PubMed Health⌕ Search

Biomedical subjects

B Doyon

Publications and source records attributed to B Doyon.

At least 19 recordsLinked to original sources
PubMed2003

Optically induced dynamics in nematic liquid crystals: the role of twist deformation and asymmetry.

We present a theoretical study of optically induced dynamics in a homeotropic nematic liquid crystal excited at normal incidence. By retaining the first symmetric and antisymmetric reorientation modes, the dynamical equations are reduced to a four-dimensional problem. The main advantage of this minimal approach is to emphasize the role of twisted mode and asymmetry of the light-induced molecular reorientation in a manner suitable for a clear physical interpretation. Theoretical results are compared with experiments in the particular case of circularly polarized light beams to show the physical origin of mode competition and of the breakdown of chiral and longitudinal symmetry. The model successfully describes previous experimental studies such as time-dependent three-dimensional molecular dynamics, light-induced stabilized helical reorientation, and in-plane precession regime in achiral nematics. While a recent experiment has revealed a new spatiotemporal transition, the model succeeds to describe all the features of such a bifurcation pointing out anew the importance of asymmetry. Finally, the first quantitative description of the appearance of a giant mirrorless optical bistability when twisted reorientation modes are excited is demonstrated. A qualitative physical interpretation is suggested for all these phenomena.

Journal Article↗
PubMed2002

Automatic grapheme processing in the left occipitotemporal cortex.

In a previous event-related fMRI study, we showed that the left occipitotemporal cortex was specifically involved in the abstract categorization of visually presented alphabetic symbols. Here, we duplicate the fMRI study by measuring visual event-related potentials to verify whether the left posterior cortex supports the processing of graphemic representations at a perceptual, prelexical level. N170 amplitudes elicited by the categorically ambiguous stimulus 'O' were about twice as small in the left as in the right occipitotemporal region, and comparable to that of other letters on the left, and to that of other geometric figures on the right. The side asymmetry suggests that a graphemic module is unilaterally implemented in the left hemisphere, where it automatically processes heteromorphic representations that do not depend on the physical characteristics of the signal, whereas the difference in amplitude suggests that this left graphemic module is in series with, precedes and does not transmit information to, or inhibits a general shape-analysis module.

Adult↗
PubMed2002

Targeting unknown and unstable periodic orbits.

We present a method to target and subsequently control (if necessary) orbits of specified period but otherwise unknown stability and position. For complex systems where the dynamics is often mixed [e.g., coexistence of regular and chaotic regions in area-preserving (Hamiltonian) systems], this targeting algorithm offers a way to not only gently bring the system from the chaotic domain to an unstable periodic orbit (where control is applied), but also to access stable regions of phase space (where control is not necessary) from within the stochastic regions. The technique is quite general and applies equally well to dissipative or conservative discrete maps and continuous flows.

Journal Article↗
PubMed2000

Information processing in large-scale cerebral networks: the causal connectivity approach.

Today, cognitive functions are considered to be the offspring of the activity of large-scale networks of functionally interconnected cerebral regions. The interpretation of cerebral activation data provided by functional imaging has therefore recently moved to the search for the effective connectivity of activated regions, which aims at understanding the role of anatomical links in the activation propagation. Our assumption is that only causal connectivity can offer a real understanding of the links between brain and mind. Causal connectivity is based on the anatomical connection pattern, the information processing within cerebral regions and the causal influences that connected regions exert on each other. In our approach, the information processing within a region is implemented by a causal network of functional primitives, which are the interpretation of integrated biological properties. Our choice of a qualitative representation of information reflects the fact that cerebral activation data are only the approximate view, provided by imaging techniques, of the real cerebral activity. This explicit modeling approach allows the formulation and the simulation of functional and physiological assumptions about activation data. Two alternative models explaining results of the striate cortex activation described by Fox and Raichle (Fox PT, Raichle ME (1984) J. Neurophysiol 51:1109-1120; Fox PT, Raichle ME (1985) Ann Neurol 17:303-305) are provided as an example of our approach.

Animals↗
PubMed1999

ERP correlates of phoneme perception in speech and sound contexts.

