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M H Giard

Publications and source records attributed to M H Giard.

At least 19 recordsLinked to original sources
PubMed1994

Auditory selective attention in the human cochlea.

According to current theories, auditory selective attention alters the sensory analysis of acoustic inputs only in the central auditory system. Despite numerous attempts, no evidence of attentional selection has been found in the auditory periphery. Measurements of evoked otoacoustic emissions (EOAEs) during a selective dichotic listening task showed that the EOAEs to tones in one ear had larger amplitude when attention was directed to this ear than when attention was directed to the opposite ear. The results indicate that genuine effects of auditory selective attention can be observed at the cochlear receptor.

Acoustic Stimulation↗
PubMed1994

Dissociation of temporal and frontal components in the human auditory N1 wave: a scalp current density and dipole model analysis.

This study reports a combined scalp current density (SCD) and dipole model analysis of the N1 wave of the auditory event-related potentials evoked by 1 kHz tone bursts delivered every second. The SCD distributions revealed: (i) a sink and a source of current reversing in polarity at the inferotemporal level of each hemiscalp, compatible with neural generators in and around the supratemporal plane of the auditory cortex, as previously reported; and (ii) bilateral current sinks over frontal areas. Consistently, dynamic dipole model analysis showed that generators in and outside the auditory cortex are necessary to account for the observed current fields between 65 and 140 msec post stimulus. The frontal currents could originate from the motor cortex, the supplementary motor area and/or the cingulate gyrus. The dissociation of an exogenous, obligatory frontal component from the sensory-specific response in the auditory N1 suggests that parallel processes served by distinct neural systems are activated during acoustic stimulation. Implications for recent models of auditory processing are discussed.

Adult↗
PubMed1994

Two separate frontal components in the N1 wave of the human auditory evoked response.

Scalp current density analysis of the auditory evoked response to 1-kHz tone bursts delivered at various interstimulus intervals (ISIs) (from 1 s to 2 min in separate runs) shows that two different frontal components can be observed and functionally dissociated in the N1 time range: one is elicited for all ISIs, peaks at about 95 ms poststimulus, and has a full recovery time below 8 s; the second is elicited only by infrequent stimuli (ISIs > 4 s), peaks around 140 ms, and significantly increases in amplitude with increasing ISIs. The first component can be considered a new obligatory component in N1 elicited simultaneously with the responses in auditory cortex; the later component could correspond to the orienting Component III of Näätänen and Picton (1987).

Acoustic Stimulation↗
PubMed1990

Precautions in topographic mapping and in evoked potential map reading.

First, we consider the main points that must be addressed when constructing topographic maps: types of projection, methods of interpolation, number and locations of recording electrodes, and color scales. Data integrity and precautions in map interpretation are then examined for the case of evoked potential data.

Brain↗
PubMed1990

Brain generators implicated in the processing of auditory stimulus deviance: a topographic event-related potential study.

The neurophysiological mechanisms underlying mismatch negativity (MMN) can be inferred from an examination of some of the brain generators involved in the process of this event-related potential (ERP) component. ERPs were recorded in two studies in which the subjects were involved in a selective dichotic listening task. Subjects were required to silently count rare stimuli deviating in pitch from a sequence of standard stimuli in one ear, while ignoring all the stimuli (standards and deviants) delivered randomly to the other ear. The results showed that, in all cases, the negative wave elicited by the deviant stimuli showed the highest amplitudes over the right hemiscalp irrespective of the ear of stimulation or the direction of attention. Scalp radial current density analysis showed that this asymmetric potential distribution could be attributed to the sum of activities of two sets of neural generators: one temporal, located in the vicinity of the primary auditory cortex, predominantly activated in the hemisphere contralateral to the ear of stimulation, and the other frontal, involving mainly the right hemisphere. The results are discussed in light of Näätänen's model: we suggest the dissociation of two functional processes on the basis of activity of distinct brain areas: a sensory memory mechanism related to the temporal generators, and an automatic attention-switching process related to the frontal generators.

Adult↗
PubMed1988

Several attention-related wave forms in auditory areas: a topographic study.

