Réponses évoquées visuelles chez les enfants voyants et mal-voyants

Abstract

Thirty-one children aged 7–12 years received intermittent photic stimulation at a slow rate (768 msec between flashes) and a fast rate (83 msec), with eyes closed and with eyes open. Twenty-two of the subjects had normal vision, nine showed a considerable bilateral visual defect of peripheral (non-cortical) origin, the remaining vision in the better eye being less than 3/10. The responses to 200 or 100 flashes (depending on the rate used) were recorded from two montages, each consisting of a line of 9 equidistant electrodes, 2 cm apart: (1) posterior median longitudinal montage going from the vertex to the inion; (2) right-left transverse montage crossing the midline 8 cm in front of the inion. After analogue-digital conversion the responses were averaged with a computer and presented in the form of integrated chronograms (r.m.s.) and spatio-temporal maps; statistically significant differences were demonstrated: (1) between normal and poor vision considered over-all, and (2) between poorly-sighted subjects as a function of remaining visual acuity, using indices to measure the amplitude and the latency of each component, their topography (place of gradient phase reversal) and the energy of the whole response or of the after-discharge studied by itself. Compared to that in children with normal vision, the response in children with poor vision is characterized by: (1) a much weaker energy, either of the over-all response, or of the afterdischarge separated by mathematical filtering; (2) a much smaller amplitude of the positive wave which culminates at about 120 msec in the most posterior regions (wave IV of Cigánek); (3) a more anterior topography (parietal instead of occipital) of this positive wave; (4) increased latency of the negative wave which culminates at about 60 msec; (5) very small differences in amplitude as a result of opening and closing the eyes, such that the differences between subjects with normal vision and poorly sighted subjects are only statistically significant with the eyes closed. In the group of children with poor vision the amplitude of the response is seen to be least when the visual acuity remaining is smallest. The site of the lesion causing the sensory defect also plays a part. Some characteristics of the visual evoked response of subjects with poor vision (small amplitude, parietal rather than occipital topography) resemble those of the alpha activity of these children, particularly if it is studied in its average form. The problem of interpreting the peculiarities of the response evoked in subjects with poor vision is discussed in relation to a specific visual mechanism, underlying background activity and synchronizing mechanisms.