INTRODUCTORY LECTURE ON
THE PHYSIOLOGY OF VISION
S. I. OGUNGBEMI
DEPARTMENT OF PHYSIOLOGY
UNIVERSITY OF LAGOS
 SPECIAL SENSES
Special senses are:
1. Vision
2. Audition
3. Olfaction
4. Gustation and
5. Special proprioception (Vestibular apparatus)
They serve as tools for learning and formation of
memory
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 CHAPTERS OF THE LECTURE
1. Functional Anatomy of the Eye
2. Optics and Optical Defects
3. The Visual Pathway
4. Photoreceptor Mechanism of Retina
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 Visual System
• Eye
• Accessory structures
– Eyebrows, eyelids, eyelashes, tear glands
– Protect eyes from sunlight and damaging
particles
• Optic nerve (II)
– Tracts
– Pathways
• Eyes respond to light and initiate afferent action
potentials
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Accessory Structures of Eye•
•
•
•
•
Eyebrows
– Prevent running
perspiration into eyes
– Shade
Eyelids or palpebrae
– Consist of 5 tissue
layers
– Protect and lubricate
Conjunctiva
– Covers inner eyelid
and anterior part of
eye
Lacrimal apparatus
Extrinsic eye muscles
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Lacrimal Apparatus
• Lacrimal apparatus
– Lacrimal Gland:
Produces tears to
moisten, lubricate,
wash
• Lacrimal Canaliculi
– Collects excess tears
• Punctum
• Lacrimal Sac
• Nasolacrimal duct
– Opens into nasal
cavity
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Extrinsic Eye Muscles
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Anatomy of the Eye
• Three coats or tunics
– Fibrous: Consists of sclera and cornea
– Vascular: Consists of choroid, ciliary body, iris
– Nervous: Consists of retina
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Anatomy of the Eye
• Fibrous tunic: Outer
– Sclera: White outer layer,
maintains shape, protects
internal structures,
provides muscle
attachment point,
continuous with cornea
– Cornea: Avascular,
transparent, allows light to
enter eye and bends and
refracts light
• Vascular tunic: Middle
– Iris: Controls light entering
pupil; smooth muscle
– Ciliary muscles: Control
lens shape; smooth muscle
• Retina: Inner
– Contains neurons
sensitive to light
– Macula lutea or fovea
centralis: Area of greatest
visual acuity
– Optic disc: Blind spot
• Compartments
– Anterior: Aqueous humor
– Posterior: Vitreous humor
• Lens
– Held by suspensory
ligaments attached to
ciliary muscles
– Transparent, biconvex
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Horizontal section of the right eye. AP, anterior pole; PP, posterior pole; VA, visual axis
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1. Functional Anatomy of the Eye
 The eye has 3 concentric coats or layers for the specialised
sense of vision.
 The sclera – the outer protective layer of fibrous coat
which is transparent anteriorly as the cornea i.e. ⅙ of
sclera
 The choroid – Middle melanin-pigmented layer which
contains the blood vessels which nourish the eye.
The specialised anterior portions are ciliary body and iris
 The retina – the innermost layer which contains the
photoreceptor cells i.e. the rods and cones.
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 The Sclera
• Sclera is the tough posterior outer coat.
• It is composed of tightly bound elastic and collagen
fibres.
• It provides adequate protection for internal contents
and components of the eye.
• It withstands high intraocular pressure of about 20
mmHg.
• The high intraocular pressure keeps structures
involved in vision in proper shape and position.
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 The Cornea
• The cornea is more convex than sclera.
• It is the anterior surface of the eyeball.
• It is made of collagen fibrils.
• It is covered anteriorly by stratified epithelium.
• It is continuous with conjunctiva covering the
exposed sclera.
• It transmits and focuses incident light to the retina.
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• Its epithelium utilises lacrymal secretion to keep
cornea in hydrated state.
• If the cornea is not hydrated, it dries up and looses
its transparency → xerophthalmia.
• It has its free nerve endings from trigeminal nerve.
• It is avascular – i.e. it has no blood vessel in itself.
