Early signs of visual categorization for biological and non-biological stimuli in humans.

In a previous experiment aimed at studying gender processing from faces, we had found unexpected early ERP differences (45-85 ms) in task-irrelevant stimuli between a condition in which the stimuli of each gender were delivered in separate runs, and a condition in which the stimuli of both genders were mixed. Similar effects were observed with hand stimuli. These early ERP differences were tentatively related to incidental categorization processes between male and female stimuli. The present study was designed to test the robustness of these early effects for faces, and to examine whether similar effects can also be generated between two classes of non-biological stimuli. We replicated the previous findings for faces, and found similar early differential effects (50-65 ms) for non-biological stimuli (grey and hatched geometrical shapes) only, however, when the two shape categories were separated by conspicuous visual characteristics. While these results can partly be explained by phenomena related to neuronal habituation in the visual cortex, they may also suggest the existence of coarse and automatic categorization processes for rapid distinction between two wide classes of stimuli with strong psychosocial significance for humans.

ERP manifestations of processing printed words at different psycholinguistic levels: time course and scalp distribution.

The aim of the present study was to examine the time course and scalp distribution of electrophysiological manifestations of the visual word recognition mechanism. Event-related potentials (ERPs) elicited by visually presented lists of words were recorded while subjects were involved in a series of oddball tasks. The distinction between the designated target and nontarget stimuli was manipulated to induce a different level of processing in each session (visual, phonological/phonetic, phonological/lexical, and semantic). The ERPs of main interest in this study were those elicited by nontarget stimuli. In the visual task the targets were twice as big as the nontargets. Words, pseudowords, strings of consonants, strings of alphanumeric symbols, and strings of forms elicited a sharp negative peak at 170 msec (N170); their distribution was limited to the occipito-temporal sites. For the left hemisphere electrode sites, the N170 was larger for orthographic than for nonorthographic stimuli and vice versa for the right hemisphere. The ERPs elicited by all orthographic stimuli formed a clearly distinct cluster that was different from the ERPs elicited by nonorthographic stimuli. In the phonological/phonetic decision task the targets were words and pseudowords rhyming with the French word vitrail, whereas the nontargets were words, pseudowords, and strings of consonants that did not rhyme with vitrail. The most conspicuous potential was a negative peak at 320 msec, which was similarly elicited by pronounceable stimuli but not by nonpronounceable stimuli. The N320 was bilaterally distributed over the middle temporal lobe and was significantly larger over the left than over the right hemisphere. In the phonological/lexical processing task we compared the ERPs elicited by strings of consonants (among which words were selected), pseudowords (among which words were selected), and by words (among which pseudowords were selected). The most conspicuous potential in these tasks was a negative potential peaking at 350 msec (N350) elicited by phonologically legal but not by phonologically illegal stimuli. The distribution of the N350 was similar to that of the N320, but it was broader and including temporo-parietal areas that were not activated in the "rhyme" task. Finally, in the semantic task the targets were abstract words, and the nontargets were concrete words, pseudowords, and strings of consonants. The negative potential in this task peaked at 450 msec. Unlike the lexical decision, the negative peak in this task significantly distinguished not only between phonologically legal and illegal words but also between meaningful (words) and meaningless (pseudowords) phonologically legal structures. The distribution of the N450 included the areas activated in the lexical decision task but also areas in the fronto-central regions. The present data corroborated the functional neuroanatomy of word recognition systems suggested by other neuroimaging methods and described their timecourse, supporting a cascade-type process that involves different but interconnected neural modules, each responsible for a different level of processing word-related information.

A systematic evaluation of the spherical model accuracy in EEG dipole localization.

This paper presents a study of the intrinsic localization error bias due to the use of a spherical geometry model on EEG simulated data obtained from realistically shaped models. About 2000 dipoles were randomly chosen on the segmented cortex surface of a particular subject. Forward calculations were performed using a uniformly meshed model for each dipole located at a depth greater than 20 mm below the brain surface, and locally refined models were used for shallower dipoles. Inverse calculations were performed using four different spherical models and another uniformly meshed model. It was found that the best spherical model lead to localization errors of 5-6 mm in the upper part of the head, and of 15-25 mm in the lower part. The influence of the number of electrodes upon this intrinsic bias was also studied. It was found that using 32 electrodes instead of 19 improves the localization by 2.7 mm on average, while using 63 instead of 32 electrodes lead to improvements of less than 1 mm. Finally, simulations involving two simultaneously active dipoles (one in the vicinity of each auditory cortex) show localization errors increasing by about 2-3 mm.

Improved dipole localization using local mesh refinement of realistic head geometries: an EEG simulation study.

A systematic evaluation of dipole localization accuracy using the boundary element method is presented. EEG simulations are carried out with dipoles located in the right parietal and temporal regions of the head. Uniformly meshed and locally refined head models are considered in both spherical and realistic geometries. An initial study determines the influence upon the localization accuracy of the dipole depth below the brain surface, of its orientation (radial and tangential), and of the global and local mesh densities. Simulated potential data are computed analytically in the spherical case, and numerically using a very fine (locally refined) model in the realistic case. Results in both geometries show that in order to get localization errors of about 2-4 mm, uniformly meshed models may be used for dipoles located at depths greater than 20 mm, whereas locally refined models should be used for shallower dipoles. Two other studies show how the localization accuracy depends upon the size of the local refinement area and upon the number of electrodes (19, 32, 63). Results show that a large number of electrodes brings significant improvements, especially for shallow and tangential dipoles.

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.

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.

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.

A behavioral and electrophysiological study of the comparison of size-discrepant shapes.

An experiment was performed to find an electrophysiological correlate of the way objects of different sizes are perceived as identically shaped. The subjects were presented with pairs of geometrical figures and were instructed to decide whether the two polygons of a pair were identical in shape regardless of size. In addition to event-related potentials (ERPs), reaction times and error rates were recorded. Reaction times increased approximately linearly with increasing size ratios. The subtraction between the ERPs of 'test' and 'control' conditions showed two main activities: a positivity localized on the occipitotemporal scalp areas in the 200-450 ms range, and a negativity localized on the posterior scalp areas in the 500-750 ms range. These different results were discussed with respect to size ratio.

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.

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.

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.

[Early somatosensory evoked potentials: a means of investigation of the lemniscal pathways (author's transl)]. / Intérêt des potentiels évoqués somesthésiques précoces dans l'exploration des voies de la sensibilité lemniscale.

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.

[A minicomputer system that extracts evoked responses from the E.E.G]. / EVøQ: systeme configurable de stimulation automatique et d'enregistrement des potentiels evoques moyens sur mini-ordinateur.

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.

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.