Physiology of Color Vision Notes

Color Vision Introduction

The human eye can recognize about 150 different colors in the visible spectrum. The discrimination and appreciation of colors depend upon the ability of receptors in the retina.

Visible Spectrum And Spectral Colors

1. Spectral Colors:

• When sunlight or white light is passed through a glass prism, it is separated into different colors. The series of colored light produced by the prism is called the visible spectrum. The colors that form the spectrum are called the spectral colors. The spectral colors are red, orange, yellow, green, blue, indigo, and violet (ROYGBIV or VIBGYOR).
• In the spectrum, the colors occupy the position according to their wavelengths. Wavelength is the distance between two identical points in the wave of light energy. Accordingly, red has got a maximum wavelength of about 8,000 A, and violet has got a minimum wavelength of about 3,000 A.

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• The light rays longer than red are called infrared rays or heat waves and the rays shorter than violet are called ultraviolet rays. But, these two extraordinary types of rays do not evoke the sensation of vision.
• The refraction of the spectral colors by the prism also depends on the wavelengths. Red is refracted less and violet is refracted more. So longer the light rays, the lesser is the refraction by the prism.

Purkinje Phenomenon:

• The Purkinje phenomenon is the shift of the brightest part of the spectrum when the intensity of illumination is changed.
  When white light is passed through a prism, it splits into spectral colors from red to violet and if the colors are viewed at high illumination, the brightest part of the spectrum is yellow, i.e. the brightest part of the spectrum is shifted to the left.
• But when the light intensity is reduced to that of twilight, the color of the spect¬rum fades. Now the brightest part of the spectrum is green, i. e. the brightest part of the spectrum is shifted to the right. It is called the Purkinje shift or effect.
• According to Purkinje, this effect is due to the maxi¬mal stimulation of cones by yellow and the maximal stimulation of rods by green.

2. Extraspectral Colors: Extraspectral colors are colors other than those present in the visible spectrum. These colors are formed by the combination of two or more spectral colors. For example, purple is a combination of violet and red. Pink is a combination of red and white.

3. Primary Colors: The primary colors are those, which when combined together produce the white. The primary colors are red, green, and blue. These three colors in equal proportion give white.

4. Complementary Colors: Complementary colors are the pair of two colors that produce white when mixed or combined in proper proportion. Examples of complementary colors are red and greenish blue; orange and cyan blue; yellow and indigo blue; violet and greenish-yellow; and purple and green.

Theories Of Color Vision

Many theories are available to explain the mechanism of perception of color by the eyes. However, most of the theories are not accepted universally. Five theories are recognized:

1. Thomas Young’S Trichromatic Theory:

• According to Thomas Young, the retina has three types of cones. Each possesses its own photosensitive substance. Each cone gives a response to one of the primary colors – red, green or blue.
• The different color sensations are produced by the stimulation of various combinations of the three types of cones. For the sensation of white light, all three types of cones are stimulated equally.

2. Helmholtz Trichromatic Theory: Helmholtz substituted the sensitive filaments of the optic nerve for the cones. The sensitive filaments of nerves give a response selectively to one or the other of the three primary colors. It is also called the Young-Helmholtz theory.

3. Granit’S Modulator And Dominator Theory: Granit observed that the ganglionic cells of the retina are stimulated by the whole of the visual spectrum. He studied the action potentials in ganglionic cells stimulated by light and obtained some sensitivity curves by using the different wavelengths of light both in light-adapted and dark-adapted eyes.

On the basis of the sensitivity curves, he classified the ganglionic cells into two groups namely, Dominators and Modulators.

• Dominators:
  • The dominators are responsible for the brightness of the light. Dominators are further divided into two types:
  • Dominators for cones, which respond in the light-adapted eye and a broad sensitivity curve is produced with the maximum response around the wavelengths 55 A
  • Dominators for rods, which respond in the dark-adapted eye and in the sensitivity curve the maximum response is given at the wavelengths of 500 A.
• Modulators:
  • The modulators are responsible for different color sensations. Modulators are of three types:
    • Modulators of blue, which are stimulated by lights with wavelengths of 450-470 A
    • Modulators of green, which are stimulated by light with wavelengths of 520-540 A
    • Modulators of red-yellow, stimulated by light with wavelengths of 580-600 A.

