2. Objectives
1 Describe the principles of optics, errors of
refraction and their correction.
2 Explain spherical and chromatic aberrations
and mechanisms of accommodation.
3 Describe type of eyeball movements and their
control centres.
4 Explain the processing of the form, movement
and colour of objects and relate to sequential
and parallel processing.
5 Describe the principles of assessment of defects
in field and colour vision.
6 Describe Visual Evoked Potentials and their
clinical applications.
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10. Basic Principles of Optics
• Speed of light in air - 3x108m/sec.
• Much slower through transparent solids and
liquids, e.g. glass 2x108m/sec.
• The refractive index of a substance is the ratio
of the speed of light in air to the speed of light
in the substance.
• When light travels from one medium to
another it changes direction providing it strikes
the surface at an angle and not at 90o. This
process is called refraction.
11. • A convex lens allows light rays to converge to a
focal point.
• A concave lens allows light to diverge.
• A cylindrical lens focuses light to a focal line as
opposed to a focal point.
• The focal length of a lens is the distance from
the lens to the focal point when the lens focuses
light form a distant source.
• The refractive power of a lens is measured in
diopters and is equal to one metre divided by
the focal length.
Basic Principles of Optics
12. The Eye
• The main function of the eye is to bring light
into focus upon the visual receptors i.e. the rods
and cones of the retina (in this way the eye is
remarkably like a camera).
• The Process: an image is formed upon the
retina light energy is transduced into
electrical signals information needed to
create the image is encoded by neurons within
the retina and the brain the information is
used by the visual cortex to create the visual
perception we call ‘seeing’.
13. How is this Achieved?
• Light must enter the eye - this occurs via an
opening in the eye called the pupil.
• The amount of light entering the eye is
controlled by the iris (a group of radial and
circular muscles that surround the pupil) and is
proportional to the square of the diameter, i.e.
the area of the pupil.
• Changing the size of the pupil also alters the
depth of field of the image and the amount of
spherical aberration produced by the lens.
14. How is this Achieved?
• Light must be focused upon the retina -
focusing is achieved by the ability of the
components of the eye to refract light.
• The majority of refraction takes place at the
interface between the cornea and the air.
• The lens is the other principal agent of
refraction.
• Accommodation is the ability of the lens to
change its refractive power thus allowing the
eye to focus on images at varying distances.
15. Terms Describing the Eye
• Emmetropia – the normal eye; when the ciliary
muscles are completely relaxed the emmetropic
eye has the ability to focus light from a distant
object onto the retina; it is also able to
accommodate to see objects at close range.
• Presbyopia – as a person ages the lens becomes
larger, thicker and less elastic. As a result the
power of accommodation decreases greatly to
about 2D at 50 years of age and 0D at 70.
16. • Hyperopia – or ‘far sightedness’ is as a result of
the lens focusing the image behind the retina.
This can be due to an eyeball that is too short
or a lens system that is too weak.
• Myopia – or ‘near sightedness’ is a result of the
image being focussed in front of the retina. This
may be due to an eyeball that is too long or a
lens system that is too strong.
• Astigmatism – the image in different planes
focuses at different distances from the retina. It
is most often due to a ‘wavy’ corneal surface.
Terms Describing the Eye
17. Other Considerations
• The design of the eye is such that light from the
focus of our gaze, i.e. the fixation point, is
always brought to focus upon a specific region
of the retina called the fovea. This is referred to
as the visual axis.
• Normal visual acuity for the human eye is
approximately 45 seconds of an arc. The
Snellen chart measures visual acuity.
• There is a region upon the retina called the
blind spot or optic disk. This region is devoid of
photoreceptors.
18. • Depth Perception is achieved by three
means:
– If one is aware of the size of the object then
the brain automatically calculates roughly
how far away the object is based upon the
visual stimulus.
– A phenomenon known as moving parallax.
– Due to binocular vision the image formed
upon each retina is slightly different. This is
called stereopsis.
Other Considerations
19. • The shape of the eye is maintained by two fluid
filled regions, one in front of the lens and one
behind the lens.
• Behind the lens the chamber is filled with a
thick jelly-like substance called the vitreous
humor.
• The region in front of the lens is divided into a
posterior and anterior chamber by the iris and
is filled with aqueous humor.
• Glaucoma is a condition in which the intra-
ocular pressure is higher than normal.
Other Considerations
22. • The choroid layer is a pigment layer that
surrounds the eye and is continuos with the iris.
