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.

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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 signLock hand in position…see which eye is still aligned by closing the otherThe eye with good alignment is your dominant eye!

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Vertebrate Eye

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What?

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