Eye: Retina and Neural Mechanisms.

1. Eye: Retina and Neural Mechanisms. Prof. K. Sivapalan

2. Critical fusion frequency. • Time resolving ability of the eye. • Rate at which stimuli can be presented and perceived as separate stimulus. Movies- 16 frames per second. Eye- Applied aspects

3. Color vision • Three different cones with different pigments. • Colors have three attributes: • Hue- shade of color • Intensity- concentration • Saturation- degree of freedom from dilution with white. • Every color has a complementary color that produces sensation of white when properly mixed. • It is said that black is a sensation produced by absence of light. It probably is a positive sensation otherwise it cannot be seen in white or colored screens. • Sensation of white or any color can be produced by mixing blue, green and red- primary colors. • Color perceived is related to color of other objects in the field. black Eye- Applied aspects

4. Color Blindness. • Rhodopsin is coded in chromosome 3, blue sensitive pigment in chromosome 7, red and green pigments are in X chromosome. • Red green blindness is sex linked recessive. • Males have only one X. If it happens to be defective gene, color blindness is expressed. • Females express defect only if both sex chromosomes have defective gene. • [Males suffer, females carry] • Yarn-matching test and Ishihara charts are used to identify color blindness. Eye- Applied aspects

5. Ishihara Charts • Normal person reads 74 but blue green color blind person reads 21. • Normal person reads 42, red blind person reads 2 and green blind person reads 4. Eye- Applied aspects

6. Eye Movements. • 4 recti and 2 oblique muscles. • High order of coordination is necessary to maintain images in the corresponding points of the retina. • Involuntary eye movements can be classified into four types: • Saccades- sudden jerky movements to bring new objects of interest into fovea, to scan the visual field and to avoid adaptation that occurs when focused on the same object for long. Coordinated by superior colliculi, secondary visual cortex. • Smooth Pursuit Movements- follow moving objects- Coordinated by cerebellum. • Vestibular Movements- movements initiated by vestibular stimuli- maintain gaze fixed while head moves. Coordinated by vestibular nuclei. • Convergence Movements- to bring visual axis towards each other on focusing on near object. Coordinated by visual association area in the cortex. Eye- Applied aspects

7. Binocular vision. • The central parts of visual fields of both eyes coincide. • The images on the retina are fused at the cortical level. • Fusion occurs when same image falls on the corresponding points in the retina- if not diplopia. • Perception of depth- • Monocular- relative size of the image of known object, movement relative to another object. • Binocular vision - adds appreciation of depth and proportion accurately. Eye- Applied aspects

8. Strabismus- squint. • When image does not fall on corresponding points. • ?congenital defects. • When image falls on non-corresponding points before the age of 6, one is suppressed and diplopia disappears. Eye- Applied aspects

9. Near Response • Eye response when changing gaze from a distant object to a near object. • Comprises convergence, pupillary constriction and accommodation. • Coordinated by visual association cortex. Efferents go through pretectal nucleus in superior colliculus • Effectors are ocular muscles, ciliary muscles and pupil. Eye- Applied aspects

10. Light reflex- Pupillary Reflex • Papillary response to light into one eye. • Fibers in optic nerve leave near the lateral geniculate body, enter the midbrain, terminate in the pretectal nucleus. • Second order neurons project to ipsilateral and contralateral Edinger-Westphal nuclei. • Causes contraction of both pupils. Eye- Applied aspects

11. Vision in Light and Dark. • Variation from darkness to bright sun light is 10 log units [factor of 10 billion] • Pupillary constriction can reduce light intensity by a factor of 16. Eye- Applied aspects

12. Dark Adaptation. • As soon as a person moves from brightly lit area to dimly lit area nothing can be seen. • Slowly vision returns. It takes 20 minutes for near maximal adaptation. • In moderate illuminatio cone adaptation occurs by reduction of threshold. • In dark adaptation of rods occurs. • Time taken for adaptation is the time taken to build rhodopsin stores. • Use of red glasses before going into dark area allows rods to dark-adapt beforerhand. Used by radiologists and pilots. • When the person moves from dark area to lit area, the light appears intensely bright until eyes adapt. • It is light adaptation [reverse of dark adaptation]. Takes about 5 minutes. Eye- Applied aspects

13. Vitamin A deficiency. • Nyctalopia- night blindness. Severe deficiency- damage to rods and cones. • Also other effects as dry cornea. Eye- Applied aspects

14. Visual acuity. • The degree to which the details and contours of objects are perceived. • Retinal factors: state of receptors, concentration, convergence, and central factors. • Image forming mechanism: refraction, adequate illumination, brightness of the stimulus, contrast and the adequate time should be provided. • Visual acuity is greatest in fovea centralis. • Measured by minimum separable- shortest distance two lines are separated and perceived as two lines. • Snellen Charts are read from 20 ft. (6 m). The Visual angle of the letters is 5 minutes and the angle of the lines 1 minute at the distance mentioned below them. Eye- Applied aspects

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16. Field of Vision. • Portion of the world visible through one eye. • It should be circular but nose, roof of the orbit and zygoma obstruct. • Perimetry: One eye is focused in the central point, other eye covered. • Small target is moved towards the central point along selected meridians. • As the target is first seen, the position is plotted as the angle of the arc. • Increased field- exophthalmia. Eye- Applied aspects

17. Field of Vision ctd. • A target screen can be used to map the blind spot and objective scotomas. • A white object moved in a black screen in the visual field. • The points where the object disappears and reappears is plotted. • Lesions:- • Optic nerve- blindness. • Optic tract- homonymous hemianopia. • Optic chiasm- heteronymous hemianopia. Eye- Applied aspects

18. Visual Perception

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