To address the question of the existence of a phonetic module for speech perception, event-related potentials were recorded using a 32 channel system in subjects performing a detection task where the target was the ambiguous, noise-like phoneme /f/ presented either among syllables (speech context) or among environmental sounds (non-speech context). Significant context effects were observed on the N2/P3 complex elicited by the target. In particular, a well localized N2b (250-280 ms) appeared at the left temporoparietal sites on the difference wave between contexts as the result of an enhanced negativity when the target was presented among non-speech stimuli. These findings suggest the involvement of the left temporoparietal region in autonomous, modular processes of speech perception.

Acoustic Stimulation↗
PubMed1999

Differential fMRI responses in the left posterior superior temporal gyrus and left supramarginal gyrus to habituation and change detection in syllables and tones.

Using a habituation-recovery paradigm adapted to functional magnetic resonance imaging, we investigated the brain responses to syllables and tones in six right-handed male subjects. We opposed a standard condition (STD) in which the subjects were listening to homogeneous sequences of four identical stimuli, to a deviant condition (DEV) in which the fourth stimulus of the sequence differed in pitch or spectral content for tones and in the initial stop consonant for syllables. The corresponding runs alternated four rest periods with two STD and two DEV conditions. In addition to a marked rightward asymmetry in the primary and secondary auditory cortex for tones and a right inferior frontal activation for the tone condition where the deviant had increased spectral content, the experiment revealed differential activations in the left posterior superior temporal gyrus and in the left supramarginal gyrus. Activations within the left posterior superior temporal gyrus were observed for the DEV condition with tones and for the STD and DEV conditions with syllables. Activation within the inferior part of the left supramarginal gyrus was only observed for the DEV condition with syllables. The analysis of the decreases and increases in the BOLD signal across the STD, DEV, and rest conditions suggests that the left posterior superior temporal gyrus is implicated in the preattentive change detection of acoustic changes in speech as well as nonspeech stimuli, whereas the left supramarginal gyrus is more specifically engaged in the detection of changes in phonological units.

Adult↗
PubMed1999

Hemispheric preponderance in categorical and coordinate visual processes.

Two experiments were conducted to address whether a left hemispheric bias would be observed for categorical processing in both 'what' and 'where' systems (experiment 1) while a reverse bias would characterize coordinate processing whatever the systems (experiment 2). Young normal subjects were tested using divided visual field tasks. The results of experiment 1 indicated that subjects made categorical judgments in both what and where systems faster when stimuli are presented to the left hemisphere. The results of experiment 2 showed a significant interaction between visual field and difficulty of processing coordinate relationships. Indeed, a left-hemisphere advantage was observed when the task required easy processing whereas a right-hemisphere advantage was noted for difficult distinctions either in location (where system) or in lightness (what system). The left-hemisphere advantage we observed for categorization in both systems confirms the Kosslyn's hypothesis (1989) for the where system and suggests that the same left-hemisphere advantage also exists for the what system. Concerning coordinate processing, our findings highlight the influence of processing difficulty on the hemispheric lateralization and evidence a right hemispheric advantage for difficult coordinate processing and a left hemispheric advantage for easy coordinate processing. The results are discussed in terms of possible link between on the one hand difficulty and coordinate processing, and easiness and categorization on the other hand.

Adult↗
PubMed1998

ERP mapping in phonological and lexical semantic monitoring tasks: A study complementing previous PET results.

Previous PET results identified distinct neural systems involved with phonology (vicinity of the left sylvian fissure) and lexical semantics (left inferior temporal, left superior frontal, bilateral inferior parietal regions). In the time domain, the phonological task was thought to involve serial parsing of pseudo-words, whereas the lexical semantic task would correspond to probabilistic automated access to meaning. Auditory event-related potentials (ERPs) elicited by the same tasks were explored on 32 channels in 12 male volunteers. Subjects had to categorize as target a second element of a pair of pseudo-words or words if a preceding target was detected in the first element. Depending on the absence/presence of target in the first element, a RELEASE condition and a HOLD condition were distinguished. RELEASE and HOLD ERPs split earlier in the semantic task than in the phonological task (300 versus 412 ms after SOT, respectively), although words lasted longer than pseudo-words. Corresponding paired t test maps showed a predominance of differences over left perisylvian regions for the phonological task and posterior bilateral regions for the semantic task. Underlying generators were investigated using BESA (Scherg, 1990) with global task ERPs. Six dipoles-constrained according to PET clusters-brought residual variance down to 0.36%, from 364 to 565 ms after SOT, in both tasks. Relative dipole amplitudes suggested a left-sided functional asymmetry for phonology. These results support the hypothesis of left perisylvian serial processing for phonology contrasting with bihemispheric parallel access for semantics and substantiates BESA for temporally tackling cognitive processes.

