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2. A Few Notes on Visual Impairment and its Causes 3. Photoreceptor Cells 4. Eye Movements Cones and rods cells have different functions in the visual system. Eye movements are another indispensable aspect of vision, since The cones encode visual information in daylight (photopic) and they modify the image formed on the retina. They are performed Visual Impairment provide colour perception. Rods, much more sensitive in dim light, by the oculomotor muscles, fxed on one end to the sclera and on and its Causes take over when the cones no longer function. They allow the retina the other end to the inner part of the orbit. They enable vertical, to adapt to darkness. Rods contain much more photosensitive horizontal and rotational movements of the globe and are controlled pigment than the cones and the renewal speed of the rhodopsin- by specifc nerve centres located in the central nervous system. containing discs is much higher. Vergence movements allow the gaze to fx on an object. In effect, Encoding and Processing Visual Information these are movements that the eyes make when you want to look at an object located within the feld of view. They allow the image The phototransduction, i.e. the transformation of the retinal image of the object to be brought onto the fovea in each eye. into nervous impulses, takes place in the external segment of the visual receptors (the internal segment looks after the cell’s life). Saccades are rapid movements of the eye. These are the move- These external segments enclose the photosensitive pigment, the ments that the eyes make when they explore the panorama. The rhodopsin, composed of a protein called opsin, to which is attached amplitude of the angle varies from a few minutes to a few degrees retinal, an aldehyde of vitamin A, derived from food and delivered and their duration is 10 to 80 milliseconds. by the circulatory system. Under the effect of light, the chemical structure of retinal is modifed, creating an electric potential diffe- A smooth pursuit is a slow, involuntary and automatic eye movement rence, which propels an electrical current through the bipolar and that you make when you follow a moving object. It is produced ganglion integration cells, and then along the optic nerve to the cortex. without latency time, so that vision is never interrupted. The cones allow for the colour vision, thanks to three types of receptor, differentiated by their absorption spectrum: the ones that essen- The (optokinetic) physiological nystagmus is a complex movement tially absorb short wavelengths (S “blue” with an absorption peak made up of pursuits and saccadic recall movements. It can be seen, of 420 nm), the ones that absorb medium wavelengths (M “green” for example, in a person who is looking at a landscape through with an absorption peak of 530 nm) and the ones that absorb long the window of a moving vehicle. The nystagmus allows part of the Practical Low Vision wavelengths (L “red” with an absorption peak of 560 nm). visual scene to be followed, before return to another fxing point, which, again, is followed, and so on. Processing the Information We should note that: The horizontal, bipolar, amacrine and ganglion cells participate in • Vision is only possible because the eyes are permanently animated the integration of the information coming from the visual cells. The by micronystagmus, a stimulation refreshment movement essen- axons of the ganglion cells constitute the optic nerve, which projects tial to perception. If there were no such constant imperceptible images onto the cortex, these images having passed through the movement of the eyes, perception would be extinguished. chiasma, lateral geniculate nucleus and superior colliculus, different • Congenital or acquired nystagmus, which we will discuss in section areas of preliminary analysis of the image (fgure 5 b). 2 E 3, is due to a diffculty in maintaining the position of the gaze, induced by cerebellar or vestibular neurological causes. Figure 5: Processing the information: Geniculate nucleus Chiasma Light Vitreous Light Ganglion cells Bipolar cells Photoreceptors Retinal Pigment © Essilor International © Essilor International Epithelium (RPE) Choroïde a) Cross-sectional view of the retina. b) Visual pathway: the left temporal and right nasal retinas are projected in the left cortex and the right temporal and left nasal retina in the right cortex. 11 Copyright © 2013 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.Copyright © 2013 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute. Copyright © 2013 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.