• Being most, it receives oxygen from its own
metabolism directly from atmosphere.
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 The Choroid, Ciliary Body and Iris.
• Choroid comprises an outer pigmented and inner
vascular layer.
• Blood vessels of the choroid nourish the inner
layers of retina by simple diffusion.
• Melanocytes are abundant in the pigmented layer.
• Black melanin pigments serve to absorb light rays
thereby preventing their reflection back to the
retina, which would blur the optical image.
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• Ciliary body arises from the anterior end of the choroid
coat.
• The ciliary apparatus consists of the ciliary muscles, the
iris and the suspensory ligament which suspends the lens.
• Ciliary muscle consists of an outer ciliary muscle and inner
ciliary processes.
• It is made up of radial (dilator) and circular (constrictor)
muscles.
• Motor supply to the ciliary muscle is parasympathetic with
cell bodies of the preganglionic neurons in the EdingerWestphal nucleus of oculomotor nerve CN III
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• Postganglionic neurons are in the ciliary ganglion.
• Contraction of ciliary muscle makes the lens more convex.
• The iris - thin pigmented contractile diaphragm arising
from the ciliary body.
• The iris varies the aperture (or pupil) of the lens in
response to contraction of the ciliary muscles.
• The pupil is the visible coloured aperture of the eye.
• The pupil which varies the amount of light entering the
eyes.
• It contains radial and circular multiunit smooth muscles
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which control the size of the pupil. Monday, July 31, 2017
• Parasympathetic stimulation causes contraction of the
ciliary circular muscles (sphincter pupillae in the iris) and
constriction of the pupil.
• Sympathetic stimulation causes contraction of radial
muscles (dilator pupillae in the iris) and dilation of the
pupil.
• Sympathetic fibres relay via the superior cervical ganglion.
 The Aqueous Humour
• It is a clear liquid located between cornea and lens
• It is continuously formed by active transport and diffusion
by
the processes of the ciliary body.
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• It nourishes the lens and cornea and also buffers acids
produced by the anaerobic glycolysis taking place in the
lens.
• Its composition is similar to CSF or plasma without
proteins.
• It is reabsorbed through the Canal of Schlemn into the
intrascleral veins at The junction of the iris with the
cornea.
• Blockage of canal increases aqueous humour volume and
intraocular pressure (which is normally 15 – 20 mmHg),
leading to glaucoma.
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• Glaucoma can damage the retina and optic nerve and
blindness may result.
 The Lens
• It is a biconvex, transparent and elastic solid disc.
• It is held in position by suspensory ligament between the
iris (in front) and the vitreous humor (behind).
• The suspensory ligament attaches it to the posterior
surfaces of the ciliary processes.
• Reduction of tension in the ligament by contraction of
ciliary muscles increases the refractive power of the lens.
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• With advancing age, the lens becomes less elastic resulting
in presbyopia.
• The function of lens is to provide a fine adjustment to the
focus.
• No blood vessels in the lens.
• The central artery which supplied it before birth atrophies
and remains as an end-artery supplying only the retina.
 The Retina
• The retina is inverted.
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• It consists of an outer pigmented layer, that separate it from
the choroid, and an inner nervous layer towards the
vitreous humour.
• The Pigmented layer of the retina and choroid are single
sheets of melanin-containing epithelial cells (melanocytes).
• The functions of the pigmented layer of the retina are:
Absorption of light to prevent reflection blurred image
in the eye.
Phagocytosis of degenerating membrane discs and
shelves from rods and cones.
Storage of large quantities of vitamin A which is
required for synthesis of visual pigment.
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• Retinal detachment :detachment of pigmented layer from
the nervous layer causes blindness.
• Treatment → laser surgical attachment of the 2 layers.
 The Nervous Layer
• It comprises rods (for poor light vision) and cones (bright
light vision) - outermost, light sensitive and abuts on the
pigmented epithelium.
• The rods and cones are the visual receptors.
– Bipolar cells – middle layer
– Ganglion cells – innermost layer
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Schematic diagram of a rod and a cone
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• In each retina, there are about:
• 100 million rods
• 7 million cones and
• 1 million ganglion cells.