Adult↗
PubMed1998

Event-related potentials elicited by passive movements in humans: characterization, source analysis, and comparison to fMRI.

Cortical areas responsive to proprioceptive stimulation were assessed by ERP technique in normals and in selected patients with stroke and were compared to fMRI data. Repetitive extension of right and left forefinger elicited a P1/N1/P2 complex wave pattern. This pattern was absent in patient with complete sensory loss and present but spatially modified in patient with recovered sensory deficit. Source localization with a simple model showed three sources starting in the contralateral rolandic area (SI), then involving the inferior parietal lobe unilaterally and the supplementary motor area (10 to 134 ms). It was followed by a bilaterally distributed pattern of two sources located in the ipsilateral parietal region and in the contralateral insula. Right and left stimulation led to very symmetrical patterns. Comparison to fMRI obtained from passive extension of the wrist in normals showed very compatible data. We described in this paper, a sequential processing of proprioceptive inputs after passive movements involving primary and secondary sensory motor areas.

Adult↗
PubMed1995

Mean-field equations, bifurcation map and chaos in discrete time, continuous state, random neural networks.

The dynamical behaviour of a very general model of neural networks with random asymmetric synaptic weights is investigated in the presence of random thresholds. Using mean-field equations, the bifurcations of the fixed points and the change of regime when varying control parameters are established. Different areas with various regimes are defined in the parameter space. Chaos arises generically by a quasi-periodicity route.

Models, Neurological↗
PubMed1992

On the existence and the role of chaotic processes in the nervous system.

Chaos theory is a rapidly growing field. As a technical term, "chaos" refers to deterministic but unpredictable processes being sensitively dependent upon initial conditions. Neurobiological models and experimental results are very complicated and some research groups have tried to pursue the "neuronal chaos". Babloyantz's group has studied the fractal dimension (d) of electroencephalograms (EEG) in various physiological and pathological states. From deep sleep (d = 4) to full awakening (d > 8), a hierarchy of "strange" attractors paralles the hierarchy of states of consciousness. In epilepsy (petit mal), despite the turbulent aspect of a seizure, the attractor dimension was near to 2. In Creutzfeld-Jacob disease, the regular EEG activity corresponded to an attractor dimension less than the one measured in deep sleep. Is it healthy to be chaotic? An "active desynchronisation" could be favourable to a physiological system. Rapp's group reported variations of fractal dimension according to particular tasks. During a mental arithmetic task, this dimension increased. In another task, a P300 fractal index decreased when a target was identified. It is clear that the EEG is not representing noise. Its underlying dynamics depends on only a few degrees of freedom despite yet it is difficult to compute accurately the relevant parameters. What is the cognitive role of such a chaotic dynamics? Freeman has studied the olfactory bulb in rabbits and rats for 15 years. Multi-electrode recordings of a few mm2 showed a chaotic hierarchy from deep anaesthesia to alert state. When an animal identified a previously learned odour, the fractal dimension of the dynamics dropped off (near limit cycles).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

[Formal and semantic lexical evocation in normal subjects. Performance and dynamics of production as a function of sex, age and educational level].

A protocol of formal lexical evocation (words beginning with a letter) and semantic evocation was applied to 168 normal subjects evenly distributed on the basis of three factors (sex, three age classes and two levels of education). Correct answers, their distribution within the allotted time (proportions of correct answers in four thirty-second periods) and errors were analysed globally and in relation to the said factors. Level of education had a decisive influence in all tests; age had no influence on performance in formal evocation, whereas the subjects in the middle age class presented the best performance in semantic evocation. Distribution of the answers over time was unrelated to any of the factors considered. Errors were related to age in half the test.

Adult↗
PubMed1990

[Standardization of a modular and hierarchic cognitive evaluation scale applicable to dementia. A French version of the Hierarchic Dementia Scale].