If green light falls on the retina, modulators of green are stimulated and the other two are less affected. Thus, according to Granit, the dominators are responsible for the brightness or intensity of light both in dark-adapted (rods) and light-adapted (cones) eyes. The modulators are responsible for color vision in light-adapted eyes.

4. Hartridge’S Polychromatic Theory: According to this theory, the human retina has seven types of receptors. All seven receptors are divided into three units.

• First Unit: It is a tricolor unit consisting of receptors for orange, green, and blue.
• Second Unit: It is a discolor unit with receptors for yellow and blue colors. Receptors for yellow and blue are complementary to each other.
• Third Unit: It is another discolor unit with red and blue-green receptors.

5. Hering’S Theory Of Opposite Colors: According to Hering, the retina has three photochemical substances. Each substance produces the sensation of a particular color by its breakdown or resynthesis.

• First Substance: It is a white-black substance. Its breakdown causes the sensation of white and resynthesis causes the sensation of black.
• Second Substance: It is a yellow-blue substance. Its breakdown causes the sensation of yellow and resynthesis causes the sensation of blue.
• Third Substance: It is a red-green substance. Its breakdown causes the sensation of red. Resynthesis causes the sensation of green. This theory explains the successive contrast and after images but not the simultaneous sensation of antagonistic colors.

Color Sensitive Areas In Retina

• The peripheral part of the retina is devoid of cones and is insensitive to color and gives sensations of white, black, and gray only. The central portion of the retina – fovea centralis has more cones so, it is more sensitive to color. In the extrafoveal regions, the cones are mingled with rods.
• The retinal area sensitive to blue is the largest and to green the smallest. Red comes next to blue and then comes yellow. All the color areas of the retina are mapped out by using the perimeter.

Contrast Effects

1. Simultaneous Contrast:

• Simultaneous contrast is the effect that intensifies the contrast (difference) between two colors which are placed against each other. When black is placed against white or white against black, these two colors set one another off, i.e. the black looks blacker, and the white looks whiter. Similarly, green is enhanced by red and red by green.
• The maximum effect of the simultaneous contrast is obtained when the complementary colors are paired. The reason for simultaneous contrast is that the stimulation of an area of the retina by one color modifies the response in the surrounding or neighboring areas. It increases the sensitivity to other colors in the surrounding receptors. The action is probably due to horizontal cells.

2. Successive Contrast:

• Successive contrast is the effect of a previously viewed color field on the appearance of a currently viewed color field. When a person looks at a green object after looking at a bright red, the green object appears to be more greenish. There is an increase in the sensitivity to the complementary color.
• The reason for successive contrast is that the stimulation of an area of the retina modifies its sensitivity to successive stimuli. Thus, there is an increase in the sensitivity to the second color.

After Image

After the image is the phenomenon in which the retention of the image occurs even after the cessation of the light stimulus. After looking at a bright object, if the eyes are closed, the image remains more distinct for some time and then fades away gradually. The after image is of two types:

1. Positive After Image:

• Positive after image is an after image persisting after the closure of eyes or turning towards a dark background.
• After looking at a bright object, if the eyes are closed or fixed on a black surface, the after-image appears to be bright and with the same color of the object.

2. Negative After Image:

• It is an afterimage that persists while turning towards a bright background. After looking at a bright object, if the eyes are fixed on a white surface (instead of closing or fixing on a black surface), the after image appears in the complementary color.
• The reason for the negative afterimage is the persistence of activity in the retina, even after the particular stimulus ceases to act.

