|CHAPTER||2||Vision–Its Physiology, Neurology, and Assessment|
The light falling upon the retina stimulates the sensory nerve endings, rods and cones. Histologically, retina consists of 10 distinct layers including three layers of cell (Fig. 2.1).
These are as follows:
•Layer of rods and cones– These are the receptors sensitive to light and serve as sensory nerve endings for visual sensations. The cell bodies of rods and cones form the outer nuclear layer (ONL).
•Layer of bipolar cells– This layer also includes horizontal cells, amacrine cells, and Muller’s cells. The cell bodies of these cells form the inner nuclear layer (INL).
•Layer of ganglion cells– They give rise to optic nerve fibers. The cell bodies of ganglion cells form the ganglion cell layer (GCL).
Functionally, the retina can be subdivided into four regions, namely, optic nerve head (ONH), fovea, retina peripheral to fovea, and peripheral retina (Table 2.1).
The rods are mainly located in the periphery, whereas the cones occupy the central region (Table 2.2).
▃Visual Process (OP1.1, PY10.17, PY10.19, PY10.20)
It is the process by which our brain forms an image from light energy. Visual process can be divided into the following stages (Flowchart 2.1):
•Initiation of visual sensation.
•Transmission of visual impulse.
■Initiation of Visual Sensation
The light falling upon retina causes two essential reactions: photochemical changes and electrical changes.
The photochemical changes concern visual pigments, that is, pigments in rods and cones. The photochemical changes in rods and cones are similar but the changes in rhodopsin have been studied in detail. Rhodopsin absorbs light with a peak sensitivity of 505 nm (green light) (Flowchart 2.2). Rods are low-resolution detectors and consist of the following:
Table 2.1 The regional variations of sensory nerve endings
1 Optic nerve head (Blind spot)
Receptors are absent, i.e., no rods and cones. So, insensitive to light.
Only cones are present and hence it is responsible for visual acuity. At fovea, there is one-to-one correspondence between photoreceptors and ganglion cells.
3 Retina peripheral to fovea
Here both cones and rods are present.
4 Peripheral retina
Mainly rods are present. This region is responsible for perception of dim light.
Table 2.2 Difference between rods and cones
•These are responsible for dim light vision (scotopic vision). These cannot detect color.
•These predominate in extra foveal region where many rods synapse with a bipolar cell. Hence, receptive field is more with less resolution.
•Rhodopsin (visual purple) is the visual pigment present in the rods.
•These are responsible for vision in bright light (photopic vision) and color vision.
•These predominate at fovea and there is one-to-one correspondence between cones and bipolar cells. Hence, resolution is more and visual acuity is better.
•There are three types of cones, each containing a specific pigment responsible for color discrimination and normal daylight vision.
•An apoprotein, opsin (called scotopsin), to which the chromophore molecule (11-Cis-retinal) is attached.
•11-Cis-retinal belongs to the carotenoid family. In the dark, it is in 11-cis form and gets converted to all-trans form in the presence of light. Retinal is derived from food sources. It is not synthesized in the body.
On exposure to light, 11-Cis-retinal is converted into all-trans-retinal isomer through short-lived intermediate products. With this change, rhodopsin loses its color (bleaching of rhodopsin). The all-trans-retinal is converted to all-trans-retinol and reaches the liver via blood. In liver, it is converted to 11-Cis- retinol. This is transformed to 11-Cis-retinal and combines with opsin to form rhodopsin (regeneration of rhodopsin). If subject immediately goes into dark after a brief exposure to light, all-trans-retinal is directly converted to 11-Cis-retinal by isomerase in the retina.
Only the first step in the bleaching sequence requires input of light. All subsequent reactions can proceed in the dark as well as in light.
When the intensity of background illumination remains relatively constant, rates of visual pigment bleaching and regeneration are in balance. This equilibrium between bleaching and regeneration of visual pigment is called visual cycle.