We tested a revised version of the Hierarchic Dementia Scale (HDS), proposed by Cole and Dastoor (1980), in order to improve its clinical usefulness and to enrich our knowledge about ageing. The scale was built with 20 subtests which covered the entire range of cognitive and motor functions. Each subtest was hierarchically organized so that success in a item implied success in inferior items. This hierarchical principle was time-saving and was validated by Cole and Dastoor. 149 control subjects performed this test. They were equally divided in 4 age-groups (55-64, 65-74, 75-84, 85-97) and 2 educational levels. None of these subjects had previous history of somatic or neuropsychiatric disease. They were completely self-sufficient in daily life. A large part of the controls failed in the most difficult items of some subtests: Learning, Calculation, Mental Control, Drawing, Recall, Similarities, Constructional Praxis. For these subtests, significantly different mean-scores were observed between age-groups and educational levels. However, the influence of each factor was variable from one subtest to another. Moreover, subgroups seem to exist in our population according to specific difficulties in some of these subtests. This study calls for caution in the interpretation of results in demented patients. Comparisons with other psychometric tools remain to be performed. This scale seems to be more useful for the quantification and follow-up of cognitive deficits than for the early diagnosis of dementia. In addition, this scale, which briefly explores many aspects of cognitive functions, seems especially useful to approach the heterogeneity of DAT.

Aged↗
PubMed1989

Abnormal ocular movements in Parkinson's disease. Evidence for involvement of dopaminergic systems.

Quantitated automated electro-oculographic data from 45 parkinsonian patients were compared with those from 30 normal control subjects. Patients were selected with idiopathic Parkinson's disease without other associated neurological disease or dementia; 20 had never received antiparkinsonian drugs and in 25 such treatment had been stopped for at least 2 days. Saccade latency, amplitude and peak velocity, smooth pursuit peak velocity, optokinetic nystagmus (OKN) maximal and mean velocities and vestibulo-ocular reflex (VOR) suppression by vision or imagination were significantly altered in patients, whereas VOR gain in darkness was normal. Alterations of saccade latency and smooth pursuit peak velocity were more severe in the more advanced stages of the disease and saccade latency directed towards the symptomatic side was slightly delayed in hemiparkinsonian patients. Saccade amplitude improved 90 min after a single oral dose of L-DOPA. These results suggest a possible dopaminergic control of some ocular movements.

Adult↗
PubMed1988

Trends in mortality from cerebrovascular disease in France from 1968 to 1978. With reference to cardiovascular and all causes of death.

Cerebrovascular disease mortality in France during the period 1968-1978 was compared with cardiovascular and all other causes of death. Our study demonstrated a 25% decline in the age-adjusted cerebrovascular disease mortality rates in both sexes and particularly in the middle-aged groups. This decline is greater than that of the general causes of mortality, which was on the order of 20%. It parallels the decline in congestive heart failure mortality but differs from that of ischemic heart disease. Nevertheless, despite the increase in the proportion of elderly people in the population, the total number of deaths from cerebrovascular diseases has remained almost unchanged, although the deaths occur at a higher age than previously. If this trend is confirmed, cerebrovascular diseases will remain one of the more frequent causes of death in the elderly and thus a social problem of crucial importance.

Age Factors↗
PubMed1987

Levodopa-induced regional cerebral blood flow changes in normal volunteers and patients with Parkinson's disease. Lack of correlation with clinical or neuropsychological improvements.

Single photon emission computed tomography was used to measure regional cerebral blood flow in six normal subjects and 12 patients with Parkinson's disease, before and after acute oral administration of levodopa. The drug induced a significant increase in cerebral blood flow both in controls and patients. Before levodopa, there was no significant difference between the groups, either in flow values or in their pattern. The clinical effects of levodopa were not related to the hemodynamic changes. The results suggest that measuring the flow response to levodopa is not appropriate to demonstrate variations in central dopaminergic receptor sensitivity in man.

Administration, Oral↗
PubMed1986

Mortality from stroke in France 1968-1982.

Mortality data collected from 1968 to 1982 were screened in order to identify all death certificates listing stroke as underlying cause of death. Annual age- and sex-specific death rates were calculated using population data obtained from the 1968, 1975 and 1982 census of the French population. Data showed that stroke mortality has been declining by more than 30%, in most age groups both in men and women. There was a slight trend for this decline to be greater between 50 and 75 years of age in both sexes. A small increase was observed in women under 40 years of age. Large and stable variations in stroke mortality were found within France.

Adult↗