Nicole Lesèvre

Attention impairment and psychomotor retardation in depressed patients: an event-related potential study

The cerebral event-related potentials (ERPs) and reaction times (RTs) of 8 drug-free depressed in-patients (mean age 58, with marked psychomotor retardation) were recorded during a dichotic listening and choice RT task and compared to those of 9 healthy, age-matched controls. The depressed showed significantly longer, more variable RTs than the controls and made more mistakes. Their ERPs for targets in the attended ear differed significantly from those of the controls by the small amplitude (or absence) of the N2 vertex component elicited by the controls in such a situation and by the presence in the same situation of a late frontal slow negativity (LFN) that did not show up in the grand average ERP of the control group. These results were discussed in terms of the two components of the processing negativity described by Näätänen (1982, 1985) and according to resource models of attention: whereas the controls would perform this easy task quickly thanks to quasi-automatic matching processes (reflected by the vertex N2), the depressed would need further effortful, controlled processing (reflected by the LFN) to perform the task.

The alpha average. I methodology and description

Abstract The spontaneous alpha rhythm has been transformed into its average form, called the “Alpha Average”, in an attempt to reduce it to a simplified picture, statistically significant and general enough to be representative of a given individual or clinical group. This paper aims at introducing and developing this concept of “Alpha Average” from a methodological point of view. After a description of the method used to obtain the average activity, the principal characteristics of the alpha average related to its spatio-temporal organization and its inter-regional relationships are described as they have been observed in normal individuals and a few patients. The alpha average is obtained by selecting all those waves that fall within the bounds of a reference located both in space and time. This reference is the alpha rhythm itself, chosen by a pattern analyser on a given channel always at the same predetermined phase, and kept within the limits of a “time-window”. Each sequence of alpha average is obtained from 100 series of alpha waves collected by 100 openings of the “time-window”. A picture of the alpha average, denoted by the term “typical alpha average” because it has been found very frequently in the normal population studied here, is characterized by: 1. a. A slight slope of the amplitude fall-off of the alpha waves throughout the time-window, which can be interpreted as a narrow dispersion of the frequencies of unaveraged alpha rhythm about the mean frequency (auto-relationships). 2. b. The presence of only one reversal of phase of the gradients (maximum of alpha potential), situated in the longitudinal montage at the parieto-occipital level, and in the transverse montage at the midline (topographical structure). 3. c. Non-random inter-regional and, especially, inter-hemispheric relationships of phase or frequency: this means that a well-developed alpha average can be seen in the regions which have not served as time reference for processing the average (crossed relationships). 4. d. Symmetry of these various properties: this can be seen on comparing the alpha average sequences obtained from a single fragment of unaveraged alpha rhythm used to trigger the averages from each hemisphere in turn. Different forms of non-typical alpha average whose statistical or clinical significance will be described elsewhere have been observed fairly frequently. These are: 1. a. Random inter-hemispheric relationships of phase or frequency (no detectable alpha average being seen from the hemisphere which did not trigger the average). 2. b. Special topographical organizations of a transverse montage: (1) a significant displacement of the line of phase reversal towards one or the other hemisphere; (2) more than one line of phase reversal; (3) no phase reversal between the two hemispheres. The alpha average is discussed in its functional significance as a “global activity”. The various properties of this global type of electrical activity are discussed in relation to the methodological approach and give rise to a mathematical model of the alpha average which is elaborated in another paper.

Potentiels evoques par l'apparition de patterns: effets de la dimension du pattern et de la densite des contrastes

Abstract This work has been performed to elicit the neurophysiological significance of each component of the visual evoked cortical response to checkerboard patterns. The variations in amplitude, polarity and chronotopographic organization of each component of the response have been studied for 8 normal subjects as a function of: checkerboard size (20°, 10°, 5°, 2°30′, 1°20′) and square size (40′, 20′, 10′, 5′). The stimulation was always binocular, the gaze being fixed in the centre of the checkerboard. After analogue-digital conversion averages were made, for each experimental situation, on 65 responses to stimuli given every 1540 msec and lasting 770 msec each. A topographic study was made with chains of bipolar derivations in 2 montages (longitudinal vertex-inion, transverse crossing the midline 2 cm above the inion). The results can be summarized as follows: The response to a 20° checkerboard made of 40′ squares (which serves as “reference response”, being obtained from 24 normal subjects) is made of 4 components: wave 0 (sink peaking at 60 msec), wave 1 (source peaking at 90 msec), wave 2 (sink peaking at 120 msec), wave 3 (long lasting source peaking towards 200 msec). Waves 0, 1 and 2 show phase reversal of gradients (maximum of potential) near the inion; wave 3 has its maximum in the parietal region. The response to a plain field of the same dimension (and of luminance twice as high as that of the checkerboard), is characterized by waves of much smaller amplitude and usually opposite polarity (in particular wave 0 is absent in plain field responses and wave 2 is either absent or replaced by a positive potential). As the checkerboard gets smaller (from 20° to 1°20′) the amplitude of each component gets smaller and the topographic organization of waves 1 and 2 changes, the maximum of potential getting more and more posterior (longitudinal montage) and lateral (transverse montage); the topography of wave 3 (longitudinal) does not change. When the square size changes (from 40′ to 5′), the chronotopographic organization of the response does not change. The amplitude of waves 0,1 and especially wave 2 does change: whatever the checkerboard size, wave 2 (sink peaking around 120 msec) is the largest with 20′ squares and the smallest with 5′ squares (for some subjects wave 2 entirely disappears in the latter case). The part played in this pattern response by the central and peripheral retina and the role of the density of contrasts, which might be related to the size of receptive fields, are discussed.

