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Examination of vision
Outline
• theoretical background
– the eyeball
– the optical system of the eye
– the visual field
• practical examination
- visual acuity
- refractive errors
- accomodation
- adaptation
- blind spot
- visual field
- color vision
- optical illusions
The visual system
• is
the part of the central nervous system which enables
organisms to process visual detail
• it interprets information from visible light to build a
representation of the surrounding world
• assimilate information from the environment (70-75%)
• the resulting perception: eyesight, sight, or vision
• 3 segments:
• receptor level (eyeball)
• visual pathways
• central level (cortical and subcortical structures)
The visual system
• complex tasks:
– reception of light
– formation of monocular representations
– construction of a binocular perception from a pair of
two dimensional projections
– identification and categorization of visual objects
– assessing distances to and between objects
– guiding body movements in relation to visual objects
– non-image forming functions (e.g. pupillary light
reflex, circadian photoentrainment)
The eyeball
• complex role: optical device, signal transducer, neural
processor
• structure: three coats – enclosing transparent structures
The eyeball
Outer layer:
• sclera – white, fibrous, protective
- contains collagen and elastic fibers
- muscles attached to the sclera
• cornea – transparent, covers the iris, pupil, and anterior
chamber
- refracts light (accounts for approximately 2/3 of the eye's
total optical power, fixed focus)
- avascular, rich sensitive innervation
• the cornea and sclera are connected by a ring called the
limbus
The eyeball
Middle layer:
• choroid - the vascular layer of eye
- contains connective tissue
- provides oxygen and nourishment to the outer
layers of the retina
- pigmented, reduces the reflexions of light
• ciliary body
- roles: accommodation, aqueous humor production
- anchors the lens in place
• iris
- regulates the amount of light entering the eye:
muscles attached to the iris expand or contract the
aperture at the center of the iris (pupil)
The eyeball
Middle layer:
• zonule of Zinn
- ring of fibrous strands
- connects the ciliary body with the crystalline lens of
the eye
• lens
- flexible
- changes the focal distance of eye
- thus the eye can focus on objects at various
distances (accommodation)
The eyeball
Inner layer:
• retina
- neural structure
- senses light
- receptor cells: rods and cones (S, M, L)
- neurons: bipolar and ganglion cells
- gets its circulation from the vessels of the choroid as
well as the retinal vessels - can be seen in an
ophthalmoscope
The eyeball
within these coats:
• aqueous humor: a clear fluid
- is contained in two areas:
- the anterior chamber between the cornea and the
iris and exposed area of the lens
- the posterior chamber, behind the iris, in front of the
zonule and rest of the lens
• vitreous body
- clear jelly
- is bordered by the sclera, zonule, and lens
The eyeball
• geometry: not properly a sphere,
rather it is a fused two-piece unit
– smaller frontal unit, more
curved (r = 8 mm)
– larger posterior unit (r =
12 mm)
– fundus (area opposite the
pupil) shows the
characteristic pale optic disk
(papilla), where vessels
entering the eye pass across
and optic nerve fibers depart
the globe.
The optical system of the eye
• image formation:
dioptre (diopter): unit of measurement of the optical power of a lens or curved
mirror, which is equal to the reciprocal of the focal length measured in metres
(that is, 1/metres)
• cornea: 42 D, lens: 18 D, total ~ 60 D
Visual pathways
Visual field
Outline
• theoretical background
– the eyeball
– the optical system of the eye
– the visual field
• practical examination
- visual acuity
- refractive errors
- accomodation
- adaptation
- blind spot
- visual field
- color vision
- optical illusions
Visual acuity
• acuteness or clearness of vision
• tested using an optotype (Snellen chart) read from
a distance of 5 (minimal accomadation)
• expressed as a vulgar fraction:
– numerator: the distance between the subject and the
chart
– denominator: the distance at which the lines that make
up those letters would be separated by a visual angle of
1 arc minute
– e. g. normal: 5/5
• lenses of varying powers are used to precisely
correct for refractive errors
Visual acuity
Tumbling E Chart
Visual acuity
Landolt C (Landolt ring)
Tumbling E Chart
Refractive (refraction) errors
• an error in the focusing of light by the eye
• a frequent reason for reduced visual acuity
• no refractive error when viewing distant objects:
emmetropia or emmetropic eye
• the eye can focus parallel rays of light (light from distant
objects) on the retina, without using any accommodation;
a distant object in this case is defined as an object 5
meters or further away from the eye
• refractive error when viewing distant objects: ametropia or
ametropic eye: when not using accommodation, the eye
cannot focus parallel rays of light (light from distant objects)
on the retina
• types of ametropia: categorized as spherical errors and
cylindrical errors
• spherical errors occur when the optical power of the eye is
either too large or too small to focus light on the retina
Myopia
• myopia (nearsightedness): when the optics are too
powerful for the length of the eyeball; can arise from a
cornea with too much curvature (refractive myopia) or an
eyeball that is too long (axial myopia)
• correction:
Hyperopia
• hyperopia (farsightedness): when the optics are too weak
for the length of the eyeball; can arise from a cornea with not
enough curvature (refractive hyperopia) or an eyeball that is
too short (axial hyperopia)
