 The human eye is sensitive to the light rays which
fall in the visible spectrum.
 Seven colors in the visible spectrum  spectral
colors  violet, indigo, blue, green, yellow, orange
and red (VIBGYOR)
 Wavelengths range from 400–750 nm.
 Human eye does not respond to electromagnetic
radiations beyond the visible spectrum  ultraviolet
(< 400 nm) and infrared (> 750 nm)
 Extra-spectral colors (brown or purple) are
obtained by mixing two or more spectral colours
 Human beings have the ability to identify more than
100 different colours
Colour can be expressed with
3 attributes –
1. Hue
2. Intensity
3. Saturation
 There are 3 Primary Colours –
1.
Red – detected by red cone ( 575 mm)
2.
Blue – Detected by Blue Cone ( 430 mm)
3.
Green - detected by Green cone ( 535 mm)
 All colours can be produced by mixing various proportions of the 3 primary
colours.
 The primary colors when mixed in equal proportions produce white color
 If two colours when mixed in proper proportions give a sensation of
white or grey, they are known as Complementary Colours
 Example  Red and Green , Black and White, Blue and Yellow
 Black colour is absence of light
 Black colour is a positive sensation and a blind person sees nothing
 White colour  equal stimulation of all the red, green, and blue cones
gives one the sensation of seeing white
 In art, primary Colours are – Red, Yellow, Green
 Mixing two primary colours gives a Secondary colour
 Red + Blue = Purple ; Blue + Yellow = Green ; Red + Yellow =
Orange  Secondary Colours
 Mixing two secondary colours gives a Tertiary Colour
 Examples  Yellow + Green = yellow green, Red + Orange =
Red Orange
COLOUR THEROY
 The color perceived depends not only on the wavelength but also other factors like illumination,
intensity and background colour.
 Effect of Illumination – Purkinje shift phenomenon 
 Rods are most sensitive to the green light
 Under dim light  the brightest part of the spectrum is green.
 Cones have maximum sensitivity in the yellow region
 High illumination (daylight)  the brightest part of the spectrum is yellow
 The brightest part of the spectrum shifts to left during scotopic vision and shifts to right during
photopic vision  Light is more sensitive to shorter wavelengths in dark and longer
wavelengths in light
 This shift of the maximum sensitivity to the visible spectrum with change in illumination is known as
Purkinje shift phenomenon.
 Effect of Background Colour
 The color perceived by the eye depends on the ambient color in the visual field.
 Red object seen as red if filed is illuminated by green or blue light
 Red object is seen as pale pink or white if field is illuminated by red light
 Effect of Intensity
 For the same wavelength of light, when the intensity of the color is increased or
decreased, different shades of the color are perceived
 Humans have 3 types of cones – Red, Green, Blue
 They differ in light absorbance and intensity
 Peak absorbance of blue cones  430 nm
 Peak absorbance of green cones  535 nm
 Peak absorbance of red cones  575 nm
 Colour pigments in cones are known as 
 Blue – Cyanolabe ( most sensitive to 430nm wavelength) – S type
 Green – Cholorabe ( most sensitive to 535nm wavelength) – M type
 Red – Erythrolabe ( most sensitive to 575nm wavelength) – L type
Mean absorption wavelength of 3
cones
 The fovea mediates color vision as it contains densely packed cones.
 Fovea contains only M and L types of cones.
 It can distinguish fine spatial details of a colored object
 Its capacity to discriminate different colors is less compared to the extra-foveal regions.
 The Visual Pathway from the Cones to the Ganglion Cells Functions Differently from the Rod Pathway.
 Visual pathway from the foveal portion of
the retina, representing the new, fast cone
system.
 Three neurons in the direct pathway: (1)
cones, (2) bipolar cells, and (3) ganglion
cells.
 Horizontal cells transmit inhibitory signals
laterally in the outer Plexiform layer.
 Amacrine cells transmit signals laterally in
the inner Plexiform layer.
Peripheral retina
Fovea
 3 different pathways project from ganglion cells to the colour blobs and visual cortex – Red-Green;
Blue-Green and Luminance and is responsible for colour vision.
 P ganglion cells receive information from cones.
 They add or subtract information from one type of cone with information from another type of cone.
 They project to the parvocellular layer of LGB
 Impulses from the LGB are transmitted to the deep layer 4 C of V1 and blob region of the visual
cortex.