The purpose of this study was to progress in the understanding of the electrogenesis of attention-related wave forms in order to highlight some of the underlying attentional processes. ERPs were recorded from 16 electrodes, from 12 subjects who attended selectively to either high or low pitch tones delivered at a constant inter-stimulus interval of 800 msec to either right or left ear, while ignoring a concurrent sequence of tones of the other pitch delivered to the other ear. The attention-related wave forms were obtained by subtracting ERPs to unattended tones from ERPs to the same tones when they were attended. These wave forms were topographically displayed by both potential maps and scalp current density maps and compared with the corresponding maps of the N1 component of the ERPs, to determine the similarity of their generators. It has been shown that the attention effect is expressed by at least two components in specific auditory areas, one of small amplitude, occurring during the ascending slope of the N1 component, sensitive to the pitch of the attended stimulus, and possibly originating in the supratemporal plane of the auditory cortex; another of large amplitude, peaking symmetrically over both hemispheres and having a different topography from that of the N1 component. As described by other authors, a third, later, component appears over frontal areas, but probably originates from deeper sources of the brain. Models of selective attention processes, particularly the 'attentional trace' concept, are discussed in the light of these results.

Adult↗
PubMed1988

Electrophysiological evidence for a shared representational medium for visual images and visual percepts.

Does mental imagery involve the activation of representations in the visual system? Systematic effects of imagery on visual signal detection performance have been used to argue that imagery and the perceptual processing of stimuli interact at some common locus of activity (Farah, 1985). However, such a result is neutral with respect to the question of whether the interaction occurs during modality-specific visual processing of the stimulus. If imagery affects stimulus processing at early, modality-specific stages of stimulus representation, this implies that the shared stimulus representations are visual, whereas if imagery affects stimulus processing only at later, amodal stages of stimulus representation, this implies that imagery involves more abstract, postvisual stimulus representations. To distinguish between these two possibilities, we repeated the earlier imagery-perception interaction experiment while recording event-related potentials (ERPs) to stimuli from 16 scalp electrodes. By observing the time course and scalp distribution of the effect of imagery on the ERP to stimuli, we can put constraints on the locus of the shared representations for imagery and perception. An effect of imagery was seen within 200 ms following stimulus presentation, at the latency of the first negative component of the visual ERP, localized at the occipital and posterior temporal regions of the scalp, that is, directly over visual cortex. This finding provides support for the claim that mental images interact with percepts in the visual system proper and hence that mental images are themselves visual representations.

Adult↗
PubMed1987

Mapping of scalp potentials by surface spline interpolation.

Evoked potentials and EEGs record punctate electrical activity at electrode sites. To represent the overall potential distribution on the entire scalp it is necessary to interpolate between these sampled values. Surface splines are mathematical tools for interpolating functions of two variables. In comparison to the classical methods of interpolation, based on linear combination of the potentials of the 4 nearest electrodes, spline methods are smoother, give more precisely located extrema and converge faster toward the 'true' potential surface when the number of recording electrodes is increased. These advantages are at the expense of lengthier computation time.

Brain Mapping↗
PubMed1986

Separate generators with distinct orientations for N20 and P22 somatosensory evoked potentials to finger stimulation?

Sequential spatial maps of scalp potentials, obtained with a 16-channel montage, were used in 12 healthy subjects in order to assess the temporal and spatial distribution of early cortical SEPs to single finger stimulation. It was found that when the contralateral parietal N20 negativity peaks there is a synchronous frontal P20 positivity, supporting the view of a tangentially orientated dipolar generator for this couple of scalp SEPs components. It was not possible to show a distribution of N20 peak on the scalp that would parallel the somatotopic finger representations in area S1; however, the orientation of the putative dipolar source of the N20/P20 complex was found to change according to the finger stimulated. A central P22 component was also constantly obtained without any synchronous negativity on the scalp surface corresponding to the electrode array; a clear somatotopic organisation was found for P22. These features favour the hypothesis that this latter component has a radially orientated generator situated in the prerolandic motor cortex, close to the scalp surface. Because of overlapping between the P20 and P22 components, the determination of P22 onset latency was hazardous in some cases, and spatial mapping was then essential to identify this component. The conclusion that the contralateral parietal N20 and central P22 could be generated by separate dipolar generators with distinct orientations is supported by recent data from combined electrical and magnetic field recording.

