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Biomedical subjects

R Ragot

Publications and source records attributed to R Ragot.

At least 19 recordsLinked to original sources
PubMed2000

Role of frontal cortex in memory for duration: an event-related potential study in humans.

Event-related potentials were recorded in order to determine how brain activity is lateralized during the encoding and the recognition of visual durations (700 and 2500 ms ranges). It is assumed in the Hemispheric Encoding Retrieval Asymmetry model that the encoding of words, faces and odours involves left frontal areas whereas their recognition involves right frontal areas. The present results indicate that, for temporal information, the hemispheric bias is different: a negativity developed over right frontal electrodes for both encoding and recognition, and for both duration ranges. Thus, the involvement of right frontal areas appears critical for time perception. Conversely to what was expected, contingent negative variation during recognition was large over both left and right frontal electrodes. These results suggest that the involvement of both hemispheres is necessary for recognition of temporal information.

Adult↗
PubMed2000

Perception of complex sounds: N1 latency codes pitch and topography codes spectra.

OBJECTIVES: This work aimed to find out whether the human cortical 'tonotopy' represents the true fundamental frequency (Fo) of complex sounds, or the center frequency CF at which harmonics peak in the audio spectrum. Indeed, complex periodic sounds (such as those of the human voice, musical instruments, etc.) comprise a 'fundamental component' (Fo) and its 'harmonics' (2Fo, 3Fo, ...nFo). These often peak around a certain frequency CF. As Fo and CF are confounded in pure (sinusoidal) tones, the question of whether Fo or CF is represented through tonotopy had been hitherto unresolved. METHODS: Whole-head recordings of brain electrical activity were obtained for 16 subjects submitted to an array of 9 different series of sounds (3 Fox3 CF). Electrophysiological data were analyzed separately for each sound and each subject with brain functional imaging and dipole reconstruction. RESULTS: Equivalent dipole sources of N1 components were, significantly for all subjects, more and more frontally oriented as CF increased, independently of Fo. CONCLUSIONS: Sounds are mapped in both the right and the left primary auditory cortices according to the spectral profiles of their harmonics (CF), rather than their fundamental frequencies (Fo).

Adult↗
PubMed1999

Automatic attentional shifts induced by a noradrenergic drug in Alzheimer's disease: evidence from evoked potentials.

Prior research showed that attentional deficits are observed in Alzheimer's disease (AD). These deficits can further impair other cognitive processes. The present experiment was designed to study the shifts in attention induced by a noradrenergic drug (S 12024-2) through their electrophysiological correlates in 12 outpatients with mild AD, using an auditory oddball paradigm. The P3a component, known to be related to automatic attentional processing, was increased by the drug, whereas no changes occurred either in PN or in P3b, which are considered to reflect conscious processing. These results point to an involvement of the noradrenergic system in the modulation of automatic attentional processing, and provide evidence for weakening of the orienting reflex in AD, due to a possible noradrenergic deficit in patients with mild AD.

Adrenergic Agents↗
PubMed1998

A dual mechanism for sound pitch perception: new evidence from brain electrophysiology.

The N1 component of the auditory brain evoked potential was used as a direct electrophysiological index, to study how the perception of pitch for complex periodic tones can be influenced by the phase of the harmonics that make up a sound, according to their ranks. Results showed that in the low pitch and/or high spectra region, N1 latency was modified by phase, whereas it was not in the high pitch and low spectra region. These data help bring evidence towards a dual mechanism for pitch perception, and demonstrate that these processes take place within 110 ms following stimulus onset. Results are compatible with a model based on the resolvability of components and on the bandwidth of auditory filters.

Acoustic Stimulation↗
PubMed1996

Heterogeneity of information-processing alterations according to dimensions of depression: an event-related potentials study.

To identify alterations in elementary cognitive operations according to dimensions of depression, two stages of information processing, namely the response choice and the motor preparation stages, were explored using an event-related potential paradigm in two subgroups of depressed patients (retarded and blunted affect versus anxious-agitated and impulsive) compared to controls. Two results are common to all depressed patients: a slow encoding of stimuli (P1 wave) and a prolonged processing of stimulus-response compatibility (after P3b). This is compensated by a global velocity increase in stimulus evaluation or decision making (P3b) in anxious-agitated patients or, on the contrary, cumulated with its velocity decrease in retarded-blunted-affect patients. Such results could provide an explanation for the massive retardation observed in blunted-affect patients, contrary to anxious-agitated patients, whose normal reaction times may come from a very high energetical involvement at the P3b level. Results as a whole suggest that impairments in blunted-affect patients concern effort mechanisms, whereas those in anxious-agitated patients concern perceptual processes.