 The cells of the blob region of visual cortex high concentration of the enzyme cytochrome oxidase.
 Information from the deep layer 4 C of V1 and the blobs is transmitted to the V8 region of the visual
cortex.
 Young-Helmholtz theory Or Trichromatic Theory
 Opponent Theory – Single and Double
 Proposed in 1801 by Thomas Young and later modified by Hermann
von Helmholtz
 Also known as Young-Helmholtz theory or Pigment Theory
 There are the three types of cones and each has a pigment with a
different absorbance spectrum
 The cones and their pigments were called Red, green and Blue.
 They are also known as L (long-wavelength), M (middle-wavelength)
and S (short-wavelength)
 They show optimal sensitivity to lights of wavelengths of 575 nm ( L
type) , 535 nm (M type ), and 430 nm (S type ).
 Each type of cone pigment can absorb a wide range of wavelengths
 A wavelength of light can stimulate more than one cone and the colour
perceived by us depends upon the degree to which different cones are
stimulated.
 The visual cortex compares the relative frequency of action potentials in
the activated cone pathways and deciphers the wavelength and thereby,
makes out the color.
 Example – Light with wavelength 580 nm will stimulate both red and
green cones to varying degrees and we can see orange colour.
 Example – Light with wavelength 700 nm will only stimulate red cones
 Example – Light with wavelength 530 nm will stimulate red, blue and
green cones in the ration 31:67:36
 At least two types of cones are required for color vision.
 Stimulation of only one type of cones does not encode any color
perception.
 According to the Young-Helmholtz theory, the color vision is due
to presence of three types of cones
 Each of the cones are maximally sensitive to one of the three
primary colors  normal color vision depends on the integrity of
the cones.
 Absence or malfunction of one or more than one type of cone
cells lead to different forms of color vision abnormality.
 Hering’s theory of color vision
 Based on the observation that there is no greenish-red or bluish-yellow color.
 The red and green colors oppose each other, and the blue and yellow colors are
opposed.
 The ganglion cells, the cells of the LGB and the neurons of the visual cortex show
color opponent property
 Single Opponent Property and Double Opponent Property
 The ganglion cells and the cells of LGB are single-opponent type.
 There are two subtypes – Red (ON and OFF center); Green (ON and OFF center)
Some cells are excited when a red stimulus is present in the center of the receptive
field
 They are inhibited when a green stimulus is present in the periphery of the receptive
field.
These cells are inhibited by a green center and excited by a red periphery.
The other type of cells shows increased discharge rate to a green center
The discharge rate decreases in response to a red periphery.
They are activated by a green periphery and inhibited by a red center
 Red-sensitive on-center cells
are excited by red center and
inhibited by green periphery.
 Red-sensitive off-center cells
are excited by red periphery and
inhibited by green center.
 Green-sensitive on-center cells
are excited by green center and
inhibited by red periphery
 Green off-center cells are
excited by green periphery and
inhibited by red center
RED
GREEN
 The cells of the visual cortex are double-
opponent type.
 They are of two types 
1.
One type of cells are excited by a red
center with green periphery; and inhibited
by a green center with red periphery.
2.
The other type of cells are excited by a
green center with red periphery; and
inhibited by a red center with green
periphery
 One type of cells are excited by a red center with
green periphery; and inhibited by a green center
with red periphery.
 The other type of cells are excited by a green
center with red periphery; and inhibited by a red
center with green periphery.
 A person with abnormality of colour vision can be –
1.
Trichromat – has all three cones
2.
Dichromat – has two cones
3.
Monochromat – has only one cone
 The abnormality can be in the form of –
1.
Anomaly or weakness in detecting a colour
2.
Complete inability to detect a colour  Colour Blindness
 Ability of the individual to recognize one of the primary colors is decreased.
 A person with this abnormality needs more brightness to properly perceive a
colour
 This weakness can involve all three colour perception
 Defects can be –
1.
Protanomaly – Weakness for red
2.
Dueteranomaly – Weakness for green
3.
Tritanomaly – Weakness for Blue
 Here the patient has all 3 cones hence are trichromats but have
weakness in one or more colour perception  one or more cones are
defective
 Red Green Weakness is the most common defect
 It is an X-linked Recessive
 Females are carriers and males are sufferers
 Occurs in males and are transmitted from maternal side
 This is called colour blindness
 Can be of 3 types 
1. Protanopia – Inability to detect red colour as red cones are absent 
confuse red with green.