• There are also 2 groups of interneurones
1. The horizontal cells which interconnect adjacent rods
and cones.
2. The amacrine cells which interconnect the ganglion
cells.
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• Interneurones have only dendrites but no axons
• The links with other neurones are both presynaptic and
postsynaptic.
• Only the ganglion cells have axons.
• Incident light must therefore pass through layers of cells,
axons, and blood vessels before reaching the rods and
cones (photoreceptors).
 Fovea Centralis
• It is located in the center of the macula lutea (yellow spot)
as a small depression in the eye visual axis.
• Fovea centralis is point of highest visual acuity in daylight.
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Neural components of the extrafoveal portion of the retina. C, cone; R, rod; MB, RB,
and FB, midget, rod, and flat bipolar cells; DG and MG, diffuse and midget ganglion
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cells; H, horizontal cells; A, amacrine cells
• Nerve fibers and blood vessels which pass to and from the
optic disc do not pass over the macula lutea but around it.
• Its photoreceptors are cones only.
The Optic Disk (Blind Spot)
• Ganglion cells converge to form the optic nerve
• Optic nerve leaves the eye at the optic disk.
• It is situated about 3 mm to the nasal (medial) side of the
fovea centralis.
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• It is white in colour because axons are myelinated.
• No photoreceptors at optic disk hence blind spot.
 The Vitreous Humor
• It is a transparent embryonic tissue of gelatinous
consistency.
• It fills the posterior chamber between the lens and retina.
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Rod and cone density along the horizontal meridian through the human retina. A plot of the
relative acuity of vision in the various parts of the light-adapted eye would parallel the cone
density curve; a similar plot of relative acuity of the dark-adapted eye would parallel the
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rod density curve.
Retina seen through the ophthalmoscope in a normal human. The diagram on the
left identifies the landmarks in the photograph on the right. Monday, July 31, 2017
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Functions of the Complete Eye
• Visible light: Portion of electromagnetic spectrum
detected by human eye
• Refraction: Bending of light
• Divergence: Light striking a concave surface
• Convergence: Light striking a convex surface
• Focal point: Point where light rays converge and
cross
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2. OPTICS AND OPTICAL DEFECTS
 Image Formation on the Retina
• Refractory surfaces of the eye from the front are:
Anterior and posterior surfaces of the cornea
Aqueous humour
Anterior surface of the lens
Posterior surface of the lens
Vitreous humour
• When all these refractive surfaces are resolved to a single
plane, it forms the principal plane.
• It lies about 1.5 mm behind the cornea just in front of the
lens.
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• The diopteric power of the eye is about 60 D.
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• Near point of the eye is the minimum distance (from the
eye) from which an object can be properly focused on the
retina.
• It is about 10 cm in children, 25 cm in adult, and it
increases with age.
• The recession of the near point with age (beyond 40 cm due
to increase plasticity of the lens) is called presbyopia.
• The eye focuses a distant object on the retina.
• The image that is formed on the retina is inverted, but is
interpreted into the upright position by the brain.
• The other defects due to refractive errors of the eye are
myopia, hypermetropia and astigmatism Monday, July 31, 2017
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• One is legally blind when his visual acuity falls
below 20/200 feet.
• In myopia (i.e. short-sightedness), the image is
formed in front of the retina due to long eye balls or
increased curvature of the cornea.
• This defect is corrected by a concave lens which
diverges the in-coming light-rays and so allows the
lens to focus the image on the retina.
• In hypermetropia (i.e. long-sightedness), the image
is formed behind the retina due to short eyeballs or
reduced curvature of the cornea.
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• The correction is by convex lens which converges the
incident rays and allows the lens of the eye to focus the
image on the retina.
• Astigmatism is due to irregular curvature of the cornea so
that the image is refracted to different foci, causing
blurring.
• It is corrected by a cylindrical lens. It may be associated
with myopia.
• Presbyopia (i.e. far-sightedness) is due to old age and has
been described earlier. It is due to recession of the near
point (i.e.) moving away from the eyes.