Applied Physiology Color Blindness

• Color blindness is the failure to appreciate one or more colors. It is common in 8% of males and only in 0.4% of females, as mostly color blindness is an inherited sex-linked recessive character. In addition to hereditary conditions, color blindness occurs due to acquired conditions also such as ocular diseases or injury or disease of the retina.
• The term ‘color blind’ does not mean that objects are seen only in black and white. Total color blindness is very rare. There are many types and degrees of color blindness. The most appropriate term for color blindness is a deficiency of color vision.

1. Causes For Acquired Color Blindness:

• Trauma: Injury to the eye due to accidents or strokes results in color blindness.
• Chronic Diseases: Color blindness is caused by chronic diseases such as:
  • Glaucoma
  • Degeneration of macula of the eye
  • Retinitis
  • Sickle cell anemia
  • Leukemia
  • Diabetes
  • Liver diseases
  • Parkinson’s disease
  • Alzheimer’s disease
  • Multiple sclerosis.
• Drugs: Frequent use of some drugs leads to color blindness:
  • Antibiotics
  • Antihypertensive drugs
  • Antituberculosis drugs
  • Barbiturates
  • Drugs are used to treat psychological problems and neural disorders.
• Toxins: Industrial toxins or strong chemicals cause color blindness. Common substances causing colorblindness are:
  • Fertilizers
  • Carbon monoxide
  • Gabon disulfide
  • Chemicals.s with high lead content.
• Alcoholism: Chronic alcoholism results in color blindness.
• Aging: Color blindness can occur after 60 years of age due to various changes in the eye.

2. Classification Of Color Blindness: Based on Young-Helmholtz trichromatic theory, color blindness is classified into three types.

1. Monochromatism:

• Monochromatism is a condition characterized by a total inability to perceive color. It is also called total color blindness or achromatopsia. Monochromatism is very rare. Persons with monochromatism are called monochromats.
• The retina of monochromats is totally insensitive to color and they see the whole spectrum in only black, white, and different shades of gray. So, their vision is similar to black and white photography. Monochromatism is divided into two types:
  • Rod monochromatic: It is the condition in which the cones are functionless and the vision depends purely on rods. So, the rod monochromats are totally color-blind. They are dazzled by light but definitely are not blind during daylight. Their visual acuity is lowered and foveal vision is absent which results in central scotoma. Central scotoma is the formation of a big blind spot in the fovea centralis due to the nonfunctioning of cones. The rods are also absent in the fovea.
  • Cone monochromatism: Cone monochromatism is the condition in which vision depends upon one single type of cone. Central scotoma does not occur in this condition.

2. Dichromatism: Dichromatism is color blindness in which the subject can appreciate only two colors. Persons with this defect are called dichromats. They can match the entire spectrum of colors by only two primary colors because the receptors for the third color are defective. The defects are classified into three groups:

• Protanopia: Protanopia is the type of dichromatism caused by the defect in the receptor of the first primary color, i.e. red. So, the red color cannot be appreciated. The persons having protanopia are called protanopes. They use blue and green to match the colors. Thus, they confuse red with green.
• Deuteranopia: It is the dichromatism caused due to the defect in the receptor of the second primary color, i.e. green. Deutera- nopes use blue and red colors and they cannot appreciate green color.
• Tritanopia: It is the dichromatism caused due to the defect in the receptor of the third primary color, i.e. blue. Tritanopes use red and green colors and they cannot appreciate blue color.

3. Trichromatism: Trichromatism is color blindness in which the intensity of one of the primary colors cannot be appreciated correctly though the affected persons are able to perceive all three colors. The persons with this defect are called trichromats. Even the dark shades of one particular color look dull for them. Trichromatism is classified into three types:

• Protanomaly: Protanomaly is the type of trichromatism in which the perception for red is weak. So to appreciate the red color, the person requires more intensity of red than a normal person.
• Deuteranomaly: Deuteranomaly is the trichromatism in which the perception for green is weak.
• Tritanomaly: It is trichromatism with a weak perception for blue.

Tests For Color Blindness

Three methods are available to determine color blindness:

1. By using Ishihara’s color charts
2. By using colored wool
3. By using Edridge-Green lantern.