The alpha average. III. Clinical application in children

Abstract The summation technique embraced by the term “alpha average” was applied to two groups of school children characterized by their clinical features. Group I, consisting of six children with normal or above normal IQ, chosen specifically for their harmonious development and homogeneous scores on verbal, motor and perceptuomotor tests. Group II, consisting of eight children presenting with severe scholastic and extra-scholastic difficulties despite a normal verbal intelligence. Psychometric testing revealed a gross discrepancy in performance between tests of verbal and motor or perceptuo-motor function or both. Three of the children also had poor visual acuity. The spatio-temporal configuration of alpha average in the records of Group I children and “normal” adults did not differ significantly as regards spatial organization, the degree of interhemispheral liaison and amplitude symmetry. The temporal organization did differ slightly: the mean frequency was lower in children and the scatter of frequencies about the mean was greater (at least in comparison with the adults analysed so far). There were, however, major points of difference between the two groups of children which enabled us to establish significant correlations between the psychological profile and spatio-temporal characteristics of the alpha average. Group II children showed: a greater spread of frequencies about the mean; a poorer spatial organization, less symmetrical and less predictable; a smaller degree of inter-hemispheral coordination (statistical liaisons of activity). On the contrary, although a certain degree of amplitude asymmetry of alpha average was present in the majority of subjects, the records of Group I children were more asymmetrical than those of Group II children. Poorly sighted children did not differ markedly from Group II sighted children as regards their alpha average, but did differ as regards their visual evoked responses. It may thus be concluded that electro-clinical correlations are of importance in the following respects: they emphasize the importance of a topographical study of cerebral electrical activity, an approach often neglected in the assessment of the conventional EEG; they show that, although visual evoked potentials are of primary interest in the exploration of visual function, nevertheless a study of the spontaneous activity of association areas is indispensable to a clear understanding of the complex functions of psycho-sensory-motor integration; finally, they give rise to pathophysiological hypotheses; e.g. , that discordant development of symbolic functions in children may be linked with the presence of a defect of cortical inter-regional and interhemispheral coordination during the waking state.

Correct and incorrect responses in a choice reaction time task and the endogenous components of the evoked potential.

Publisher Summary This chapter discusses correct and incorrect responses in a choice reaction time task and the endogenous components of the evoked potential. The results clearly demonstrate that, when the subject has to deal successively with auditory and visual stimuli within one same run, an endogenous negative wave develops and peaks at the same location as that of the modality-specific potential obtained in response to relevant stimuli. These modality-specific negative potentials (MSNPs) are the only ones to be highly correlated with RTs. Some more points must be emphasized that may contribute to the interpretation of the vertex non-specific negative wave observed for visual stimuli: (1) this wave is followed by a P300 and looks like the N2-P3a complex in response to novel irrelevant stimuli, (2) its amplitude increases for unexpected stimuli whatever the modality (P= 0.01 across modalities, sign test), and (3) in the case of incorrect responses, some preliminary results obtained from a trial-by-trial analysis showed two kinds of vertex activity in response to auditory stimuli: the first one looked like the usual AEmP and the second one, of much higher amplitude, is similar to the K-complex described long ago as representing a surprise effect.