2. Dueteranopia - Inability to detect green colour as green cones are absent
 confuse green with red or blue.
3. Tritanopia - Inability to detect blue colour as blue cones are absent. Very
rare.
 These patients lack one of the cones  2 cones are present  Dichromats
 A person may have only one cone ( usually blue)
 They are known as Monochromats
 They do not see colour as at least two cones are required to perceive
colour
 They see varying grades of brightness with one cone  they see
everything in black and white or varying shades of grey.
 Can be inherited or Acquired
 Inherited/Genetic –
 X-linked recessive disorder  females are the
carriers and males are the sufferers.
 All the daughters of a colorblind person become
carriers and half of their sons suffer from the
disease  color blindness manifests in every
second-generation males.
 Gene for human rhodopsin is
located on chromosome 3.
 The gene for the blue-
sensitive cone pigment is
located on chromosome 7.
 The genes for the red-
sensitive and the green Deuteranomaly and Protanomaly are the most
common defects.
 Tritanomaly and Tritanopia are rare and occur
equally in both males and females
sensitive cone pigments are
arranged in tandem on the q
arm of the X-chromosome.
Acquired –
 Lesions – lesion of the striate cortex (area V8)  suffers from complete
loss of color vision  Achromatopsia
 Drugs – Sildenafil inhibits the enzyme Phosphodiesterase individuals
develop transient blue-green color weakness till the effect of the drug
lasts
 Ishihara’s Chart
 Edridge-Green Lantern Test
 Holmgren’s Wool-matching Test
 RETINA – LAYERS
 NEUROPHYSIOLOGY OF VISION
 VISUAL ACUITY
 ACCOMODATION
 VEP
Parasympathetic stimulus
Both sets of ciliary muscles contract
 Process by which visual apparatus is adjusted to
focus on a near object.
 Mainly consists of increase in power of lens via
Suspensory ligaments relax
Lens become more convex
suspensory ligament. This is brought about by the
action of both sets of ciliary muscles ( circular and
Power of lens increases from +20 D to +34 D in a child (change
decreases with age)
meridional)
Absence of parasympathetic stimulus
 It is under the control of parasympathetic nerve
Ciliary muscles relax
supply via Edinger Westphal Nuclei of Occulomotor
Suspensory ligament becomes taut
nerve ( third cranial nerve)
Lens become flat
Power of lens decreases
While focusing on a near object ->
1. Ciliary Muscles contract
2. Suspensory ligament slacken
3. Lens become thicker
4. Focal length decreases and power increases
While focusing on a distant object ->
1. Ciliary Muscles relax
2. Suspensory ligaments become taut
3. Lens become flat
4. Focal length increases and power
decreases
 The examiner keeps their index finger 10-15 cm in front of the nose of the
subject
 The subject is asked to look at a distance ( up to infinity)
 Now the subject is asked to look at the tip of the index finger of the
examiner – done to change from distant vision to near vision.
 The following changes occur –
1.
Convergence of the eyeball ( via Medial Rectus)
2.
Constriction of the Pupil ( via Occulomotor nerve)
3.
Bulging of the anterior surface of the lens/Ant surface of lens becomes
more curved ( by ciliary muscles)
 The first two changes can be easily seen but observation of the third
PURKINJEE SAMSON IMAGES
When a strong beam of light falls on
the eye, 4 images are formed on–
1. 1st image – Anterior surface of
cornea. This is the brightest erect
image.
2. 2nd Image – Posterior surface of
lens. This is a faint erect image.
May not be visible.
3. 3rd Image – Anterior surface of
lens. This is the largest erect
image.
4. 4th Image – Posterior surface of
lens. This is an inverted image
When accommodation occurs, the anterior
surface of the lens becomes more convex
and the third image becomes smaller.
Near Response consists of –
1.
Increased curvature of lens – Increases the converging power of lens. This converges
the diverging rays coming from a near object
2.
Constriction of Pupil – So that refraction only occurs by the central part of lens which
has a higher refractive index than periphery
3.
Convergence of eyeball – This brings corresponding retinal points in focus.