• correction:
Accommodation
• the process by which the vertebrate eye changes optical
power to maintain a clear image (focus) on an object as its
distance changes
• reflex, but can also be consciously controlled
• by changing the form of the elastic lens using the ciliary
body
Accommodation
•the amplitude of accommodation declines with age: by the
fifth decade of life the accommodative amplitude has
declined so the near point of the eye is more remote than
the reading distance = presbyopia
• emmetropic eye (does not require optical correction for
distance vision) needs an optical aid for near vision
• myopic eye (nearsighted, requires an optical correction
for distance vision) – less correction needed
• hyperopic eye (farsighted) - needs a correction for both
distance and near vision
Accommodation
• curtain experiment: one can look through the window at a
distant tree, or can adjust our eyes so as to see the curtain,
bot never in the same time
• Scheiner’s experiment: uses a card with two minute holes,
separated from each other by less than the diameter of the
pupil, one looks at a pin; at a short distance from the eye
the pin appears double; as it is moved from the eye a point
is found where it appears single, and beyond which it
remains single for the emmetropic eye, but for the myopic
eye it soon again becomes double
3.5 mm from
centre to centre
Holes 1.5 mm
in diameter
Accommodation
• Purkinje-Sanson experiment: three images of the same
object, produced by reflections from the surface of the
cornea and the anterior and posterior surface of the lens,
the images on the two anterior surfaces are virtual and
noninverted, and the image on the posterior surface is real
and inverted (the viewer adapts to this phenomenon and
ignores these normal images)
Adaptation
• the human eye can function from very dark to very
bright levels of light; its sensing capabilities reach
across nine orders of magnitude
• pupillary light reflex, adjusting the
• two mechanisms:
amount of light that reaches the
retina
- myosis:
contraction of the pupil
- mydriosis:
dilatation of the pupil
• changes in the sensitivity of rods
and cones in the eye
Adaptation
Recovery time of a retinal pigment:
• a strong white light is projected in the eye for 3-4 sec, then is
turned off and a weak light is turned on (one of the basis colors)
• the time is measured from the moment of turning off the white
light to the moment when the colored light is detected (normal:
< 10 sec)
Latency:
• the time from the application of the light stimulus to the
appearance of activity in the optic nerve (~1/10 sec)
Persistance:
• the sensation of light remains for ~0.15 sec after the end of the
stimulus
Flicker fusion threshold (or flicker fusion rate)
• the frequency at which an intermittent light stimulus appears to
be completely steady to the observer
Astigmatism
• cylindrical refraction error
• the optical power of the eye is too powerful or too weak
across one meridian (as if the overall lens tends towards a
cylindrical shape along that meridian)
• blurred vision - due to the inability of the optics of the eye
to focus a point object into a sharp focused image on the
retina
• Placido keratoscope
Blind spot
• Mariott's test
• close your left eye and stare at the square mark in the
diagram with your right eye, off to the right you should be
able to see the spot
• slowly move toward the computer screen (or paper),
keep looking at the square mark while you move
• at a particular distance the spot will disappear (it will
reappear again if you move even closer)
• the spot disappears because it falls on the optic disc
• for the left eye, stare at the spot and notice the
disappearance of the square
Visual field
• perimetry is the systematic measurement visual field detection of the presence of test targets on a defined
background
Visual field
Visual field loss
• due to disease or disorders of the eye, optic nerve, or
brain
• scotoma
Visual field loss
• due to disease or disorders of the eye, optic nerve, or
brain
• hemianopia / quadrantanopia
Color vision
• the capacity of an organism or machine to distinguish
objects based on the wavelengths (or frequencies) of the
light they reflect, emit, or transmit
• dyschromatopsia color vision deficiency is the decreased
ability to perceive differences between some of the colors
that others can distinguish
Optical illusions
Optical illusions
• negative afterimage
Stare, unfocused, at the red cross for 10 seconds then look at white wall!
Optical illusions
• negative afterimage
Optical illusions
• negative afterimage
Stare, unfocused, at the flag for 10 seconds then look at white wall
Optical illusions
• negative afterimage
Cyan
Magenta
Yellow
Optical illusions
• lilac chaser
Optical illusions
• cognitive illusions are assumed to arise by
interaction with assumptions about the world
• Necker cube
Optical illusions
• cognitive illusions are assumed to arise by
interaction with assumptions about the world
• Rubin vase
Optical illusions
• cognitive illusions are assumed to arise by
interaction with assumptions about the world
• ambiguous image
Optical illusions
• cognitive illusions are assumed to arise by
interaction with assumptions about the world
• Kanizsa triangle
Optical illusions
• cognitive illusions are assumed to arise by
interaction with assumptions about the world
Optical illusions
• distorting illusions are characterized by distortions of
size, length, or curvature
Müller-Lyer illusion
Optical illusions
• distorting illusions are characterized by distortions of
size, length, or curvature
cafe-wall illusion
Optical illusions
Optical illusions
• simultaneous contrast
Optical illusions
Optical illusions
Optical illusions