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Explore the Duplicity Theory of Retinal Functions and mechanisms of dark and light adaptation in eye physiology. Learn about colour vision and the Young-Helmholtz Theory.
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بسم الله الرحمن الرحيم ﴿و ما أوتيتم من العلم إلا قليلا﴾ صدق الله العظيم الاسراء اية 58
Physiology of Vision By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology
Duplicity Theory of Retinal Function • states that retinal function is double in nature depending on the properties of its 2 photoreceptors • assumes the existence of 2 different inputs from the retina • each input works maximally under a different condition of illumination
Duplicity Theory of Retinal Function Scotopic Vision Photopic Vision
Duplicity Theory of Retinal Function 1. Evidence for the duplicity theory Nocturnal animals such as rats have rods only 2. Day-life such as chickens have cones only 3. Vitamin A deficiency which causes dysfunction of the rods leads to night blindness (nyctalopia) 4. eye shows both dark and light adaptation
Duplicity Theory of Retinal Function 2. Objections to the duplicity theory Guinea pigs have only rods Snakes have only cones 3. Modification of this theory
Retinal Adaptation Definition • It means that the retina can change its sensitivity according to the intensity of light. So, retinal sensitivity increases in dark and decreases in light (i.e. automatic adjustment to the changes in illumination).
Retinal Adaptation Value of light and dark adaptation The difference between the star light and sun light is about 10 billion times The eye can change its sensitivity as much as 500,000-1000, 000 times
Dark Adaptation Definition • It is the ability of the retina to become more sensitive to faint light Example On entry of room After 5 minutes After 30 minutes
Dark Adaptation Time • 30 min Value • The sensitivity of the retina increases by 10,000 - 100,000 times Types • Cone adaptation (complete after 5 min) • Rod adaptation (complete after 30 min)
Dark Adaptation Mechanism • Slow process due to regeneration of the photosensitive pigments (rhodopsin and iodopsin)
Dark Adaptation Changes in the eye on dark 1. Dilatation of the pupils which allows more light to enter the eye and stimulates more rods
Dark Adaptation Changes in the eye on dark 2. Regeneration of the pigments in the rods and the cones which increases the retinal sensitivity to the dim light
Dark Adaptation Changes in the eye on dark 3. Decreased visual acuity so that the fine details and colours of the object cannot be seen 4. The blue-green part of the spectrum becomes the most luminous part
Light Adaptation Definition • It means a decrease in the retinal sensitivity which occurs in bright light. Example On entry of room After 5 minutes
Light Adaptation Mechanism • due to degeneration of the photosensitive pigments (rhodopsin and iodopsin). • Duration: • complete within 5 minutes
Light Adaptation Eye changes in Light 1. Constriction of the pupils (miosis) to reduce the amount of light entering the eye
Light Adaptation Eye changes in Light 2. Breakdown of the pigments in the rods and cones, so photopigments are decreased • 3. Decrease in the retinal sensitivity due to a parallel decrease in the amounts of the photosensitive pigments of the rods and cones
Light Adaptation Changes in the eye in light 4. Yellow green part of the spectrum becomes the most luminous part.
Light Adaptation Changes in the eye on light 5. Electrical changes (electroretinogram or ERG).
Colour Vision • It is the ability of the eye to perceive the different types and characters of colours. Types of colours: Primary colours • They are red, green, and blue • When they are mixed together in the same proportion they give white colour • When mixed by different proportion they give other colours Complementary colours These colours when mixed together they give white colour e.g. deep blue and yellow; red and cyano
Colour Vision Characters of colours: Hue means the wave length e.g. red light (wavelength 723-647 nm), green light (575-492 nm), and blue light (492-450 nm). 2. Intensity or saturation means the purity of the colour i.e. it is pure or mixed with other colours 3. Brightness means the amount of light in the colour
Mechanism of Colour Vision • 2 theories • One the level of receptors or retina → retinal mechanism → Young-Helmholtz Theory • The other at the level of pathway →neural mechanism →Herring Theory
Young-Helmholtz Theory • It is the most accepted theory of colour vision • It postulates that there are 3 kinds of cones
Young-Helmholtz Theory • Equal stimulation of the 3 cone systems produce white sensation • Unequal stimulation produce another color e.g.. yellow colour is perceived when red cone stimulated by 83%, green cone by 83%, and blue cone by 0%.