Electrical changes: The biochemical reactions result in generation of receptor potential. The process by which light energy is converted into receptor potential is known as phototransduction.
■Transmission of Visual Impulse
The processing of visual information takes place at the following three levels:
2.At LGB (lateral geniculate body).
3.At visual cortex.
The changes in electrical potential are transmitted through bipolar cells, ganglion cells, and optic nerve fibers to brain via visual pathway (visual pathway comprises optic nerves, optic chiasma, optic tracts, lateral geniculate bodies, optic radiations, and visual cortex in brain Flowchart 2.3).
Processing at Retina
Bipolar cells, horizontal cells, and amacrine cells participate in lateral inhibition (a form of inhibition in which activation of a particular neural unit is associated with inhibition of the activity of nearby units). Lateral inhibition prevents spreading of excitatory signal widely in the retina and improves the contrast of borders and edges of an object.
Retinal ganglion cells are of two types:
•Magno cells (M cells).
•Parvo cells (P cells).
Magno cells are larger cells and concerned with black and white response, perception of movement, and rough sketch of the object.
Parvo cells are smaller cells and predominate in the macular region. These cells are color sensitive and concerned with color vision and finer details of the object.
Two separate pathways start from ganglion cells, one from parvo cells (parvocellular pathway) and another from magno cells (magnocellular pathway). Both pathways are involved in the parallel processing of the image.
The action potentials (impulse) developed from these cells are conducted to lateral geniculate bodies (LGBs).
Processing at Lateral Geniculate Body (LGB)
The retina has point-to-point representation in a LGB. A LGB consists of six layers. Magnocellular pathway from magno cells terminate in layers 1 and 2 of LGB. Parvocellular pathway from parvo cells terminate in layers 3, 4, 5 and 6 of LGB. On each side, layers 1, 4 and 6 receive input from contralateral eye, while layers 2, 3 and 5 receive input from the ipsilateral eye.
From LGB, two separate pathways project to the visual cortex. Magnocellular pathway from layers 1 and 2 carries signals for detection of movement, depth, and rough sketch of the object. Parvocellular pathway from layers 3, 4, 5, and 6 carry signals for color vision and finer details of the object (Fig. 2.2).
■Visual Perception (Processing at visual cortex)
Visual cortex consists of two areas:
•Primary visual cortex or striate cortex which transforms information received from LGB and transmits it to the secondary visual cortex.
•Secondary visual cortex or extra striate cortex which transmits information received from primary visual cortex to the higher visual areas.
Visual sensations resulting from the stimulation of retina by light are of four types:
•Sense of contrast.
It is the perception of light in all its gradations of intensity. The intensity of light required to perceive it is called the light minimum. The light is no longer perceived if the intensity of light is reduced below the point of light minimum. The eye functions normally in a wide range of illumination by adjustment to such changes (called visual adaptation). Visual adaptation involves dark adaptation (adaptation to dim illumination) and light adaptation (adaptation to bright illumination). If we move from bright sun light into a dim light in the room, we cannot perceive the objects in the room until sometime has elapsed to adapt the amount of illumination by the eyes. The time taken to see in dim light is called dark adaptation time. The rods are much more sensitive to low illumination, so that rods are used in dim light at dusk (scotopic vision). The cones come into play in bright illumination (photopic vision). Bats have few or no cones; hence, it is a nocturnal animal. Squirrels have no rods and is therefore a diurnal animal. Human beings have rods and cones both.
During dark adaptation, the following changes take place in the eye:
•Vision changes from cones to rods (photopic vision to scotopic vision). This is called Purkinje shift.
•Sensitivity of receptors to light increases.
•Photopigments are resynthesized and so their concentration increases. Since vitamin A is required for the synthesis of both rod and cone pigments, deficiency of this vitamin produces visual abnormalities.
•Visual acuity decreases.
During light adaptation, the following changes take place in the eye:
•Vision changes from rods to cones (scotopic to photopic vision).