• Visual acuity is tested by Snellen’s Charts.
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Common defects of the optical system of the eye. In hyperopia, the eyeball is too short and light rays come to a focus
behind the retina. A biconvex lens corrects this by adding to the refractive power of the lens of the eye. In myopia, the
eyeball is too long and light rays focus in front of the retina. Placing a biconcave lens in front of the eye causes the
light rays to diverge slightly before striking the eye, so that they are brought to a focus on the retina.
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Other Visual Tests
Visual Acuity
• In visual acuity, distance vision is tested using Snellen’s
chart.
• Snellen’s chart is a board on which rows of letters are
printed with the larger letters at the top.
• The figure that mark each of the rows indicates the
distance at which the thickness of the line of the individual
letter subtends an angle of 1’’ and the height of the letters
5’’ at the eye.
• At that distance, the normal eye conveniently distinguish
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the letters of that particular row.
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• The test is read at a distance of 6 m at which the normal
eye will read the row of the letters marked with figure 6.
• The card is placed in a good white light.
• One eye is tested at a time while the other eye is kept
covered with an opaque disc in a spectacle frame.
• Visual acuity is expressed by a fraction of which
numerator represents the test distance in meters i.e. the
reciprocal of the visual angle in minute.
• The denominator the smallest row of letters which can be
read at the distance.
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• Thus normal vision is ⁶⁄₆ viz. 6 is the smallest row of
letters that can be read at 6 meters. at which distance.
• If the subject has hypermetropia, he will be able to read the
same line with and without a convex (or positive) lens.
• If the subject has myopia, a negative (or concave) lens will
improve his acuity.
• Inversion of the Retinal Image
• Prick a hole in a piece of black paper, hold it in the left
hand about 3 inches from one while the other eye is closed.
• Look at the sky through the hole.
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• Hold a pin in your right hand so that its head is close to the
eye between the paper and the eye.
• It appears to be upside down.
• The light coming through the hole in the paper casts a
direct shadow of the pin’s head on the retina and this
shadow is the same way up as the pin itself.
 Near Point
• Hold an open book in front of the bare eye and bring it
nearer just before the point it can no longer be seen
clearly.
• Measure the distance to the eye.
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 Focusing and Accommodation Process
• Hold a pencil between one eye and the corner of the room
and keep the other eye closed.
• Attempt to focus both the corner of the room and pencil at
the same time.
• We can only focus on one thing at a time.
• If an open book is placed about 2 feet from the eyes and
the viewer sees it through a screen or fine net held at 6
inches from the eyes.
• He can see either the mesh of the screen or net or the
letters in the book with clarity, but not at the same time.
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• When the mesh is in sharp focus, the book is blurred but
when the letters are in sharp focus, the mesh is blurred.
• Accommodation enables man to shift his gaze from near
to far objects with ease and greater speed than the finest
camera of a most skilled photographer.
 Retina Vessels
• Look at the sky through a pin hole in a piece of black
paper held close to the eye.
• Close the other eye.
• Move the paper up and down and side to side.
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• The shadow of retina vessels will be seen
• The group of blood vessels seen depends on the direction
of the movement.
 Blind Spot
• Make 2 black circles of ⅛ in diameter and 4 in apart.
• Hold up the paper in front of the right eye at arm’s length.
• Close the left eye and fix the right eye on the left arm
mark and bring the paper slowly to the face.
• The right hand mark will disappear (i.e. when focused on
blind spot) and then reappear as it is brought nearer (i.e.
when focused on the fovea centralis). Monday, July 31, 2017
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 Scheiner’s Experiment
• Make 2 pin holes in a card of about 2 mm apart (i.e. less
than pupil diameter).
• Place the card close to one eye while the other is closed
and look through the hole at a distant object, say, across
the window.
• Hold a pencil with its point about 10 ins in front of the eye
such that it comes into the field of vision.
• While looking at the distant object, the pencil point
appears double.
• Carefully, slide along an opaque card to cover only one of
the pin holes and then focus on the pencil point.
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• The window cross-piece will then appear double.
