صدق الله العظيم الاسراء اية 58

By Dr. Abdel Aziz M. Hussein Lecturer of Physiology Member of American Society of Physiology

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

Photopic Vision Scotopic Vision

Photopic VisionScotopic Vision ReceptorsConesRods SiteCentral part of the retinaPeripheral part of the retina ThresholdIs highIs low Sensitivity to light LowHigh AccuracyHigh (can perceive the fine details of the object) Less (cannot perceive the fine details of the object) Colour visionCan perceive coloursCannot perceive colours AdaptationIs rapidIs slow

1. Evidence for the duplicity theory 1.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

2. Objections to the duplicity theory 3. Modification of this theory Guinea pigs have only rods Snakes have only cones

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). Definition

Value of light and dark adaptationValue of light and dark adaptation The eye can change its sensitivity as much as 500, , 000 times The difference between the star light and sun light is about 10 billion times

Example It is the ability of the retina to become more sensitive to faint light Definition On entry of room After 5 minutes After 30 minutes

Types 30 min Time The sensitivity of the retina increases by 10, ,000 times Value Cone adaptation (complete after 5 min) Rod adaptation (complete after 30 min)

Mechanism Slow process due to regeneration of the photosensitive pigments (rhodopsin and iodopsin)

Changes in the eye on darkChanges in the eye on dark 1. Dilatation of the pupils which allows more light to enter the eye and stimulates more rods

Changes in the eye on darkChanges 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

Changes in the eye on darkChanges 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

Definition It means a decrease in the retinal sensitivity which occurs in bright light. Example After 5 minutesOn entry of room

Mechanism due to degeneration of the photosensitive pigments (rhodopsin and iodopsin). Duration: complete within 5 minutes

Eye changes in LightEye changes in Light 1. Constriction of the pupils (miosis) to reduce the amount of light entering the eye

Eye changes in LightEye 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

Changes in the eye in lightChanges in the eye in light 4. Yellow green part of the spectrum becomes the most luminous part.

Changes in the eye on lightChanges in the eye on light 5. Electrical changes (electroretinogram or ERG).

It is the ability of the eye to perceive the different types and characters of colours. Types 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

Characters of colours:Characters of colours: 1.Hue means the wave length e.g. red light (wavelength nm), green light ( nm), and blue light ( 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

2 theories 1) One the level of receptors or retina → retinal mechanism → Young-Helmholtz Theory 2) The other at the level of pathway →neural mechanism → Herring Theory

It is the most accepted theory of colour vision It postulates that there are 3 kinds of cones

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%.

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