•Photopigments are bleached and so their concentration decreases.
•Sensitivity of receptors to light decreases. This happens due to decreased concentration of photopigments.
•Visual acuity increases. Both the photoreceptors work together at the midrange of illumination, which is the mesopic range.
It is the ability which enables us to perceive the shape of objects. Cones play the major role in form sense, so it is most acute at the fovea having cones. It decreases very rapidly toward the periphery due to a decrease in the number of cones. Visual acuity is the ability to see fine details of objects in the visual field. Assessment of visual acuity is discussed in the latter part of this chapter.
Sense of Contrast (Contrast Sensitivity)
It is the ability to perceive slight changes in the luminance between the regions which are not separated by definite borders. It indirectly assesses the quality of vision as loss of contrast sensitivity may disturb the patient more than the loss of visual acuity.
It is the ability to distinguish between different colors excited by the light of different wavelengths. The appreciation of colors (color vision) is a function of cones. Therefore, it occurs only in photopic vision. In dark adapted eyes, where the rod function dominates, colored objects appear as gray differing in brightness. There are three types of cones with three different pigments which absorb wavelengths of light in the spectrum corresponding to red, green, and blue colors. The three pigments are:
Pigment sensitive to red light (long wave pigment): It absorbs maximally in the yellow portion with a peak of 570 nm. The cone that contains this pigment is called “L” cone.
Pigment sensitive to green light (middle wave pigment): It absorbs maximally the green portion of spectrum with a peak of 535 nm. The cone that contains this pigment is called “M” cone.
Pigment sensitive to blue light (short wave pigment): It absorbs maximally blue violet portion of spectrum with a peak of 445 nm. The cone that contains this pigment is referred to as “S” cone (Fig. 2.3).
Any given cone pigment may be deficient or entirely absent.
Deficiency of red pigment is called protanomaly (red weakness) and its entire absence is called protanopia (red blindness).
Deficiency of green pigment is called deuteranomaly (green weakness) and its entire absence is called deutranopia (green blindness).
Deficiency of blue pigment is called tritanomaly (blue weakness) and its entire absence is called tritanopia (blue blindness).
Trichromats possess all three types of cones. Absence of one type of cone renders the individual dichromat, while absence of two types of cone renders the individual monochromat.
The red, green, and blue colors are called primary colors. These in different proportions will give a sensation of white or any other color shades. Hence, normal color vision is called trichromatic. For any given color, there is a complimentary color which when mixed will produce white. If a person stops looking at a color, he or she may continue to see it for a short time (positive after image), or he or she may see its complimentary color (negative after image). When a colored light strikes the retina, the response of the cones depends on the color mixture. The response in the form of local potentials gets transmitted into the bipolar cells which, in turn, activates ganglion cells. The signals from the three cones after processing by the ganglion cells are conducted to LGB in three different ways:
•Red green pathway via parvo cells.
•Blue yellow pathway via parvo cells.
•Luminance (white black) pathway via magno cells.
The cells of LGB process the color sensation in a similar fashion as that of ganglion cells and conduct the impulses to primary visual cortex. Red–green and blue–yellow pathways take relay in the layers 3 to 6 (Parvo cells) and white–black take relay in layers 1 and 2 (Magno cells). The primary visual cortex contains clusters of color-sensitive neurons. The color information from these neurons is projected to area 37 (secondary visual cortex) which converts color input into the sensation of color.
■Theories of Color Vision
The following theories have been proposed to explain the mechanism of color vision:
•Young-Helmholtz theory (trichromatic theory).
•Hering theory (Opponent process theory).
•Postulates: There are three primary colors– red, green, and blue.
•There are three types of cones with different pigments, each maximally sensitive to one of the primary colors, although each color receptor also responds to the other two primary colors.
•All other colors are assumed to be perceived by combinations of these, that is, sensation of any given color is determined by the relative frequency of impulse reaching the brain from each of the three cone systems.
