, and the right inferior occipital gyrus. These findings confirm our hypothesis concerning the existence of learning related changes at early levels of visual processing in human adults and suggest that mechanisms resulting in neuronal activity decreases might be involved in the present kind of learning.

Adult↗
PubMed1999

Studies of technetium-99m nitridobisdithiocarboxylate leucocyte specific radiopharmaceutical: [99mTcN(DTCX)2], DTCX = CH3(CH2)8CS2. The cellular and subcellular distribution in human blood cells, and chemical behaviour. Synthesis of the analogous rhenium-188 radiopharmaceutical.

The distribution of the radiopharmaceutical ([99mTcN(DTCX)2], DTCX = CH3(CH2)8CS2) in the leucocyte population determined by a density separation with double gradient Polymorphprep was studied. Microautoradiographic analysis showed a subcellular distribution of the radiomarker in human blood cells. This technique confirmed the observed lymphocyte selectivity (69%) and revealed that the uptake was predominantly cytoplasmic around the nucleus. A labeling mechanism by passive endocytosis could be proposed involving a required lipophilicity of the radiopharmaceutical for lymphocyte targeting. Finally, we describe the new synthesis with an efficient yield and radiochemical purity of the analogous radiopharmaceutical [188ReN(DTCX)2].

Autoradiography↗
PubMed1998

PET study of human voluntary saccadic eye movements in darkness: effect of task repetition on the activation pattern.

Using H2(15)O 3D Positron Emission Tomography (PET), regional cerebral blood flow (rCBF) was measured in six human subjects under two different conditions: at rest and while performing self-paced horizontal saccadic eye movements in darkness. These two conditions were repeated four times each. First, the comparison between the four saccadic and four resting conditions was investigated in a group and a single subject analysis. Saccades elicited bilateral rCBF increases in the medial part of the superior frontal gyrus (supplementary eye field), precentral gyrus (frontal eye field), superior parietal lobule, anterior medial part of the occipital lobe involving striate and extrastriate cortex (lingual gyrus and cuneus), and in the right inferior parietal lobule. At the subcortical level, activations were found in the left putamen. These results mainly replicate previous PET findings on saccadic control. Second, the interaction between the experimental conditions and their repetition was examined. When activations throughout repetition of the same saccadic task are compared, the supplementary eye fields show a progressive increase of activation. On the contrary, the activation in the cerebellum, left superior parietal lobule and left occipital cortex progressively decreases during the scanning session. Given the existence of such an interaction, the pattern of activations must be interpreted as a function of task repetition. This may be a factor explaining some apparent mismatch between different studies.

Adult↗
PubMed1997

New bis(dithiocarboxylato) nitridotechnetium-99m radiopharmaceuticals for leucocyte labelling: in vitro and in vivo studies.

Dithiocarboxylate ligands were synthesized and characterised. New nitrido 99m-technetium complexes were obtained with these ligands and identified by thin layer chromatography. The nitrido complexes were tested in vitro in whole blood for leucocyte labelling and the design of the ligand was optimized. Best results were obtained with aliphatic linear ligands, containing 9 to 11 atoms of carbon. The in vivo experiment failed because an inflammated area could not be visualized by gamma imaging, the cell labelling mechanism being probably different.

Animals↗
PubMed1997

Rhenium-188 and technetium-99m nitridobis(N-ethoxy-N-ethyldithiocarbamate) leucocyte labelling radiopharmaceuticals: [188ReN(NOET)2] and [99mTcN(NOET)2], NOET = Et(EtO)NCS2: their in vitro localization and chemical behaviour.

In this study, we have investigated the preparation of rhenium-188 nitridobis(N-ethoxy-N-ethyldithiocarbamate) [188ReN(NOET)2] (NOET = Et(EtO)NCS2), analogous to the known technetium-99m radiopharmaceutical. The new 188Re complex was synthesized in good yield with a satisfactory radiochemical purity, using a kit method. The subcellular localization of both radiopharmaceuticals in granulocytes was observed by microautoradiography. The uptake was independent of the radionuclide and predominantly nuclear. Furthermore, HPLC was used to characterize the 99mTc complex before and after blood cell labelling and revealed that the intact radiopharmaceutical was involved.

Autoradiography↗
PubMed1996

Smooth eye movements evoked by electrical stimulation of the cat's superior colliculus.

