Physiology of Vision
Light is the visible portion of electromagnetic radiation spectrum to which the human eye is sensitive. Light waves travel at a speed of 300,000 kilometers per second. White light is the combination of seven wavelengths and each
wavelength has its own colour. This can be demonstrated by passing the white light through a glass prism, which refracts it, i.e. it bends the rays of the different colours to a greater or lesser extent, depending on their wavelength. Red light has the longest wavelength while violet has the shortest
The Spectrum of Light
The visible spectrum extends from 700 nm at the red end to 400 nm at the violet end. Beyond the long end (red), there are infra-red (heat), radar and radio waves, whereas beyond the short end (violet) there are Ultraviolet (UV), X-rays and cosmic waves. The UV light is normally invisible because it is absorbed by a yellow pigment in the lens .
A specific colour is perceived, when the object reflects only one wavelength and all the others are absorbed, for example an object appears red, when only the red wavelength is reflected. Objects appear white, when all wavelengths are reflected, and black, when they all are absorbed.
MECHANISM OF VISION
In order to achieve clear vision, light reflected from objects within the visual field passes through the following stages: 1. The light rays interring the eye are focused onto the retina by mechanism of:
- Phototransduction-initiation of vision by photo receptors (rods and cones).
- Processing and transmission of visual sensation by image processing cells of retina and visual pathway.
- Visual perception of processed image in visual cortex.
Focusing of Image on Retina
Light rays coming from a distance (beyond 20 feet) are parallel and do not focus on retina, they need to bend or refract to get focused on retina, to form a clear image of the outside world. This refraction is done by cornea, aqueous, lens and vitreous, which are denser than air.
The light rays coming from a nearby object (less than 20 feet) are divergent, and need additional refractive power to focus on the retina. This additional power is provided by accommodation mechanism of the eye, which includes:
i. Change in the curvature of lens to increase the refractive power.
ii. Change in the size of pupil to control the amount of light.
iii. Convergence of eyes to see an object nearer to the eye.
The photoreceptors, rods and cones serve as the sensory endings, for visual sensation. Light falling on the retina causes photochemical change in visual pigment of rods and cones, which in turn triggers a cascade of biochemical reactions that result in generation of electrical changes called receptor potential.
This phenomenon of conversion of light energy into receptor potential (visual impulse) is known as phototransduction.
Processing and Transmission of Visual Impulse
The receptor potential generated in the photoreceptor is transmitted by electronic conduction (i.e. the direct flow of electric current, and not as action potential) to other cells of the retina via bipolar cells, horizontal cells, amacrine cells and ganglion cells.
The ganglion cells transmit the visual signal by means of action potential to the neurons of lateral geniculate body, and finally, to the primary visual cortex.
The visual impulse i.e. the action potential is perceived in the visual cortex, which results in the following visualperception:
a. Light sense
b. Form sense
c. Contrast sense
d. Colour sense
a. Light Sense
It is the awareness of light in all its gradation of intensity
It is the minimum intensity (brightness) of light appreci by the retina.
It is the ability of the eye to adapt itself to decreas illumination. When one goes from bright sunshine in dim light room, one cannot perceive the objects in ro until sometime has elapsed. During this time, e adapting to low illumination. The time taken to see ind illumination is called dark adaptation. The rods are m more sensitive to low illumination than cones.
The ability to see in dim light (dark). the function of rods. The loss of rods causes night blinde fc.g. retinitis pigmentosa.
The ability to see in bright light. It is function of cones. The loss of cones causes day blindn e.g. cone-dystrophy.
b. Form Sense
It is the ability to discriminate between the shapes of objects. Cones play a major role in this facity. The sense is most acute (sharp/clear) at fovea, which con maximum number of cones. Visual acu measure of the form sense. Loss of cones uses decr in visual acuity c.g. cone-dystrophy.
C. Sense of Contrast
It is the ability of the eye to perceive changes in the luminance between regions which are not separated by definite borders. Loss of contrast sensitivity results in fogginess of vision.
Contrast sensitivity is affected by refractive errors, glaucoma, amblyopia, diabetic retinopathy, optic nerve diseases and lenticular changes.
Contrast sensitivity may be impaired in the presence of normal visual acuity.
It is assessed by various charts e.g. Pelli-Robson chart, Vistech chart or Cambridge chart.
d. Colour Sense
It is the ability of the eye to discriminate between different colours excited by light of different wavelength. Colour vision is a function of the cones and thus, better appreciated in photopic vision.
It postulates that there are three types of cones, each containing different photo pigment, which is maximally sensitive to one of the three colours (wavelength) via red, green and blue. These three colours are called primary colours. All other colours are formed by varying mixture of the three primary colours. When all the three types of cones are equally stimulated, they produce white colour.
Colour vision is tested by:
- i. Ishihara chart
- ii. The lantern test
- iii. Holmgren wool test
- iv. Nagel’s anomaloscope
- v. The Farnsworth-Munsell 100 hue test
It is the inability to recognize the colours. It may be:
It occurs due to the absence of red, green or blue pigment in the cones. It is a bilateral and incurable..
- Protanope: The red sensation is deficient.
- Deuteranope: The green sensation is deficient.
- Tritanope: There is absence of blue sensation. It is rare.
It occurs due to disease of macula and optic nerve.
- Macular disease e.g. macular degeneration and toxic amblyopia. It tends to produce blue yellow defect.
- Optic nerve disease e.g. optic neuritis. It tends to produce red green defect.