• Again slide in a card to obscure the same pin hole as before
and notice that the double images disappear.
 After-Images: after-images are of same shape and size
with original stimulus
• Look intently at a bright circle of white light for 20-30 s in
the dark room.
• Turn off the light and look fixedly at a black surface where
an after-image of the original stimulus will be seen.
• This is positive after-image owing to the lack of second
stimulus.
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• Repeat the stimulation and quickly transfer the gaze to the
centre of white area larger than the original source of light.
• Owing to the second stimulus (the white area), the afterimage will be negative and appear as a dark area on the
white ground.
• Put a piece of red glass in the illuminated box and transfer
the gaze to a white area and the after-image will still be
negative and tinged with complementary colour of the first
stimulus.
• The negative after-image of white is black; of red is green;
of blue is yellow, and the longer and stronger the primary
stimulus, the stronger will be the after-image.
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 Streoscope Vision
• Thread a needle, note how you do it and how long it takes.
• Close one eye and repeat this experiment without your
hand touching each other along visual axis and at right
angle to the visual axis.
• It is easier done by the 2 eyes.
 Field of Vision
• The subject stands facing the examiner with his back to
the light at 2 ft.
• Each eye must be examined separately.
• The examiner closes his eye opposite the closed eye of the
subject.
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• The subject open eye should look fix on the examiner’s
open eye while the examiner holds his hand midway
between himself and the subject.
• The examiner then moves his outstretched forefinger from
the periphery towards the centre of the visual field.
• The subject is then asked to say when he sees the
movement of the finger as the same moment as the
examiner, provided they both have normal visual field.
• The movements of the hand are repeated in all the different
meridian of the field.
• Thus, the examiner’s field is compared with the that of the
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subject.
• The examiner constantly observe the subject’s eye in case
of any wandering from the point of fixation, which should
be corrected immediately.
• The perimeter is a more accurate tool for measuring visual
field.
 Chromatic Aberration
• Look at a bright circle of light through a piece of cobalt
glass, which transmit only red and blue rays.
• A halo of 1 colour will be seen round the light because, the
eye cannot focus at once long and short wavelengths.
• The halo will be elliptical in shape if the eye has
astigmatism.
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 Astigmatism
• Cover 1 eye and look at the astigmatic fan with the
other eye.
• The set of radiating lines on the card is astigmatic
fan.
• All line should appear black, but if some lines
appear grey or blurred, astigmatism is present.
• It is easier to say which lines are better seen than
which lines appeared blurred.
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 Simultaneous and Successive Contrasts
• Lay a piece of grey paper on a large piece of blue paper
and cover both with tissue paper.
• The grey will be tinged with yellow other colour
background could be used – simultaneous contrast.
• Look fixedly at the centre of a small red square on a grey
background; then look quickly at a piece of grey paper.
• The after image of the red square will be blue-green –
successive contrast.
• Other colours can also be used.
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3. The Visual Pathways
 Visual Field is the view seen by one eye without the
movement of the head.
• Lateral half of the visual field is called the temporal field
• Medial half of the visual field is called the nasal field.
• The retinal fibres also are similarly divided into nasal and
temporal fibres.
• Light from the temporal half of the visual field falls on the
nasal half of the retina.
• Light from the nasal half of the visual field falls on the
temporal half of the retina.
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• Light rays from upper right quadrant of the visual field
fall on the lower left quadrant of the retina.
• Light from the center of the visual field falls on the
macula lutea (fovea centralis).
• In this way, there is a total inversion of the image
formed at the retina.
 Field of Vision
• Place field of vision disk it against your forehead and
stare intently, straight ahead, at the target on the disk.
• Have your partner move the mobile target, first to one
side, then to the other while you continue to stare
straight ahead.
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• Record the exact angle at which the mobile target
completely disappears from your field of vision on the
Data/Analysis Sheet.
 Functional Structure of the Visual Pathways
• Fibers from the nasal half of each retina decussate at
the optic chiasma and run in the contralateral optic
tract.
• Fibers from the temporal half of each retina (rays from
the nasal field) run in the ipsilateral optic tract.
