Antoine Rémond

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.

Studies on the mechanism of the action of visual afferents on motor cortex excitability

Abstract 1. 1. The growing interest in the effect of sensory stimulation and especially of light stimulation on epileptic and myoclonic phenomena raises two questions of particular importance. The first is whether light stimulation produces its effect on the motor system by affecting the motor cortex or whether the effect is injected into the motor system at lower levels. The second question is by what pathways the visual afferent impulses reach the motor system in order to affect its threshold. A study of these two questions has been carried out on 30 cats under various types of anaesthesia including chloralose and semicarbazide. In order to test the excitability of the motor cortex, bipolar stimulating electrodes were placed on it and recording electrodes were placed in the pyramidal tract at the medullary pontine junction. The height of the response recorded from the pyramidal tract was used as an index of motor cortex excitability. 2. 2. The response recorded from the pyramidal tract after a single stimulus to the motor cortex is a complex one of prolonged duration. Our analysis of the pyramidal discharge suggests that it contains two sharply differentiated components. The fast component is probably due to direct stimulation of cells and perhaps fibres which contribute directly to the pyramidal tract (D response). The second slow component (I response) is probably due to stimulation of internuncial cortical cells which fire onto cells whose fibres contribute to the pyramidal tract. 3. 3. The effect of a single light flash in the eye or of lateral geniculate electric stimulation is to produce a prolonged period of facilitation of the motor cortex. The time course of this excitability increase is shown in figure 2 and the nature of the effect on the components of the pyramidal discharge is shown in figure 1. In some animals the light flash alone produces a descending volley in the P.T. It is concluded that the light flash affects both the cortical cells of origin of the pyramidal tract and the cortical interneurons. Some preliminary evidence suggests that other sensory systems may produce a similar phenomenon. 4. 4. It is shown that the effect of visual afferents on the excitability of the motor cortex is unaffected by complete removal of the occipital lobe cortex. It is concluded that neither the primary visual receiving area nor the visual association area need be present for the faciliting effect of visual stimulation to occur in the motor cortex. 5. 5. Complete removal of the cortex of the superior colliculus did not affect the time course or size of the facilitating effect of a visual afferent volley on the excitability of the motor cortex. 6. 6. Lesions placed in the pretectal region abolished the facilitatory effect of visual stimulation. 7. 7. It is therefore concluded that neither of the two main visual projection systems are responsible for the facilitory effect of visual afferent volleys on the motor cortex. The possible role of other visual pathways especially the ventral division of the lateral geniculate (the pregeniculate system) is discussed.

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.

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.

Réponses évoquées par les mouvements oculaires d'exploration (réponse lambda): Rôle des afférences intervenant à divers moments du processus oculo-moteur

Experiments are reported the aim of which was to elucidate the cause of each of the components of the lambda response, and particularly to evaluate the role of ‘on’ and ‘off’ visual effects which appear at various times during the oculomotor process and also the possible influence of non-visual mechanisms. Eight subjects with normal sight were studied under the following conditions: (i) horizontal eye movements of 12° were guided by fixation points placed on a dimly-lit uniform black field of 20°; a checkerboard of 6° aperture was placed in this field so as to be integrated into the oculomotor process at different times—at the beginning, during and at the end of the eye movement; (ii) successive horizontal eye movements of 3°, 7° and 11° scanned a checkerboard of 20°, each square of which had a 40′ aperture; (iii) the checkerboard was moved with an amplitude and period similar to those of the eye movements in (ii), but this time with gaze fixed. Horizontal and vertical movements of both eyes were recorded with an EOG. An EEG of the parieto-occipital regions was obtained using eight linked bipolar derivations in line on two montages, median longitudinal and right-left transverse. The EEG and EOG data were digitalized and a numerical programme of waveform recognition was used to identify the beginning of the saccade which triggers the averaging out of the EEG before (100 ms) and after (500 ms) the eye movement. A discussion of the results, taking into account the latency of the different components and their reinforcements or inhibition depending on experimental conditions, suggests that the two initial components of lambda response (including the initial portion of the classical lambda wave) might be due to visual effects (‘off effect’) that arise at the start of the movement or slightly before it at the time that the saccadic suppression begins. The later components could be attributed to visual effects brought into play towards the end of the movement (‘on effect’), when perception becomes normal again. It is, however, difficult to explain some of the results related to the amplitude of lambda components without bringing in a mechanism of non-visual origin (corollary discharge).

Réponses évoquées par le nystagmus spontané

A digital programme of wave form recognition was applied to a nystagmogram so as to pave the way for an average chronotopographic study of the occipital EEG activity triggered by the start of the rapid stage of the nystagmus. The programme was used in a clinical case of congenital nystagmus associated with a visual deficiency (coloboma of the optic nerve), with the following results: (i) in the patient examined, no average evoked response was observed either with eyes closed or with eyes open in the dark; (ii) an average response of small amplitude was observed with eyes open centred on a homogeneous illuminated field; this response was reinforced (greater amplitude and more complex structure) by a patterned field (checkerboard); (iii) the response to nystagmus showed characteristics similar to those of lambda response in the same subject. These results appear to confirm the conclusions of Bender and Shanzer (1964) according to which the calcarine activity in monkeys in response to experimental nystagmus is “light and vision dependent and correlates with the visual input and not with the oculomotor output”. Differences in amplitude, latency, and structure between the various responses recorded in the patient examined (response to nystagmus, to appearance of lighted checkerboard, and lambda response) and the same responses in normal subjects are considered and discussed in relation to factors associated with visual deficiency.

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.