Adult↗
PubMed1996

Brain potentials as objective indexes of auditory pitch extraction from harmonics.

Complex tones such as vowels expressed by the human voice vary both in fundamental pitch (harmonic periodicity) and spectrum shape (vowel type). Three different pitches and spectra were combined into nine auditory stimuli. Latencies of cerebral evoked potential components decreased significantly as pitch increased, whereas they remained constant with spectrum shape. Because the fundamental (direct clue about pitch value) was purposely omitted from stimuli, these results reflect feature extraction processes. They also demonstrate that brain potentials can be used as objective indexes of perceived pitch.

Adult↗
PubMed1994

Mental processing during reactions toward and away from a stimulus: an ERP analysis of auditory congruence and S-R compatibility.

In its first usage, stimulus-response (S-R) compatibility defined strict S-R relationships. The later use of more complex two-dimensional stimuli led to the formulation of the Simon effect as a tendency to respond toward the side of stimulation. Ragot and Guiard (1992, European Journal of Cognitive Psychology, 4, 219-232) observed an inversion of the Simon effect in the auditory modality. The present study was undertaken to present a more thorough observation of this seemingly paradoxical inversion through the use of brain event-related potentials (ERPs). Two experimental variables were manipulated: (a) stimulus congruence, expressing the degree of correspondence between two randomly varying conflictual or nonconflictual attributes of the stimulus (spoken word droite or gauche presented to the right or left ear), and (b) S-R compatibility as such, expressing the relationship between the relevant stimulus attribute (word) and the responding hand. Results show that the nature of the spoken word acts on N100 latency and is therefore the first stimulus attribute to be processed. Data on P300 indicate that S-R compatibility is the last variable to be taken into account. The observed results are integrated into a cascade/parallel model.

Adult↗
PubMed1993

Negative necrotaxis.

We studied necrotaxis in several strains of protists and compared the reaction of living cells in the vicinity of cells killed by a ruby laser. Negative necrotaxis was observed for the unicellular green alga Euglena gracilis, whereas Chlamydomonas was shown to exhibit positive necrotaxis. The cellular colony Pandorina morum exhibited no reaction to the killing of nearby colonies. Both the colorless cryptomonad Chilomonas paramecium and the ciliate Tetrahymena pyriformis exhibited negative necrotaxis following the lysis of vitally stained specimens of their own species. They also exhibited negative necrotaxis following the lysis of Euglena cells. It was also demonstrated that the cellular content of Euglena cells lysed by heat or by a mechanical procedure acts as a repellent to intact Euglena cells. These results suggest that the negative necrotaxis provoked in Euglena by the laser irradiation is probably due to the chemotactic effect produced by the release of cell content in the extracellular medium. This cell content could, according to its chemical composition, act either as a repellent, an attractant, or be inactive. The sensitivity of cells (specific or nonspecific ion channels or chemoreceptors) are also of prime importance in the process.

Animals↗
PubMed1992

Effect of target position on the sequential organization of processing stages.

In order to study the organization of processing stages taking place in a choice reaction time (RT) situation, the three following experimental variables were manipulated: (1) S-R compatibility; (2) movement direction; and (3) time uncertainty, these variables being known to influence distinct information processing stages. Both behavioural (RT and movement time, MT) and electrophysiological (N200 and P300 latencies) indices of processing time were analysed. All three experimental variables showed significant, additive effects on RT. Only the preparatory period had a significant effect on N200 and on P300 latencies. These results support the construction of a serial model. Furthermore, the analysis of the time intervals between electrophysiological and behavioural indices allows one to infer the relative order of the different stages in this model, as follows: motor pre-initiation, motor pre-programming, response selection and programming, and motor initiation.

Adult↗
PubMed1988

[Changes in various activation indices as a function of the value given by a stimulus in anhedonic and deprossogenic subjects].

