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Biomedical subjects

J F Echallier

Publications and source records attributed to J F Echallier.

At least 19 recordsLinked to original sources
PubMed1995

Improved forward EEG calculations using local mesh refinement of realistic head geometries.

A method for semi-automatically constructing realistic surface meshes of 3 head structures--scalp, skull and brain--from a stack of MR images is described. Then an evaluation is given for both spherical and realistic dipolar models, using the boundary element method (BEM). In both cases, locally refined models were considered. Two characteristic mesh parameters were defined: the global and the local mesh densities (in triangles per cm2). In spherical geometries, numerical and analytical solutions were compared, and in the realistic case, all models were compared to a highly refined one, considered as a reference. Both geometries gave comparable results. It was found that for "deep dipoles" located at more than 20-30 mm under the brain surface, meshes with a global density of 0.5 tri/cm2 gave "acceptable" results, whereas for more superficial dipoles (2-3 mm < depth < 20-30 mm), it was necessary to locally refine meshes near the source location up to a local density of about 5-8 tri/cm2, to get comparable results.

Electroencephalography↗
PubMed1995

Tonotopic organization of the human auditory cortex: N100 topography and multiple dipole model analysis.

The tonotopic organization of the human auditory cortex has been investigated by means of scalp potential mapping and dipole modelling of the evoked response occurring around 100 msec after the stimulus onset. The major characteristics of the topographical changes observed with increasing stimulus frequency were statistically demonstrated. Using a 3-concentric sphere head model, the scalp potential distributions can be explained in first approximation by two equivalent current dipoles, located in the supratemporal plane and mimicking the activity of both auditory cortices. To take into account the temporal aspects of the brain activities, 3 time-varying dipole strategies were tested. Frequency dependence of the dipole orientation has been evidenced in both hemispheres with the 3 models, whereas no significant change in dipole position was found. The tilt in dipole orientation could be related to the folding geometry of Heschl's gyrus, which varies with depth. In agreement with previous MEG findings, this brings new evidence for a tonotopic organization of the auditory cortical area involved in the N100 wave generation. Moreover, distinct frequency dependences of the equivalent current dipoles were observed in the early and the late parts of the N100. This study demonstrates that simple dipolar models, applied on electrical data, make it possible to reveal functionally distinct cortical areas.

Adult↗
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↗
PubMed1992

Computer-assisted placement of electrodes on the human head.

A system has been studied with 3 purposes: digitization of the head and mathematical representation of the scalp surface, assistance for electrode placement, and digitization of the exact 3-D position of each electrode after placement. The system has been validated in several ways, mainly by comparing the electrode locations obtained using the classical manual procedure based on the international 10-20 system of electrode placement, and through the assisted procedure based on the described system. The main result is improved reproducibility of the assisted procedure which is 3 times better than in the manual procedure.

Brain Mapping↗
PubMed1989

Spherical splines for scalp potential and current density mapping.

Description of mapping methods using spherical splines, both to interpolate scalp potentials (SPs), and to approximate scalp current densities (SCDs). Compared to a previously published method using thin plate splines, the advantages are a very simple derivation of the SCD approximation, faster computing times, and greater accuracy in areas with few electrodes.

Computer Simulation↗
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↗
PubMed1981

[Early somatosensory evoked potentials: a means of investigation of the lemniscal pathways (author's transl)].

The recording of the somatosensory evoked potentials (SEP's) elicited by stimulation of the median nerve is a non painful and non invasive mean to investigate the function of the different levels of the lemniscal pathways: peripheral nerves, dorsal root ganglia, dorsal funiculi, brainstem, thalamus, and somesthetic cortex. Recording technique, normative data in adults and abnormalities due to neurological diseases are reviewed. This investigation appears to be of peculiar interest in lesions of dorsal roots, cervical spinal cord and brainstem. SEP's recording may also be helpful for the functional testing of the somesthetic pathways in patients with disorders of consciousness.

Adolescent↗
PubMed1980

[A minicomputer system that extracts evoked responses from the E.E.G].

EVøQ is a minicomputer system that enables one to extract average evoked responses from the E.E.G. from a large number of analog channels, and can, therefore, be oriented towards a topographic study of the responses. It allows high-frequency sampling of the signal, in order to make possible a study of the brain stem evoked responses. This system consists of three programs: a configuration-editor which allows a pre-configuration of several kinds of experiments; an acquisition program, of monitoring, calibration, signal processing and automatic control of the stimulators; finally, a management and processing program of the resulting files.

Brain Stem↗
PubMed1979

Optimal response of eye and hand motor systems in pointing at a visual target. I. Spatio-temporal characteristics of eye and hand movements and their relationships when varying the amount of visual information.

In a task requiring an optimal hand pointing (with regards to both time and accuracy) at a peripheral target, there is first a saccade of the eye within 250 ms, followed 100 ms later by the hand movement. However the latency of the hand movement is poorly correlated with that of the eye movement. When the peripheral target is cut off at the onset of the saccade, there is no correlation between the error of the gaze position and the error of the hand pointing. This suggests an early parallel processing of the two motor outputs. The duration of hand movement does not change significantly when subjects either see or not see their hand (closed or open loop). In the open loop situation, the undershoot of the hand pointing increases with target eccentricity, whatever the subjects are allowed or not to do a saccade toward the target. It suggests that the encoding of eye position by itself is a poor index for an accurately guided movement of the hand.

Adult↗