The alpha average. II. Quantitative study and the proposition of a theoretical model

Abstract In the spatio-temporal display of alpha average derived from EEG, the configurations obtained depend upon the interplay of a number of properties of the spontaneous activity. These are the mean frequency and its scatter, the mean amplitude, the inter-regional phase relationships and the stability of these relationships. Reduction to an average from constitutes a first step towards the global appreciation of the changing qualities of the spontaneous activity, in that many random events of secondary significance are thereby eliminated. An attempt has been made to define the parameters of the alpha average by indices of spatial symmetry, symmetry of amplitude, frequency stability and global organization. These indices, although based on experimental data, are derived from a theoretical concept of the spatio-temporal properties of the alpha rhythm. The manifestation of typical and reproducible components in the alpha average justifies a physico-mathematical approach to this activity. Hence a model of alpha average was proposed, with the following dual objectives: 1. (1) to explain the alpha average patterns in terms of the properties of the underlying spontaneous activity (which are often difficult to assess in the conventional EEG); 2. (2) to interpret this information by using a limited number of mathematical concepts and the simplest fundamental hypothesis. It is shown that the electrostatic model, composed of a limited number of precisely located charges which evolve in a sinusoidal fashion, fulfils these requirements. Furthermore, a number of typical and atypical observed features of the alpha average can be artificially imitated by varying the parameters of the model.

Hemisphere asymmetry of alpha burst sequential organization in depression

A new quantitative EEG index based on the sequential variability of the frequency of occurrence of alpha bursts (alpha-BVI) was utilized for investigating the respective role of the two hemispheres in depression and their relationship with two clinical dimensions of this illness: psychomotor retardation and blunted affect. The EEG (at P3 and P4 referred to Fz) was recorded during rest periods in two groups of patients selected according to their scores on various clinical scales: one consisted of 12 patients characterized by psychomotor retardation (PMR group), the other of 9 patients characterized by blunted affect (BA group). A control group of 12 normal subjects was recorded in the same conditions. All subjects were dextral. The following main results were obtained: (1) in both groups of patients the right and the left alpha-BVI were, before treatment, significantly lower than those of the controls. (2) In controls, the sequential alpha burst variability was identical on both hemispheres. (3) In patients, before treatment, the right hemisphere alpha-BVI was significantly lower than the left. (4) Electro-clinical correlations were also observed: (A) in the BA group, before treatment, (a) between the degree of blunted affect and the decrease of the right alpha-BVI, (b) between ideoverbal retardation and the decrease of the left alpha-BVI (these correlations disappeared after treatment); (B) in the PMR group, ideoverbal retardation was, on the contrary, correlated to a right alpha-BVI decrease, this correlation persisting after treatment. These results are discussed according to the role of each hemisphere in depression.

Potentials evoked by patterns: Effects of pattern dimensions and contrast density

Abstract This work has been performed to elicit the neurophysiological significance of each component of the visual evoked cortical response to checkerboard patterns. The variations in amplitude, polarity and chronotopographic organization of each component of the response have been studied for 8 normal subjects as a function of: checkerboard size (20°, 10°, 5°, 2°30′, 1°20′) and square size (40′, 20′, 10′, 5′). The stimulation was always binocular, the gaze being fixed in the centre of the checkerboard. After analogue-digital conversion averages were made, for each experimental situation, on 65 responses to stimuli given every 1540 msec and lasting 770 msec each. A topographic study was made with chains of bipolar derivations in 2 montages (longitudinal vertex-inion, transverse crossing the midline 2 cm above the inion). The results can be summarized as follows: The response to a 20° checkerboard made of 40′ squares (which serves as “reference response”, being obtained from 24 normal subjects) is made of 4 components: wave 0 (sink peaking at 60 msec), wave 1 (source peaking at 90 msec), wave 2 (sink peaking at 120 msec), wave 3 (long lasting source peaking towards 200 msec). Waves 0, 1 and 2 show phase reversal of gradients (maximum of potential) near the inion; wave 3 has its maximum in the parietal region. The response to a plain field of the same dimension (and of luminance twice as high as that of the checkerboard), is characterized by waves of much smaller amplitude and usually opposite polarity (in particular wave 0 is absent in plain field responses and wave 2 is either absent or replaced by a positive potential). As the checkerboard gets smaller (from 20° to 1°20′) the amplitude of each component gets smaller and the topographic organization of waves 1 and 2 changes, the maximum of potential getting more and more posterior (longitudinal montage) and lateral (transverse montage); the topography of wave 3 (longitudinal) does not change. When the square size changes (from 40′ to 5′), the chronotopographic organization of the response does not change. The amplitude of waves 0,1 and especially wave 2 does change: whatever the checkerboard size, wave 2 (sink peaking around 120 msec) is the largest with 20′ squares and the smallest with 5′ squares (for some subjects wave 2 entirely disappears in the latter case). The part played in this pattern response by the central and peripheral retina and the role of the density of contrasts, which might be related to the size of receptive fields, are discussed.