Adult↗
PubMed1985

Sequential colour mapping system of brain potentials.

We present a colour mapping system for the visualization of both the spatial scalp distribution and the temporal evolution of brain potentials. The system is applicable to recordings of auditory, visual and somatosensory potentials. It uses a Tektronix 4113 colour terminal connected to a Solar 16 (SEMS) mini-computer. The brain potentials are recorded on up to 16 scalp electrodes. The gain and the baseline are corrected separately on each channel. At each point of the scalp the potential is reconstructed by a linear interpolation of the measured potentials of the four nearest electrodes. Simultaneously n2 (1 less than n less than 8) colour maps can be presented on the screen. This allows the study of the temporal evolution of full scalp evoked potentials. The user chooses the two extreme latencies defining the time window to be explored and the latencies of the maps are regularly time-spaced within this window. In a typical case, in which four maps are desired, the latencies of the maps can be chosen independently. The 16-colour palette is predetermined but the user has three possibilities to establish the correspondence between the electrical potential and the colours. Examples are shown in the visual and somatosensory stimulation modalities. The advantages and limitations of such a representation are discussed.

Brain↗
PubMed1985

[Sequential spatial maps of visual potentials evoked by checkerboard-pattern reversal: effect of the retinal field stimulated on response topography].

Sequential color maps of visual potentials evoked by the reversal of various checkerboard patterns were recorded in 10 young adults using a 16 channel montage. It was found that each of the components of the N75-P100-N145 occipital complex had a specific spatial distribution on the scalp and was selectively influenced by the size, the spatial frequency, the luminance and possibly the wave length of the stimulus. Component N75 was found to be elicited by the more peripheral area of the TV stimulus (12 degrees X 16 degrees). Component P100 was associated with a frontal negativity of similar latency favoring the hypothesis of a dipolar occipital generator. With half-field stimulations the dipole orientation was modified, leading to a 'paradoxical' lateralization of P100 in most cases. However the reverse situation (P100 contralateral to the stimulated half-field) was observed in 4 and 3 subjects out of 10 with left and right half-field stimulations respectively. Thus VEP to full-field TV pattern reversal cannot be recommended to investigate hemianopic patients. Component N145 was of maximal amplitude when elicited by the reversal of small foveal patterns (2.18 degrees), especially red light emitting diodes.

Adult↗
PubMed1985

[Rare cause of myoclonus with giant SEP's: methyl bromide poisoning. Apropos of a case with unilateral predominance].

Giant and asymmetric SEPs were recorded in a patient with predominantly unilateral, spontaneous and intention myoclonus due to voluntary intoxication with methyl bromide as soon as day 3 after intoxication. The N10 Erb's point potential, cervical N13 and scalp recorded P15 potentials were found to be normal in latency, morphology and amplitude. The somesthetic informations could be considered as normally processed up to the subcortical levels of the somatosensory pathways. The parietal cortical potentials N20 and P25 and the frontal cortical potentials P22 and N30, contralateral to myoclonus, were abnormally large. This suggests that myoclonus could be related with an abnormal reactivity of somatomotor and somatosensory cortices to the afferent volleys triggered by voluntary movements. The prerolandic components P22 and N30 were found to be relatively more enhanced than the parietal N20 and P15.

Cerebral Cortex↗
PubMed1984

The temporal component of the auditory evoked potential: a reinterpretation.

The auditory evoked potentials in man cannot be explained by a single source even though a strong influence of the primary areas in the supra-temporal plane has been pointed out in different works. In 26 normal adults we mathematically extracted the greater part of the experimental AEPs explicable by such an origin. The residual part obtained by subtracting this first component from the experimental data is in agreement with an origin in the precentral motor cortex.

Brain Mapping↗