The aim of this experiment was to investigate, in "normal" subjects, the relationship between personality characteristics (anhedonia and depressogenic attitudes) and various tonic and phasic activation indices (EEG power spectra, CNV (at Fz and Cz), heart rate and reaction time) recorded during auditory stimulations, and the influence of the affective value of stimuli on phasic activation indices. Eighteen subjects were divided into two opposite groups according to their scores on two self-rating questionnaires (the Chapman Physical Anhedonia Scale and the Beck-Weissman Dysfunctional Attitude Scale): the A group (anhedonic with depressogenic attitudes) and the H group (hedonic and non-depressogenic). The experiment was divided into three phases. The first and third phase utilised an identical classical CNV paradigm. During the second phase, two of the three neutral warning tones of the first phase were given, by conditioning, a positive or a negative value. The results showed that: 1) before conditioning, when all stimuli were neutral, all activation indices (tonic and phasic) were significantly higher in the A group than in the H group; 2) after conditioning, the two groups differed mainly by their cortical reactivity to the positively conditioned sound: the amplitude of the two CNV components increased in the H group whereas a tendency to a CNV decrease was seen in the A group; 3) all the between-group CNV differences were significant only at Fz. These results were discussed in terms of differences of optimal level of activation.

Acoustic Stimulation↗
PubMed1987

Electrophysiological changes elicited by auditory stimuli given a positive or negative value: a study comparing anhedonic with hedonic subjects.

The present experiment investigates in 'normal' subjects the relationship between personality characteristics (anhedonia versus hedonia) and the influence of the affective value of acoustic stimuli (positive, negative, neutral) on various electrophysiological indices reflecting either tonic activation or phasic arousal (EEG power spectra, contingent negative variation: CNV, heart rate, skin potential responses: SPR) as well as on behavioural indices (reaction time: RT). Eighteen subjects were divided into two groups according to their scores at two self-rating questionnaires, the Chapman's Physical Anhedonia Scale (PAS) and the Beck-Weissman's Dysfunctional Attitude Scale (DAS) that quantifies cognitive distortions presumed to constitute high risk for depression: 9 with high scores at both scales formed the A group (Anhedonic-dysfunctional), 9 with low scores at both scales, the H group (Hedonic-adapted) The electrophysiological indices were recorded during 3 situations: the first one was a classical CNV paradigm with a motor reaction time task in which one of 3 tones of different pitch represented the warning stimulus S1; during the second, conditioning phase, two of these tones were associated with either a success (and reward) or a failure (and punishment) during a memory task in order to make them acquire either a positive or a negative affective value; the third situation consisted in the repeating of the first CNV paradigm in order to test the effect of the positive and the negative stimuli versus the neutral one on RTs and electrophysiological data. Significant between-group differences were found regarding tonic activation as well as phasic arousal indices from the very beginning of the experiment when all stimuli were neutral ones, the anhedonics exhibiting higher activation and arousal than the hedonics at the cortical (increased CNV amplitude, increased power in the beta frequency band), cardiovascular (higher heart rate habituating more slowly) and behavioural (faster RTs) levels. Significant between-group differences were also found concerning reactivity to affective stimuli during the third situation: both the orienting response (but only at the cortical level: early CNV) and the motor preparation processes (late CNV) were in the A group significantly less reactive to affective stimuli (especially to the positive one) than in the H group, in particular concerning the frontal (Fz) data.

Adult↗
PubMed1982

Onset and offset of brain events as indices of mental chronometry.

Analysis of single-trial electroencephalogram waveforms in a reaction time task demonstrated that the onset and offset values of event-related potentials can be used as indices of the duration of information processing. Two negative waves have been identified which peak at different times in different regions of the scalp, with the second overlapping the last part of the first. These waves are related in different ways to the duration of perceptual processing.

Brain↗
PubMed1982

Ideomotor apraxia and P300: a preliminary study.

Event-related cerebral potentials and reaction time were studied in two patients with life parietal lesions inducing ideomotor apraxia and in seven control subjects. Results showed no marked increase of P300 latency in the patients with respect to the control group, whereas the RT was largely lengthened. This suggests that the decision-making processes are left intact in IMA and that only the motor execution stage is impaired.

Adult↗