• Fibers of each optic tract synapse in the lateral
geniculate body of the thalamus.
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• Geniculocalcarine fibers pass by way of optic radiations
(geniculocalcarine tract) to the primary visual cortex
calcarine sulcus occipital lobe (Broadman’s area 17, also
known as V1).
Ganglion cell projections from the right hemiretina of each eye to the right lateral geniculate body and from
this nucleus to the right primary visual cortex. Note the six layers of the geniculate. P ganglion cells project
to layers 3–6, and M ganglion cells project to layers 1 and 2. The ipsilateral (I) and contralateral (C) eyes
project to alternate layers. Not shown are the interlaminar area cells, which project via a separate
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component of the P pathway to blobs in the visual cortex.

Lesions of the Optic Pathway
i.
Optic nerve lesion results in total blindness in the
affected eye (monocular blindness).
ii. Optic tract lesion interrupts crossed nasal fibres from
the opposite eye and temporal fibres from the same
side.
• The result is loss of visual field in the temporal side of
the opposite eye (nasal fibres) and nasal field of the
same side (temporal fibres) resulting in Homonymous
hemianopia.
• If the right or left optic tract is damaged, it results in left
or right homonymous hemianopia respectively.
• Homonymous hemianopia refers to the side of the body
viz. left- or right-sided visual sides of the body.
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iii. Optic chiasma lesion interrupts fibres from nasal part of
both retinae and hence both temporal fields resulting in
bitemporal hemianopia.
•
Bitemporal hemianopia in optic chiasma lesions is an
example of heteronymous hemianopia since both
temporal fields are affected i.e. the left temporal field is
left of the body and the right temporal field is the right
side of the body i.e. left and right sided or heteronymous.
iv. Optic radiation – results in homonymous hemianopia but
the macula fibres are not affected. The macular fibres run
separately from the peripheral visual fields and they also
have large number of fibres and so are not easily affected
by occipital lesions.
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Visual pathways. Transection of the pathways at the locations indicated by the letters
causes the visual field defects shown in the diagrams on the right (see text). Occipital
lesions may spare the fibers from the macula (as in D) because of the separation in the
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brain of these fibers from the others subserving vision . Monday, July 31, 2017
Visual Pathways
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Eye Disorders
• Myopia: Nearsightedness
– Focal point too near lens,
image focused in front of
retina
• Hyperopia: Farsightedness
– Image focused behind
retina
• Presbyopia
– Degeneration of
accommodation, corrected
by reading glasses
• Astigmatism: Cornea or lens
not uniformly curved
• Strabismus: Lack of
parallelism of light paths
through eyes
• Retinal detachment
– Can result in complete
blindness
• Glaucoma
– Increased intraocular pressure
by aqueous humor buildup
• Cataract
– Clouding of lens
• Macular degeneration
– Common in older people, loss
in acute vision
• Diabetes
– Dysfunction of peripheral
circulation
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 Connections of the Optic Tract or Visual fibres
i. As above to the lateral geniculate body and then
to the calcarine/visual cortex for vision.
ii. Pass from the optic chiasma to the
suprachiasmatic nucleus of the hypothalamus for
control of circadian rhythm.
iii. From the lateral geniculate nucleus to the pretectal
nucleus of the midbrain which then relays through
the occulomotor nerve and Edinger-Westphal
(pre-tectal) nucleus to mediate the pupillary and
consensual light reflexes.
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iv. From the lateral geniculate nucleus to the superior
colliculus which then relays to the visual cortex
and medial longitudinal bundle to cause eye
movements synchronous with that of the body
during postural adjustments.
v. Into the ventral lateral geniculate nucleus of the
thalamus, to help control some of the body’s
behavioral functions.
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 Responses in the Visual Pathways
• 2 types of ganglion cells are found in the retina
• Large ganglion cells (magno or M cells) are
concerned with movement and sterropsis
• Small ganglion cells (parvo or P cells) are
concerned with color, texture and shape.
• Axons of ganglion cells (optic nerve) project a
detailed spatial representation of retina on the
lateral geniculate body.
