arXiv subjects

Guy Ropars

Publications and source records attributed to Guy Ropars.

5 recordsLinked to original sources

arXiv2025

Direct observation of the crossed interhemispheric transfer of the left-right mirror-images in human vision

Symmetry breaking is common in animal and human brains where the lack of asymmetry often perturbs behavioral and cognitive functions. In particular, the ubiquity of mirror-image confusion in young children, which often persists in dyslexia, is established. However, the very existence of these symmetric mirror-images and their perceptual or memory nature remain controversial. Here, using the noise-activated afterimage method, we demonstrate that a dyslexic with mirror-images is an ideal candidate for solving the mystery. Indeed, after a monocular fixation, the primary afterimages are perceived alone through this eye, while the mirror-images are also perceived alone, but exclusively through the other eye which has remained closed. We deduce that the callosal interhemispheric connections are necessarily projected on the dominance columns of layer 4 of the primary cortex, the only layer where segregation is strict, and are furthermore crossed. Our results show that perceived primary and mirror images are spatially and temporally resolved.

physics.bio-ph↗

arXiv2024

The lack of asymmetry of the Maxwell centroids, and of ocular dominance, in persons with dyslexia

While the existence of an asymmetry between the two Maxwell centroids at the centre of the two foveas recorded using a foveascope, leads to the ocular dominance in good readers, the lack of asymmetry in most of the observers with dyslexia leads to their non-dominance and their difficulties in reading and writing. Indeed, the lack of asymmetry between the two main roads to the brain, i.e. the two optical nerves, leads to perturbations in the brain central connectivity, namely between the two hemispheres inducing too robust interhemispheric visual connections, beyond the critical period of 7-8 years. The symmetrical mirror-connections like b-d (observed for about 60% of children with dyslexia) or the non-symmetrical connections like b-b (35%) induce confusions and duplications in observers with dyslexia but remain erasable thanks to pulsed systems (glasses, lamps, screens. . .), using Hebbian processes in primary cortex synapses. The effect is instantaneous, non-invasive and compensates the lack of asymmetry. These systems help most of the observers with dyslexia to overcome their difficulties. The orthoptists and speech therapists can acquire and improve the mechanism to help the children to catch up, and perhaps be able to use the foveascope for an early diagnosis with young children.

q-bio.NC↗

arXiv2023

Hebbian control of fixations in a dyslexic reader

During reading, dyslexic readers exhibit more and longer fixations than normal readers. However, there is no significant difference when dyslexic and control readers perform only visual tasks on a string of letters, showing the importance of cognitive processes in reading. This linguistic and cognitive processing demand in reading is often perturbed for dyslexic readers by perceived additional letter and word mirror-images superposed to the primary images on the primary cortex, inducing an internal visual crowding. Here we show that whereas for a normal reader, the number and the duration of fixations remain invariant whatever the nature of the lighting, the excess of fixations and total duration of reading can be controlled for a dyslexic reader using the Hebbian mechanisms to erase the extra images in an optimized pulse-width lighting. The number of fixations can be reduced by a factor of about 1.8, recovering the normal reader records.

q-bio.NC↗

arXiv2022

The structure of the Maxwell spot centroid

The dark entoptic Maxwell spot centroids seen through a blue filter, which coincide with the blue cone-free areas centered on the foveas, are shown to exhibit a structure. When observing through the green part of a blue-green exchange filter in a foveascope, after a fixation through the blue part, a small orange disc is seen around the centre of the pale green memory afterimage corresponding to the blue cone-free area. Using artificial pupils with different diameters, we show that this small circular pattern corresponds to the Airy disc due to the Fraunhofer diffraction through the pupil. Typically, for an eye with a 3 millimeter diameter pupil, the Airy disc exhibits a diameter of about 8 micrometers at the centre of the usual 100-150 micrometer Maxwell centroid. Fixation tests show that the towering central maximum of the Airy pattern irradiance corresponds to the preferred locus of the fixation of the eye near the centre of the blue cone-free area of the fovea. The locus of fixation in human vision thus appears to be located in the only area of the fovea where the large chromatic dispersion is cancelled, optimising the eye acuity.

physics.bio-ph↗

arXiv2020

Postural instability in a young dyslexic adult improved by Hebbian pulse-width modulated lighting

Postural stability is linked to vision in everyone, since when the eyes are closed stability decreases by a factor of 2 or more. However, in persons with dyslexia postural stability is often deficient even when the eyes are open, since they show deficits in motor as well as specific cognitive functions. In dyslexics we have shown that abnormal symmetry between retinal Maxwell's centroid outlines occurs, perturbing the interhemispheric connections. We have also shown that pulse-width modulated lighting can compensate for this lack of asymmetry, improving the reading skills. As the postural stability and the vision are correlated, one may wonder if the excess of the postural instability recorded in a young adult with dyslexia can also be reduced by a pulse-width modulated light controlling the Hebbian synaptic plasticity. Using a force platform we compared the postural responses of an observer without dyslexia with the responses of a subject with dyslexia, by measuring their respective standing postures with eyes open looking at a target in a room with either continuous or pulse lighting. There was no effect of changing the lighting conditions on the postural control of the subject without dyslexia. However, we found that the postural stability of the subject with dyslexia which was actually impaired during continuous light, but was greatly improved when a 80 Hz pulsed light frequency was used. Importantly, the excursions of the surface area of the center of pressure on the force platform were reduced by a factor of 2.3.

physics.med-ph↗