Functions include:
– It is designed to absorb excess light entering
the eye and prevents scattering of light.
– Phagocytosis of the end of the outer
segments of the rods that are continually
being removed.
– Conversion of the utilized photopigments
back to an active form.
– Storage of vitamin A.
The Retina
23. • This is a multi-step process designed to convert
the light energy of photons into electrical
impulses in the rods and cones.
• Steps:
1 Light energy is ‘captured’ by the visual
pigments called opsins.
2 Rhodopsin is composed of the protein scotopsin
+ retinal (derived from vitamin A) in its 11-cis
form. Only this form can bind with scotopsin to
form rhodopsin.
Phototransduction
24. Phototransduction
3 Light energy is absorbed by rhodopsin and this
initiates a series of reactions due to the
conversion the 11-cis-retinal to all-trans-retinal.
4 All-trans retinal is converted back to the cis
form via an isomerase directly or indirectly via
conversion to retinol a form of vitamin A.
• How does this alter the membrane potential of
the rods?
25. Phototransduction
5 Under dark conditions the outer segments of the
rods and cones are extremely permeable to
sodium ions.
6 The decomposition of rhodopsin decreases the
permeability of the outer segment to Na+
resulting in cell hyperpolarization as the inner
segment is still pumping sodium out of the cell.
7 Metarhodopsin II transducin
phosphodiesterase cGMP. cGMP is
responsible for the leaky nature of the outer
segment membrane to sodium.
26. Visual Processing
• Begins within the retina itself, continues in the
Lateral Geniculate Nucleus (LGN) and is
completed in the visual cortex.
• Ganglion Cells - the output cells of the retina.
• The axons of ganglion cells coalesce to form the
optic nerve of each eye.
• Ganglion cells transfer information by firing
action potentials. Photoreceptors, horizontal
and bipolar cells all respond with graded
changes in their membrane potentials.
27. • Each ganglion cell has a receptive field, that is a
particular area upon the retina which when
stimulated by light modulates the activity of the
ganglion cells.
• The receptive fields of ganglion cells are:
– circular
– divided into two regions: a central circular
zone, the centre, and the region around it
called the surround.
• Ganglion cells process information along two
parallel pathways.
Visual Processing
28. Types of Ganglion Cells
• X cells - most numerous in the retina, medium size,
10-15mm, transmit at a rate of 14m/s, small receptive
fields. Mainly responsible for the visual image (fine
detail) and colour vision.
• Y cells - as large as 35mm and transmit at 50m/s or
greater, broad dendritic fields and are believed to be
responsible for detecting rapid changes in light
intensity or rapid movements across the visual fields.
They make up only 5% of the total.
• W cells - 10mm in size and transmit signals at 8m/s.
Receive most input from the rods, via inter-neurons,
have wide extensive dendritic trees and are believed
to be responsible for detecting directional movement.
29. Visual Processing
• Information is transferred from the
photoreceptors to the ganglion cells via the
inter-neurons, i.e. the bipolar, horizontal and
amacrine cells.
• Bipolar cells have a receptive field arrangement
with centre-surround organization. On centre
bipolar cells depolarize in response to light
while off-centre bipolar cells hyperpolarize.
• To achieve this glutamate is released from the
cones and this in turn activates ionotropic and
metabotropic receptors respectively.
30. The Visual Pathway
• The visual field is the view seen by the two eyes
without movement of the head. There are also a
corresponding left and right visual fields.
• Ganglion cells converge upon the optic disk
where they are myelinated and leave as the
optic nerve.
• This sends projections to three sub-cortical
regions after crossing at the optic chiasm, the
LGN (visual processing), the superior colliculus
(eye movements) and the pretectal region
(pupillary reflexes).
31. Lateral Geniculate Nucleus
• The vast majority of neurons from the retina
terminate here.
• Due to crossing at the optic chiasma the left
hemifield of vision is represented by the right
optic tract and ends in the right LGN.
• The LGN has a visuotopic map, i.e. a specific
point on the retina always stimulates the same
point in the LGN.
• Neurons from the fovea and the regions around
it make up approximately half of the neural
mass in the LGN.
32. • The LGN has a very orderly arrangement.
• The first two layers are called the
magnocellular cells layers and receive input
from the M or Y cells.
• The other four are called the parvocellular
layers and receive input from the X or P cells.
• A layer receives input from one eye only, with
the contralateral nasal hemiretina ending in
layers 1, 4 and 6 and the ipsilateral temporal
retina in layers 2,3, and 5.