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• Each geniculate body or nucleus contains 6 well
defined layers
• Layers 1 and 2 have large cells and are called
magnocellular
• Layers 3-6 have small cells and are called
parvocellular
• On each side, layers 1, 4 & 6 receive input from
the contralateral eye
• Layers 2, 3 & 5 receive input from the ipsilateral
eye
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• In each layer there is a precise point–to–point
representation of the retina.
• Only 10-20% of inputs to the lateral geniculate
nucleus comes from the retina.
• Major inputs occur from the visual cortex and other
brain regions.
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Ganglion
cells
P
M
Lateral
geniculate
MagnoParvo-cellular
Inter- Via
cellular
laminar dendrites laminae
laminae
(layers 3-6)
region
(layers 1 & 2)
Visual
Cortex
Superficial
layer 4C
Blobs
Deep layer 4C
Function Movement,
depth, flicker
Color
vision
Color, texture,
shape, fine detail
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ORGANISATION OF THE VISUAL PATHWAYS
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 The Visual Cortex
• The Primary Visual Cortex
• It is also called Broadman’s area 17 or VI
• It receives point-to-point representation from the
lateral geniculate body
• It processes input in terms of orientation, edges etc
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Medial view of the human right cerebral hemisphere showing projection of the
retina on the occipital cortex around the calcarine fissure.
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 Other Cortical Areas Concerned with Vision
• The primary visual cortex projects to several other
areas of the occipital cortex and the brain.
• These areas are as listed below.
1. V2,V3, VP – Continued processing, larger
visual fields
2. V3A – Motion
3. V4v – Unknown
4. MT/V5 – Motion, control of movement
5. LO – Recognition of objects
6. V7 – Unknown
7. V8 – Color vision
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Some of the main areas to which the primary visual cortex (V1)
projects in the human brain. Lateral and medial views
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 Visual Reflexes
i. Pupillary light reflex
ii. Consensual light reflex
iii. Accommodation reflex
i. Pupillary (Direct) light reflex
• When bright light shines into one eye, it results in
pupillaryconstriction in the affected eye.
• Pathway: retinal cells → pretectal nucleus →
Edinger-Westiphal nucleus → occulomotor nerves
→ pupillary constriction.
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ii. Consensual light reflex
• When bright light is shines into one eye, it results
in pupillary constriction in both eyes.
• Pathway: retinal cells → Edinger-Westiphal
nucleus → occulomotor nerves (of both eyes) →
pupillary constriction of both eyes.
• The pupils of both eyes are constricted by the
contraction of their pupillary circular muscle
(sphincter pupillae).
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iii. Accommodation Reflex
• Accommodation reflex is the reaction of both eyes when
they are focused on a near object.
• It consists of:
i. Pupillary constriction
ii. The contraction of the ciliary muscles in order to relax
the pull on the suspensory ligament, increasing the
curvature of the lens, thus allowing focusing on the
retina.
iii. The convergence of the eyeballs in order to enable
same part of the object to be focused simultaneously
on same part of both retinae.
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Accommodation. The solid lines represent the shape of the lens, iris, and ciliary body at
rest, and the dashed lines represent the shape during accommodation.
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Focus and Accommodation
• Emmetropia:
Normal resting
condition of lens
• Far vision: 20 feet
or more from eye
• Near vision: Closer
than 20 feet
– Accommodation
– Pupil
constriction
– Convergence15-79
Decline in the amplitude of accommodation in humans with advancing age. The different symbols
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identify data from different studies
The Retina
• Provides black
backdrop for
increasing visual
acuity
• Sensory retina and
pigmented retina
• Photoreceptors
– Rods: Noncolor
vision
• Rhodopsin
reduction:
Light
adaptation
• Rhodopsin
production:
Dark adaptation
– Cones: Color
vision
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 Photoreceptive Mechanism of the Eye
• Cones are for daylight and colour vision.
• They have high threshold and are therefore used for bright
light.
• Rods are for monochromatic and twilight or night vision.
• They have low threshold (scotopic vision).
• The visual pigment in the rods in rhodopsin derived from
the proteins - opsin and retinal, the aldehyde form of
vitamin A or retinol.
