10. Light perception.Adaptation.Methods of examination.Disturbances in adaptation PDF

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10. Light perception. Adaptation. Methods of examination. Disturbances in adaptation FUNCTION OF RETINA Light perception is the main function of the retina. The light sensitive cells are the rods and cones. Light rays cause chemical changes in photosensitive pigments in these cells and they emit nerve impulses which pass to the occipital lobes of cerebrum via the optic nerves. The rods are more sensitive than the cones. They are stimulated even in low intensity or dim light, e.g. by the dim light in the interior of a darkened room (scotopic vision). The cones are only sensitive to bright light; therefore, colour can only be seen in daylight. The different wavelengths of light stimulate photosensitive pigments in the cones, resulting in the perception of different colours. In bright lighting, the light rays are focused on the macula lutea. The rods are more numerous towards the periphery of the retina. Rhodopsin (Visual purple) is a photosensitive pigment present only in the rods. It is bleached by bright light and when this occurs the rods cannot be stimulated. Rhodopsin is quickly reconstituted when an adequate supply of vitamin A is available. When the individual moves from an area of bright light to one of dim light, there is a time period when it is difficult to see. The rate at which dark adaptation takes place is dependent upon the rate of reconstitution of rhodopsin. In dim evening light, different colours cannot be distinguished because the light intensity is insufficient to stimulate colour sensitive pigments in cones. VISUAL PERCEPTIONS

Def: Subjective visual sensations, caused by the action of light on the visual system The functions of the eye include: a) Light perception, b) Perception of details and form c) Color perception, d) Stereoscopic (3D) vision

a. Light Sense: Ability to perceive light in all the grades of intensity. (Unit = Candella) 

light perception: Light minimum is the minimum intensity of light appreciated by the retina. If the light which is falling on the retina is gradually reduced in intensity, a point comes when light is no longer perceived.

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Dark Adaptation: Dark adaptation is the ability of the eye to adapt itself to decreasing intensity of light, i.e. going from a brightly lit room into a dimly lit room, one cannot perceive the objects in the room until sometime has elapsed. This time interval is known as dark adaptation.

Methods of evaluating adaption include: 

Orienting response (Also called orienting reflex), is an organism's immediate response to a change in its environment, in this case response to light changes.

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Adaptometry: Evaluation of light sensitivity by apparatus. Adaptometer of Forster, and adaptometer of Krakow-Wishnewski.

b. Form Sense: Ability to perceive the shape of objects. Visual acuity is a record of form sense, measured most commonly using the Snellen chart or LogMar chart.

c. Colour Sense: Colour sense is that faculty which helps us to distinguish between different colours as excited by light of different wavelengths. Three important factors influence colour vision such as: •

Color tone (hue) – it is determined by the wavelength

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Saturation – it is determined by the quantity of white light added to the main tone

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Brightness (luminance) – it is determined by the intensity of light

The normal colour vision is called “trichromatic” (red, green, blue) and it is the basis of the Young- Helmholtz theory. When red, green, and blue portion of spectrum mix together, they produce white colour. Thus red, green, and blue are known as primary colours. The exact nature of the defect is tested by:

Isochromatic chart; coloured lithographic plates in which bold numbers are represented in dots of various colours, e.g. Japanese Ishihara lithographic plates, American H-R-R test.

d. Sense of Contrast The ability to perceive slight changes in luminance between regions which are not separated by definite borders. (Units = Luminosity – lux). Contrast is measured by Contrast sensitivity charts.

DISTURBANCE OF ADAPTATION TO DARKNESS 1. Hemeralopia – disturbance of the adaptation to darkness a. Essential – alimentary, in hepatic diseases, metabolic diseases(diabetes), toxic damage by heavy metals etc. b. Symptomatic – in pigment retinal degeneration, high myopia, glaucoma, retinal detachment etc. 2. Nictalopia – disturbance of the adaptation to brightness...