Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. Let’s see the physiology of vision.
नेत्र क्रिया शारीर :-
इंद्रिय ज्ञान के लिए इंद्रिय अर्थ, इंद्रिय, मन व आत्मा में संबंध होना चाहिए, नेत्र में रहने वाला पित्त जिसका काम रूप आलोचन का है और तारक में रहता है इसलिए उसे आलोचक पित्त कहते है। आचार्य भेल ने इसे दो प्रकार का बताया है:-
- चक्षु वैशेषिक :- रुप का प्रत्यक्ष होता है
- नेत्रहीन में हानि मिलती है
- बुद्धि वैशेषिक :- बुद्धि व्यापार, विभिन्न परिस्थितियों में दृष्टिकोण, सूक्ष्म अर्थ, अध्ययन, ज्ञान ग्रहण, स्मृति के कारण, योजना, ध्यान, एकाग्रता
- मूर्ख या बुद्धि हीन व्यक्ति में हानि
- भ्रु मध्य श्रृंगाटक मे स्थित
नेत्र पंचक :-
- इंद्रिय – चक्षु
- अधिष्ठान – नेत्र
- बुद्धि – रूप ज्ञान
- द्रव्य – तेज
- अर्थ – रुप
Physiology of Vision :-
Physiology of vision is a complex phenomenon which is still poorly understood. The main mechanism involved in physiology of vision are:
- Initiation of vision (Photo transduction), a function of photoreceptors (rods and cones)
- Processing and transmission of visual sensation, a function of image processing cells of retina and visual pathway and,
- Visual perception, a function of visual cortex and related areas of cerebral cortex.
Photo Transduction –
The rods and cones serve as sensory nerve endings for visual sensation. Light falling upon the retina causes photochemical changes which in turn trigger a cascade of biochemical reactions that result in generation of electrical changes. Photochemical changes occurring in the rods and cones are essentially similar but the changes in rod pigment (rhodopsin or visual purple) have been studied in more detail. This whole phenomenon of conversion of light energy into nerve impulse is known as Photo transduction.
Photochemical Changes –
It includes- Rhodopsin bleaching. Rhodopsin refers to the visual pigment present in the rods—the receptors for night (scotopic) vision. Its maximum absorption spectrum is around 500 nm. Rhodopsin consists of a colourless protein called opsin coupled with a carotenoid called Retinine (Vitamin A aldehyde or 11-cis-retinal). Light falling on the rods converts 11-cis-retinal component of rhodopsin into 11-cis-retinal through various stages. The alt-trans-retinal so formed is soon separated from the opsin. This process of separation is called Photo decomposition and the rhodopsin is said to be bleached by the action of light.
Rhodopsin Regeneration:
The 11-cis-retinal is regenerated from the alt-trans-retinal separated from the opsin (as described above) and Vitamin-A (retinal) supplied from the blood. The 11-cis-retinal then reunites with opsin in the rod’s outer segment to form the rhodopsin. This whole process is called Rhodopsin regeneration. Thus, the bleaching of the rhodopsin occurs under the influence of light, whereas the regeneration process is independent of light, proceeding equally well in light and darkness.
Visual cycle:
In the retina of living animals, under constant light stimulation, a steady state must exist under which the rate at which the photochemicals are bleached is equal to the rate at which they are regenerated. This equilibrium between the photodecomposition and regeneration of visual pigments is referred to as Visual cycle.
Electrical changes –
The activated rhodopsin, following exposure to light, triggers a cascade of complex biochemical reactions which ultimately result in the generation of receptor potential in the photoreceptors. In this way, the light energy is converted into electrical energy which is further processed and transmitted via visual pathway.
Processing and Transmission of Visual Impulse:
The receptor potential generated in the photoreceptors is transmitted by electronic conduction (i.e. direct flow of electric current, and not as action potential) to other cells of the retina via horizontal cells, amacrine cells, and ganglion cells. However, the ganglioncells transmit the visual signals by means of action potential to the neurons of lateral geniculate body and the later to the primary visual cortex.
Visual Perception –
It is a complex integration of light sense, form sense, sense of contrast and colour sense. The receptive field organisation of the retina and cortex are used to encode this information about a visual image.
- Light sense =
It is awareness of the light. The minimum brightness required to evoke a sensation of light is called the light minimum. It should be measured when the eye is dark adapted for at least 20-30 minutes. The human eye in its ordinary use throughout the day is capable of functioning normally over an exceedingly wide range of illumination by a highly complex phenomenon termed as the visual adaptation. The process of visual adaptation primarily involves - Dark adaptation (adjustment in dim illumination), and
- Light adaptation (adjustment to bright illumination).
Dark adaptation –
It is the ability of the eye to adapt itself to decreasing illumination. When one goes from bright sunshine into a dimly-lit room, one cannot perceive the objects in the room until sometime has elapsed. During this period, eye is adapting to low illumination. The
time taken to see in dim illumination is called ‘Dark adaptation time’.
The rods are much more sensitive to low illumination than the cones. Therefore, rods are used more in dim light (scotopic vision) and cones in bright light (photopic vision).
Light adaptation –
When one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises. The process by means of which retina adapts itself to bright light is called light adaptation. Unlike dark adaptation, the process of light adaptation is very quick and occurs over a period of 5 minutes. Strictly speaking, light adaptation is merely the disappearance of dark adaptation.
Form sense –
It is the ability to discriminate between the shapes of the objects. Cones play a major role in this faculty. Therefore, form sense is most acute at the fovea, where there are maximum number of cones and decreases very rapidly towards the periphery. Visual acuity recorded by Snellen’s test chart is a measure of the form sense.
Sense of contrast –
It is the ability of the eye to perceive slight changes in the luminance between regions which are not separated by definite borders. Loss of contrast sensitivity results in mild fogginess of the vision.
Colour sense –
It is the ability of the eye to discriminate between different colours excited by light of different wavelengths. Colour vision is a function of the cones and thus better appreciated in photopic vision. In dim light (scotopic vision), all colours are seen grey and this phenomenon is called Purkinje shift.