Head-fixed gaze shifts were evoked by electrical stimulation of the deeper layers of the cat superior colliculus (SC). After a short latency, saccades were triggered with kinematics similar to those of visually guided saccades. When electrical stimulation was maintained for more than 150-200 ms, postsaccadic smooth eye movements (SEMs) were observed. These movements were characterized by a period of approximately constant velocity following the evoked saccade. Depending on electrode position, a single saccade followed by a slow displacement or a "staircase" of saccades interspersed by SEMs were evoked. Mean velocity decreased with increasing deviation of the eye in the orbit in the direction of the movement. In the situation where a single evoked saccade was followed by a smooth movement, the duration of the latter depended on the duration of the stimulation train. In the situation where evoked saccades converged towards a restricted region of the visual field ("goal"-directed or craniocentric saccades), the SEMs were directed towards the centre of this region and their mean velocity decreased as the eye approached the goal. The direction of induced SEMs depended on the site of stimulation, as is the case for saccadic eye movements, and was not modified by stimulation parameters ("place" code). On the other hand, mean velocity of the movements depended on the site of stimulation and on the frequency and intensity of the current ("rate" code), as reported for saccades in the cat. The kinematics of these postsaccadic SEMs are similar to the kinematics of slow, postsaccadic correction observed during visually triggered gaze shifts of the alert cat. These results support the hypothesis that the SC is not exclusively implicated in the control of fast refixation of gaze but also in controlling postsaccadic conjugate slow eye movements in the cat.

Animals↗
PubMed1995

Evidence for high-velocity smooth pursuit in the trained cat.

It is generally accepted that in cats smooth pursuit velocity of the eye never exceeds a few degrees per second. This is in contrast with observations in primates, where smooth pursuit velocity can reach values as high as 100 degrees/s. Cats were trained to fixate and pursue spots of light appearing on a translucent screen. Spots were moved in the horizontal and vertical planes at different constant velocities up to 80%. Eye position was recorded with the scleral search coil technique. Naive cats did not pursue moving targets with high efficiency. Smooth eye movement velocity saturated at 5 degrees/s. After a few days of training, smooth-pursuit eye velocity increased with target velocity and saturated at 25 degrees/s on average. However, velocities twice as high have been observed frequently. When the target was unexpectedly extinguished, smooth eye movement velocity dropped to values close to 0 degree/s in approximately 350 ms. After a short training period (usually 5 times the same target presentation), the eye continued to move smoothly until the target reappeared. These data suggest that smooth pursuit eye movements of the cat are qualitatively similar to those of primates, but reach lower velocities and are more variable in their characteristics.

Animals↗
PubMed1993

Slow correcting eye movements of head-fixed, trained cats toward stationary targets.

Inspection of eye saccades made by head-fixed, trained cats revealed the existence of many eye shifts at an approximately constant velocity during the deceleratory phase of the saccade or at the end of it. Slow eye movements occurring at the end of a saccade are usually referred to as "postsaccadic drifts". It is shown that the duration and mean velocity of these "drifts" are related to the amplitude of the movement. The kinematics of these slow eye movements are nevertheless different from those of saccades. Slow movements at the end of the gaze shift have longer durations than those occurring during the intersaccadic interval between a saccade and a reacceleration of the eye. A closer study of the drifts of three trained cats showed that they play an important corrective role in reducing the residual error at the end of a saccade or during an intersaccadic interval. This functional corrective role was demonstrated by relating the amplitude of the slow movement to the amplitude of the residual error when the slow velocity eye shift began. It is therefore proposed to name these eye shifts "slow correcting movements".

Animals↗
PubMed1992

Experimental study and modeling of vestibulo-ocular reflex modulation during large shifts of gaze in humans.

An experimental study of head-free and head-fixed gaze shifts explores the role of the vestibulo-ocular reflex (VOR) during saccadic and slow phase components of the gaze shifts. A systematic comparison of head-free and head-fixed gaze shifts in humans revealed that while the VOR is switched off as soon as the saccade starts, its function is progressively restored during the terminal phase of the saccade. The duration of this restoration period is fairly constant; therefore, the faster the gaze saccade, the sooner the VOR function starts to be restored. On the basis of these experimental data, a new eye-head coordination model is proposed. This model is an extension of the one proposed by Laurutis and Robinson (1986) where VOR gain is a function of both the dynamic gaze error signal and head velocity. This extension has also been added to another eye-head coordination model (Guitton et al. 1990). Both modified models yield simulation results comparable to experimental data. This study pinpoints the high efficiency of the gaze control system. Indeed, a fixed period of time (approximately 40 ms) is needed to restore the inhibited VOR; the gaze control system thus must have a knowledge of its own dynamics in order to be able to anticipate the end of the saccadic movement.