This theory fails to explain the sensation of black color. It also has difficulty in explaining
color confusion and complimentary color after images.
The Hering theory assumes three sets of receptor systems: red–green, blue–yellow, and black–white. Each system is assumed to function as an antagonistic pair. The stimulation of one results in inhibition of the opposite receptor in the pair, for example, red light stimulates the red receptors and simultaneously inhibits the green. This concept can explain color contrast (if a piece of blue paper is laid up on a yellow paper, the color of each of them is heightened due to color contrast) and color blindness.
The most widely accepted theory (stage theory) incorporates both theories which help in explaining how our color vision system works.
1.The first stage is the receptor stage. The trichromatic theory operates at the receptor level which consists of three photopigments.
2.The second stage is the neural processing stage for color vision in which signals are recorded into the opponent process form by the higher level neural system.
▃Assessment of Visual Function
Each eye must be tested separately throughout for all forms of visual perceptions (form sense, field of vision, light sense, and color sense). Visual perceptions can be assessed by:
Subjective tests– These tests require the patient’s subjective expression of visual function which include:
•Assessment of visual acuity.
•Assessment of field of vision.
•Assessment of dark adaptation.
•Assessment of contrast sensitivity.
•Assessment of color vision.
Objective tests– These tests are independent of patient’s expression and achieved by electrophysiological tests which include:
•Visual evoked potential (VEP).
Assessment of Visual Acuity (OP1.3)
Visual acuity of the eye is an estimation of its ability to discriminate between two points. It must be tested both for distance and near. Distant visual acuity is tested by:
Snellen’s Test Types
The basic principle of Snellen’s test types is the fact that two objects can be perceived separately only when they subtend a minimum angle of 1 minute at the nodal point of the eye. The Snellen’s test type consists of a series of letters arranged in lines, each diminishing in size from above downward. Each letter is so designed that it can be placed in a square, the sides of which are 5 times the breadth of the constituent lines. Hence, at a given distance, whole letter subtends an angle of 5 minute at the nodal point of the eye. The letter of the top line subtends an angle of 5 minutes at the nodal point of eye if it is 60 m from the eye. The letters in subsequent lines subtend an angle of 5 minutes if they are 36, 24, 18, 12, 9, and 6 m away from the eye, but at 6 m, a 6/6 letters subtend an angle of 5 minutes, a 6/12 letter subtends 10 minutes, and a 6/60 letter subtends an angle of 50 minutes (Fig. 2.4). The illumination of chart should not fall below 20 foot candles.
Recording of Visual Acuity
The patient should be seated at a distant of 6 m from the chart. At this distance, the rays of light are practically parallel and accommodation is thus negligible. The patient is asked to read test types with each eye separately and the visual acuity is expressed as a fraction, in which the numerator is the distance of the chart from the patient (6 m) and denominator is the distance at which a person with normal vision ought to be able to read, for example, if a patient reads only the top line, his visual acuity will by 6/60 as a normal person ought to have read this line from a distance of 60 m, and when a patient reads the 7th (last) line of the chart, his visual acuity is recorded as 6/6 which is a normal person’s vision.
When the top letter cannot be read, the patient is asked to move toward the chart till he reads the top line, for example, if he reads the top line from a 3 m distance, his visual acuity will be 3/60.
If the patient cannot read the top letter even from a distance of less than 1 m, he is asked to count the fingers of the examiner and visual acuity is recorded as FC- 3′, FC- 1’.
When the patient fails to count fingers, the examiner moves his or her hand and observes whether the patient appreciates hand movements (HMs) or not. If he or she appreciates HMs, his or her visual acuity is recorded as HM +ve.
In absence of the recognition of HMs, see whether the patient can perceive the light, and the visual acuity is recorded as PL +ve or PL –ve.
In illiterate individuals, “E” test types or Landolt broken ring (C) should be used.