PATHWAY OF ACCOMODATION
MORE DETAILS UNDER VISUAL
PATHWAY
Retina
Optic nerve
Optic Chiasma
Optic Tract
Lateral geniculate
Body (LGB)
Edinger Westphal
Nucleus (EWN)
Corticospinal tract
Association fibers to
frontal eye field
(Area 8)
Occipital Cortex
(Area 17 of occipital
lobe)
Optic Radiation
Ciliary Ganglion
Post Ganglionic
parasympathetic
fibers
Ciliary Muscles (
Accommodation),
Sphincter pupillae
(Pupillary
constriction)
Pre Ganglionic
Parasympathetic
fibers
Third Cranial Nerve
(Occulomotor)
 This is seen in Neurosyphillis
 Here the pretectal region is damaged ( Pretectal region takes part in light reflex but not in
accommodation reflex)
 Thus light reflex is lost while Accommodation reflex is preserved.
Pathway of light reflex will be discussed
under vsual pathway and reflexes of the
eye
• Also known as Punctum Proximum
• Minimum distance from which an object can be seen
clearly by a normal eye ( without changes in the
accommodative apparatus)
• It is taken to be 25 cm for an young adult with normal
vision.
• It gradually increases with age with rapid increase
after 40 years due to age related changes in the
accommodative apparatus.
•
Also known as Punctum Remotum
•
It is the maximum distance at which an object can be
seen clearly without adjustment of visual apparatus
•
It is infinity for normal eye
•
It is considered to be 6 Meter/20 Ft for all practical
purposes.
•
In hypermetropic eye, far point is virtual and lies behind
the eyes while in myopic eye, the far point is real and
lies in front of the eye
•
The difference between the dioptric power needed to
focus at near point and far point is called Amplitude of
accommodation.
•
Range of Accommodation – Distance between far point
and near point
1. Presbyopia
2. Insufficiency of accommodation – premature presbyopia
3. Paralysis of accommodation – maybe drug induced (due to atropine),
internal ophthalmoplegia ( thirds cranial nerve palsy) or may be due to
any orbital cause
4. Spasm of accommodation – Maybe drug induced, due to excessive
work, bad illumination, anxiety, children trying to compensate for
uncorrected refractive errors
What is presbyopia ?
 Condition characterized by physiological insufficiency of
accommodation with defective near vision.
Pathophysiology of Presbyopia –
Decrease in accommodative power of crystalline lens
 As discussed earlier, Near Point recedes with age.
 Near Point at 10 years – 7 cm, at 40 years – 25 cm, at
45 years – 33 cms.
 Presbyopia can also be described as - Condition of
with increasing age is due to –
1. Decrease in elasticity and plasticity of crystalline
lens ( age related sclerosis)
2. Age related decrease in power of ciliary muscles
failing near vision due to age related decrease in
amplitude of accommodation/ increase in near point.
Treatment –
Premature Presbyopia can occur in -
• Convex glasses ( bifocal) for near work.
•
Uncorrected hypermetropia
• Convex glasses bend the divergent rays coming
•
Premature sclerosis of crystalline lesns
•
Presenile weakness of ciliary muscles ( as in general
debility)
•
Chronic simple glaucoma
from near object and help focus them on retina.
 Angle subtended by an object at the nodal
point of the eye.
 It is expressed in degrees of arc.
 For an object to be seen clearly the object has
to subtend an angle of 1 minute at the nodal
point. This forms a retinal image of 4.5 micron
which will stimulate at least two cones.
 As the object moves further the size of the
object has to be increased to maintain the
same visual angle.
 Concept is applied to Snellen’s Chart
• Chart with letters of different sizes used to
measure visual acuity of distant vision (The
numbers indicate the distance from which a normal
person can read the letter)
• Patient asked to stand in front of the chart with one
eye closed and then asked to read the letters. The
smallest letter read is noted. if the first line cannot
be read the person is asked to move forward.
• Visual Acuity is expressed in terms of – Distance of
chart from subject / The distance at which a normal
person is able to read the lowermost line read by
the subject
Snellen’s Chart for Visual acuity
measurement
 It indicates how clearly a subject can see the details of an
object.
 Tested for Near Vision and Distant Vision
 Visual Acuity for Distant Vision – tested by Snellen’s chart ,
Landolt C chart
 Visual Acuity for Near Vision is tested by – Jaeger’s Chart
JAEGER’S CHART
 It is the ability to distinguish two points as separate
 Human eye can see an object if it is more than equal to 200 micron.
 If two points are separated by 200 micron, the human eye will be able to distinguish it as
two different points.
 In comparison –
Resolution of Light Microscopy – 0.2 micron
Resolution of Electron Microscope – 0.002 micron