Download Use both eyes

3-D Vision
• One person holds
test tube at arms
length
• Other holds pencil in
arm upright
• Try to swing down
lower arm to place
pencil directly in
test tube
• Repeat, with one
eye closed
Visual Acuity
• Visual acuity refers to the sharpness of
one’s eyesight. Acuity is reported as a
fraction.
– Numerator – distance from the subject to the chart.
– Denominator – size of the symbol the person can see
at 20 feet.
– Example – 20/40, at 20 feet the eye sees at a 20 foot
distance what a normal eye sees at 40 feet.
Testing Distance
Visual Acuity
General Procedures:
• Make sure the room is well lit.
• Choose appropriate chart.
• Chart at eye level.
• Mark off appropriate distance for chart.
• Child position their heels on the line.
• Use an occluder. Make sure the child does not peek.
Vision Charts
Vision Charts (cont.)
Testing Distance
Visual Acuity (cont.)
• Watch carefully to observe if the child is peeking,
tilting their head, or squinting.
• Instruct the child to keep both eyes open and read the
selected letter or line of letters with the uncovered
eye. Do not use a marking device as a pointer.
• Child must identify or match greater than 60% of
the letters on the critical line.
Watch Out!
• Watch for
peeking
* Resource – MA Preschool Vision Screening Protocol
WHAT IS ASTIGMATISM?
• An optical defect.
– Vision is blurred.
– Inability to focus.
• Cause
– An irregular curve in the lens.
Types of Astigmatism?
• Regular
– Three types
• With-the-Rule
• Against-the-Rule
• Oblique
– Caused by an
irregularity in the
cornea or crystalline
lens.
• Irregular
– Caused by a corneal
scar or scattering in the
crystalline lens.
Demonstration of Blind Spot
Basic Phenomenon
• The next slide has a cross on the left and a
circle on the right.
• Close or cover your left eye, then look at the
cross with your right eye.
– Move your head slowly either toward the screen
or away from the screen, staring at the cross
with your right eye, until the circle disappears
– When the circle disappears, you have found
your right eye’s blind spot.
Variations on Basic Phenomenon
• The next slide has a colored background
with a brightness gradient that is darker
behind the cross and lighter behind the
circle
• Find your right eye blind spot
• Is the brightness at the blind spot darker,
like the gradient behind the cross, or
lighter, like the gradient behind the circle?
Variations on Basic Phenomenon
• The next slide has a cross on the left and a
circle on the right
• The circle has been marked off by a ring
• Find your right eye blind spot
• Determine whether or not you can see the
ring. If you cannot see the ring, adjust its
size to make it larger.
Variations on Basic Phenomenon
• The next slide has a cross on the left and a
circle on the right
• The circle has been marked off by a ring
filled with color
• Find your right eye blind spot
• What color is the area where the circle was
before it disappeared?
Variations on Basic Phenomenon
• The next slide has a cross on the left and a
circle on the right
• The circle has been marked off by a ring
filled with a textured coloring
• Find your right eye blind spot
• What texture is the area where the circle
was before it disappeared?
Variations on Basic Phenomenon
• The next slide has a cross on the left and a
circle on the right
• The circle has been marked off by a ring
filled with colored stripes
• Find your right eye blind spot
• What happens to the lines that go through
the blind spot?
Variations on Basic Phenomenon
• The next three slides have a cross on the left
and a circle on the right
• The circle has been marked off by a ring
filled with a color
• The ring around the circle is smaller in each
of the slides
• Find your right eye blind spot
• What happens as the ring gets smaller?
Concluding Remarks
• What do your observations suggest
about the visual processing
mechanisms that produce the blind
spot?
Blind Spot and Fovea
Use both eyes…at arm’s length, center target within finger OK sign
Lock hand in position…see which eye is still aligned by closing the other
The eye with good alignment is your dominant eye!
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Copyright Norton Presentation Manager
Vertebrate Eye
senses
image
focuses
image
blind spot
What?
controls light
sends image to brain
Use both eyes, stare at area shown in blue for 15 seconds or so
What do you see on this blank white slide? Blink if needed!
This is called an “after image”
Does it move around as you move your gaze?
Experiments:
Visual Accomodation
• Near Point of Vision (Presbyopia)
– Test one eye at a time
– Place meter stick on bridge of nose
– Focus on pencil tip
– Draw tip along meter stick towards eye
– Point at which tip just begins to become
fuzzy = near point of vision.
Myopia
• The most common
of the refractive
errors. Formerly
called
“nearsightedness”.
Unable to see
clearly at
distance.
Hyperopia
• A refractive error in which the light rays
from an incoming visual image have not
converged by the time they reach the
retina. Formerly called “farsightedness”.
Near vision may be blurry.
• A mild degree of hyperopia is normal and
asymptomatic in young children. High
degrees of hyperopia can cause poor
near acuity and/or strabismus.
Strabismus
• A manifest deviation of one
or both eyes from the visual
axis of the other so they are
not simultaneously directed
to the same object. Also
referred to as heterotropia,
or tropia.
Amblyopia
• An ocular condition in an otherwise
healthy eye, in which there is an
abnormality of cortical response in the
occipital lobe of the brain due to
insufficient or inadequate stimulation
of the fovea, neural pathway, and
cortex that may result in unilateral
vision loss if untreated.
Amblyopia
Good eye
Amblyopic eye