PHYSIOLOGY OF VISION-audio

Physiology of vision is a complex phenomenon which is still poorly understood. The main mechanisms involved in physiology of visions are:

Initiation of vision ( Phototransduction) , 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.
PHOTOTRANSDUCTION

The rods and cones serves 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 phototransduction.

Photochemical changes

The photochemical changes include:

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 all-trans-retinal through various stages (Fig. 2.1). The all-trans-retinal so formed is soon separated from the opsin. This process of separation is called photodecomposition and the rhodopsin is said to be bleached by the action of light .

Rhodopsin regeneration. The 11-cis-retinal is regenerated from the all-trans-retinal separated from the opsin ( as described above) and vitamin-A (retinal) supplied from the blood. The whole process is called rhodopsin regeneration (Fig. 2.1). 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 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 ( Fig. 2.2).
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 electrical current, and not as action potential) to other cells of the retina viz. horizontal cells, amacrine cells, and ganglion cells. However, the ganglion cells transmit the visual signals by means of action potential to the neurons of lateral geniculate body and the later to the primary visual cortex.

The phenomenon of processing of visual impulse is very complicated. It is now clear that visual image is deciphered and analyzed in both serial and parallel fashion.

Serial processing. The successive cells in the visual pathway starting from the photoreceptors to the cells of lateral geniculate body are involved in increasingly complex analysis of image. This is called sequential or serial processing of visual information.

Parallel processing. Two kinds of cells can be distinguished in the visual pathway starting from the ganglion cells of retina including neurons of the lateral geniculate body, striate cortex, and extrastriate cortex. These are large cells ( magno or M cells) and small cells (parvo or P cells). There are striking differences between the sensitivity of M and P cells to stimulus features ( Table 2.1)

Table 2.1. Differences in the sensitivity of M and P cells to stimulus features

Stimulus feature		Sensitivity
M cells	P cells
Colour contrast	No	yes
Luminance contrast	Higher	Lower
Spatial frequaency	Lower	Higher
Temporal frequaency	Higher	Lower
The visual pathway is now being considered to be made of two lanes: one made of the large cells is called magnocellular pathway and the other of small cells is called parvocellular pathway. These can be compared to two-lanes of a road. The M pathway and P pathway are involved in the parallel processing of the image i.e., analysis of different features of the image.

VISUAL PERCEPTION

It is a complex integration of light sense, form sense, sense of contrast and colour sense. The receptive field organization of the retina and cortex are used to encode this information about a visual image.

The 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:

a. Dark adaptation (adjustment in dim illumination), and

b. 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 some time 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.

2. The 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.

Components of Visual Aquity. In clinical practice, measurment of the threshold of discrimination of two spatially separated targets ( a function of the fovea centralis) is termed visual acuity. However, in theory, visual acuity is a highly complex function that consists of the following components:

Minimum Visible. It is the ability to determine whether an object is present or not.

Resolution (ordinary visual acuity). Discrimination of two spatially separated targets is termed resolution. The minimum between the two points , which can be discriminated as two, is known as minimum resolvable. Measurement of the threshold of discrimination is essentially an assessment of the function of the fovea centralis and is termed ordinary visual acuity. Histologically, the diameter of a cone in the foveal region is 0.004 mm and this, therefore, represents the smallest distance between two cones. It is reported that in order to produce an image of minimum size of 0.004 mm (resolving power of the eye) the object must subtend a visual angle of 1 minute at the nodal point of the eye. It is called the minimum angle of resolution (MAR).

The clinical test determining visual acuity measure the form sense or reading ability of the eye. Thus, broadly, resolution refers to the ability to identify the spatial characteristics of a test figure. The test targets in these tests may either consist of letters (Snellen’s chart) or broken circle (Landolt’s ring). More complex targets include gratings and checker board patterns. Recognition. It is that faculty by virtue of which an individual not only discriminates the spatial characteristics of the test pattern but also identifies the patterns with which he has had some experience. Recognition is thus a task involving cognitive components in addition to spatial resolution. For recognition , the individual should be familiar with the set of test figures employed in addition to being able to resolve them. The most common example of recognition phenomenon is identification of faces. The average adult can recognize thousands of faces.

Thus, the form sense is not purely a retinal function, as, the perception of its composite form is largely psychological.

MINIMUM DISCRIMINABLE: refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity. The best example of minimum discriminable is vernier acuity, which refers to the ability to determine whether or not two parallel and straight lines are aligned in the frontal plane.

3.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.

Contrast sensitivity is affected by various factors like age, refractive errors, glaucoma, ambylopia, diabetes, optic nerve diseases and lenticular changes. Further , contrast sensitivity may be impaired even in the presence of normal visual acuity.

4.COLOUR SENSE:

It is the ability of the eye to discriminate between different colours excited by light of different wavelenths. 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

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