Lateral Geniculate Nucleus
33. The Visual Cortex
• From the LGN fibres pass to an area termed
the primary visual cortex or visual area 1
(Brodmann’s area 17) or the striate cortex.
• It is located in the region of the calcarine
fissure, extending to the occipital pole on the
medial aspect of the occipital cortex.
• The secondary visual area surrounds this
region, is located in Brodmann’s area 18 and is
also called the V-2 region. This area is
responsible for the analysis of visual signals.
34. • The cells from the LGN synapse in layer 4C on
spiny stellate cells. This layer responds to the
small circular spots of light similar to those that
have activated the visual system so far.
• The layers above and below this respond to
more complex patterns of activation.
• Their receptive fields are rectangular as
opposed to circular. Two types of cells have
been identified depending upon their pattern of
activation, simple cells and complex cells.
The Visual Cortex
35. • Simple cells are similar to those found in the
LGN, they have on and off zones, but a line of
light activates them and their receptive field is
larger. In addition the on zone is always in a
particular axis of orientation.
• Complex cells have a larger receptive field and
the axis of orientation is also important.
However they do not have distinct on and off
zones and so the precise position of the stimulus
in the receptive field is not as important.
The Visual Cortex
36. Colour Vision
• Colour greatly enhances contrast between and
within objects and so improves visual
processing.
• Colour vision is achieved by the existence of
three different photo-pigments with different
but overlapping absorption spectra. This is
called trivariancy.
• The first makes a strong contribution to blue
light (B or S – 420nm). The second contributes
mostly to green (G or M – 531nm) and the third
contributes most to red (R or L – 558nm).
37.
38. • Although colour vision greatly enhances our
ability of contrast it is limited in two ways.
• The first of these is due to the fact that if an
object only stimulates a single cone then no
contrast can be achieved.
• The second is a property of short wavelengths
of light termed chromatic aberration. Images
produced by such light are inherently blurred.
Colour Vision
39. • Colour opponent theory –certain colours can
never be perceived, e.g. a reddish green or a
bluish-yellow colour. This is believed to be due
to an opponent system. The antagonistic pairs
are red-green; blue-yellow and black-white.
Presumably this is because light that excites a
green cone when processed inhibits a red cone.
• It is important to appreciate that the brain
computes colour perception by not only
analyzing the response of the cones that are
stimulated by the incident light but by
analyzing the response of all the cones to light.
Colour Vision
40. Role of the Pretectal Area
• Light shone directly into either eye initiates a
reflex constriction of the pupil - the direct
response. At the same time there is also a reflex
constriction of the pupil in the other eye - the
indirect response.
• These reflexes are mediated by the ganglion
cells that detect overall changes in brightness
and project to the pre-tectal area.
• Efferent pathway: Pre-tectal area Edinger
Westphal nucleus ciliary ganglion
postganglionic fibres smooth muscle of the
iris.
41. Eye Movements and the
Superior Colliculus
• Three pairs of extra-ocular or extrinsic muscles
control eye movements, namely, the medial and
lateral recti, the superior and inferior recti, and
the superior and inferior obliques.
• These muscles are reciprocally innervated and
connected via the medial longitudinal fasiculus.
This allows one muscle of each pair to contract
while the other relaxes. Cranial nerve 4
innervates the superior oblique, 6 the lateral
rectus and 3 the rest.
42. • Fixation movements - 2 types voluntary and
involuntary.
• A bilateral region in the pre-motor cortex of the
frontal lobe controls voluntary movements.
• The secondary visual cortex via the superior
colliculus achieves involuntary fixation.
• There are three types of movements involved in
involuntary fixation.
Eye Movements and the
Superior Colliculus
43. 1 A continuous tremor of 30-80Hz due to
successive contractions of the extra-ocular
muscles.
2 A slow drift across the fovea.
3 A sudden flicking movement that occurs when
the spot of light reaches the edge of the fovea
ands is designed to bring it back into focus.
• Saccadic movement is the sudden jumping of
the eyes to maintain focus upon a continual
changing visual field, e.g.s driving in a car or
reading.
Eye Movements and the
Superior Colliculus
44. • Saccades occur extremely rapidly and the
visual image is suppressed during the eye
movement.
• Another type of movement is smooth pursuit
and involves the eyes following the course of
movement of an object.
• The superior colliculi have a topographical
map of both the retina and the somatic
sensations of the body and thus are responsible
for turning the eyes and the head towards or
away from a particular light source.
Eye Movements and the
Superior Colliculus