• Light rays cause decomposition of rhodopsin leading to the
development of an action potential.
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Rhodopsin Cycle
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Range of luminance to which the human eye responds, with the receptive mechanisms involved.84
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• The pigment for cones is not well-known.
• The cone pigments are for red sensitivity
(erythrolabe), green sensitivity (chlorolabe) and
blue sensitivity (cyanolabe).
• Rhodopsin = Scotopsin (an Opsin) and retinal.
• Retinal exists as cis-retinal in the dark.
• When light falls on the retinal cell, cis-retinal is
converted to all-trans-retinal.
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• This leads to the formation of intermediates including
metarhodopsin II, which then initiates the development of
an action potential in the photoreceptor cell via closure of
Na+ channels hyperpolarisation
• There is decreased transmitter release and excitation of
bipolar and ganglion cells causing impulse transmission to
the brain.
• This mechanism involves cyclic GMP gated ion channels.
• Deficiency of vitamin A in the diet will result in nightblindness (nyctalopia) since it is a component of
rhodopsin.
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Sensory Receptor Cells
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Rod Cell Hyperpolarization
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Effect of light on current flow in visual receptors. In the dark, Na+ channels in the outer segment are
held open by cGMP. Light leads to increased conversion of cGMP to 5'-GMP, and some of the
channels close. This produces hyperpolarisation of the synaptic terminal of the photoreceptor. 89
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Initial steps in phototransduction in rods. Light activates rhodopsin, which activates transducin to bind
GTP. This activates phosphodiesterase, which catalyzes the conversion of cGMP to 5'-GMP. The
resulting decrease in the cytoplasmic cGMP concentration causes cGMP-gated ion channels to close.
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Sequence of events involved in phototransduction in rods and cones
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 Colour Vision
• Young-Helmboltz or trichromatic theory proposes
three types of cones response to light of 3 different
wavelengths/colours i.e. 450 nm (Blue), 550 nm
(Green) and 600 nm (Red).
• The colour perceived is a combination of the
degree to which each of these cones or primary
colours are stimulated.
• Equal stimulation of the three cones leads to a
white colour.
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Absorption spectra of the three cone pigments in the human retina. The S pigment that peaks at 440 nm senses
blue, and the M pigment that peaks at 535 nm senses green. The remaining L pigment peaks in the yellow portion
of the spectrum, at 565 nm, but its spectrum extends far enough into the long wavelengths to sense red
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• Colour blindness can occur when any or all (total
colour blindness) of the three cone types are
absent.
• These are dichromats and monochromats.
• Trichromats have all the 3 cones and so have
normal colour vision.
• Monochromats are completely colour blind.
• They only have one cone type and see only black
and white or grey.
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• Dichromats have two cone types losing either that
to Red (protanopia) or Green (deuteranopia).
• They confuse red or green and perceive colours in
shade of yellow and blue.
• Blue cone loss (tritanopia) is rare.
• Colour blindness is a sex-linked recessive gene
and therefore is more common in males.
• Colour blindness is tested by Ishihara charts.
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 Dark Adaptation
• When the eye is exposed to poor light it adapts and
becomes increasingly sensitive to the dark
resulting in increased visual acuity.
• There are two components:
An initial rapid adaptation within 5 minutes
though with poor detail.
• This is accomplished by the cones.
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Then follows a more prolonged adaptation which is
about 60% of the total adaptation.
• This results to an increase in acuity and is due to
the Rods.
• Hence the rods contribute a quantitatively greater
proportion to visual adaptations in the dark, due to
their higher content of photopigment.
•
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Dark adaptation. The curve shows the change in the intensity of a stimulus
necessary to just excite the retina in dim light as a function of the time the
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observer has been in the dark.
EFFECTS OF AGING ON THE SPECIAL SENSES
• Slight loss in ability to detect odors
• Decreased sense of taste
• Lenses of eyes lose flexibility
• Development of cataracts, macular degeneration,
glaucoma, diabetic retinopathy
• Decline in visual acuity and color perception
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