Adult↗
PubMed1989

Neck muscle activity in eye--head coordinated movements.

The electromyographic (EMG) activity of different neck muscles in relation to gaze orientation has been studied in alert trained cats. When the head is kept fixed, the activity of these muscles is proportional to eye eccentricity in the horizontal as well as in the vertical planes. On basis of this tonic activity, a preferential orientation can be attributed to each muscle: upward and lateral for biventer, rectus and complexus, and downward and lateral for longissimus, splenius and obliquus capitis cranialis. Fluctuations in this modulation of the EMG activity by eye position can be observed. When the head is free to move, the muscles show phasic discharges having similar preferential orientations. For a given muscle, this orientation covers a quite large angle: many muscles contribute to a given movement. The timing of the discharge of the different muscles as a function of the direction of the head movement was examined. It was found that the latency, i.e. the delay between the discharge and movement onset, progressively increases as the movement direction diverges from the preferential orientation of the muscle. It has been noted that the muscles having an upward preferential orientation may show, in relation to downward movements, inhibition occurring prior to the onset of the head movement. The same muscles may also increase their activity around the midcourse of downward movements. Thus, the head motor system controls the direction and amplitude parameters not only by selectively activating the appropriate muscles but also by sequencing their activity in a subtle way to start, control the trajectory and stop the movement, reminiscent of what has been described for limb movements.

Action Potentials↗
PubMed1983

Development of fixation and pursuit eye movements in human infants.

Visual fixation and pursuit abilities of human infants were tested during their first year of life. Eye as well as head position was measured. Results show that the fixation of visual targets is accomplished by a head rotation accompanied by a series of small eye saccades. The number of these saccades increases with target eccentricity but progressively decreases with age. Pursuit of a moving visual target is performed by a smooth eye and head movement only if the target velocity is low. The maximum speed of pursuit progressively increases with age. The results are compatible with the relatively late development of the fovea.

Eye Movements↗
PubMed1980

Stimulation of the superior colliculus in the alert cat. I. Eye movements and neck EMG activity evoked when the head is restrained.

Electrical stimulation of the cat superior colliculus (SC), in conjunction with the accurate measurement of elicited eye movements and histologically verified electrode positions, has revealed a striking antero-posterior variation in collicular organization. Three zones could be defined in the SC on the basis of eye movement patterns and associated neck muscle EMG activity evoked from the deeper layers. The Anterior zone was coextensive with the central 25 degrees of the visual retinotopically coded map contained in the superficial layers. Saccades evoked from this zone were also retinotopically coded, and the latency of EMG activity depended on the position of the eye in the orbit. A similar observation applies to the entire monkey SC. The Intermediate zone was coextensive with the 25 degrees--70 degrees of visual projections. Saccades evoked from this region were "goal-directed" and were associated with invariant, short latency EMG responses. The Posterior zone was found in the extreme caudo-lateral portion of the SC. Eye movements evoked from this zone were centering saccades associated with constant latency EMG activity. The present results in conjunction with previously demonstrated antero-posterior variations in projections to the SC, suggest that the motor strategies controlling gaze shifts toward visual targets vary depending on the location of the target in the visual field.

Animals↗
PubMed1980

Stimulation of the superior colliculus in the alert cat. II. Eye and head movements evoked when the head is unrestrained.

Electrical stimulation of the superior colliculus (SC) in alert cats free to move their head, evoked coordinate eye and head movements. The characteristics of these movements as well as their mode of coordination differed according to the collicular region being explored. Three zones were distinguished. In the anterior zone, evoked eye saccades were retinotopic and the accompanying head movements were slow and small in amplitude. The vestibular slow phase velocity signal was continuously added to the eye saccadic command so that the evoked gaze shift was identical, with the head fixed or free. In the intermediate zone, evoked eye saccades were goal-directed and the synchronous head movements fast and of large amplitude. The vestibular slow phase signal was cancelled during the eye saccade so that the evoked gaze shift was the result of the eye plus head angular displacement. In the posterior zone, the evoked head movements were goal-directed. The pattern of eye movements was similar to a vestibular nystagmus. This zone probably directly commands body orienting movements. A model of SC function in gaze orienting behavior is proposed, calling upon at least two different modes of eye-head coordination.

Animals↗