Lecture 3 : Studies of abnormal visual development

Aim

• To understand abnormal development of Binocular Vision from animal and human studies

Critical Periods

• the period of development during which the anatomy and physiology of the visual system is plastic

Critical Periods

• term first coined by Hubel and Wiesel to refer to the period of age during which suturing the eyelids of a kitten closed had an effect on the ocular dominance of cells in the visual cortex

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Cat: artificially induced alternating strabismus

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RE closed for 9 days at birth: 4 years later there is little recovery

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RE closed at birth suturing is then reversed at 5 weeksat 6 months the cortical cells preferred the RE

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RE closed at 7 days for 1 year thenreversed for another yearLE still dominates

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Effect of reverse suture at 3 weeks of age on labelling pattern in the right cortex.Right eye sutured then opened and left eye sutured. Right eye was injected. RE dominated layer IVC As layer was still plastic at 3 weeks. IVC white as critical period was over (Le Vey et al., 1980)

Comparison of critical periods for direction selective and ocular dominance changes.Animals reared RE open movement right and then LE open movement left. Varied the age of the switch (Daw IOVS 1994).

• It has become apparent the critical period depends on factors such as– The species – Part of the visual system being recorded– Type of deprivation– Previous history

• Therefore one can not talk about one critical period

• These results show that ocular dominance changes can be produced after the initial development of ocular dominance columns has largely finished

• This seems to be the case in all visual properties

• Visual properties usually approach the adult level in an exponential manner making it difficult to say exactly when adult level is reached

• However we can say that modifications can occur after the time that they are so close to adult level that one cannot tell that they are different from the adult value

Daw - 3 stages in the development of each visual property

1 Sensitivity of property is refined to adult level (or no measurable difference)

2 Cells in visual cortex can still be changed by a deprivation that affects the property

3 Properties cannot be changed by visual deprivation except if previously changed during steps 1 or 2 then may be altered again

• The visual system is more plastic and remains plastic for a longer period of time at higher levels of processing

• Retina is largely hard wired at birth

• Some plasticity in the LGN

• Output layers of the primary visual cortex are more plastic than the input layers

• The visual cortex projects to the inferior temporal cortex where faces and objects are recognised and plasticity continues for a substantial period of time

• Visual memories are also stored in the temporal cortex and plasticity continues indefinitely

• Temporal cortex projects to the hippocampus which is known to be plastic in the adult

• Different visual properties have different critical periods eg direction and ocular dominance

Examination of Children

• Recommedation on Vision Screening

• American Optometric Association Board of Trustees (1993)– Infants/Toddlers (birth-2yrs): examine by 6 months

– Pre-school (2-5years): examine at age 3 and age 5

– School age : examine annually

Examination of Children

• Must be able to properly examine children of all ages

• Appreciate what constitutes normal and abnormal visual development

• MAXIMUM information• SHORTEST time

Examination of a <1 year old

• History and Symptoms– Gain as much information from the parent

– History of patient

– Reason for visit

– Difficulties with birth

– General health

– Patients ocular history

– Family ocular history

Preliminary testing

• Pupils: small until 6-12 months• Visual field: confrontation difficult• Bruckner test: pupil of the most turned eye will be

brighter and whiter• Tonometry: tonopen

Visual acuity

• <6 months: alteration of fixation - vertical prism dissociation

• > 6 months– Preferential looking

– Child will look at a patterned target rather than a blank screen

– No difference in average brightness between targets and its background

– If child looks at towards target consistently then they can resolve the detail within

Preferential looking

• Series of grey cards with a central peephole• Black and white grating is positioned at one side

of the peephole• Grating varies in SF throughout the series• Examiner views the child’s responses• Proceed down the card series until examiner

cannot make the right judgement (2 correct responses mean that you proceed to the next scale)

Preferential looking• Teller Cards

– Have a square wave grating format than can give rise to an edge artifact - edge of grating is visible even when individual stripes are not

• Keeler Cards– Grating lies within a narrow white circular band (blank white

band on other side)

– Takes longer as examiner has to allow time for child to look at both sides of the card

– Judges position of the grating from the length of time fixated

Alternate Occlusion

• If there is a big difference in acuity between the two eyes

• Occlude each eye in turn and gauge the child’s response

Hirschberg technique

Corneal reflections central and symmetrical

Rule: 1mm = 20∆ (approx. 11°)

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Corneal reflections displaced temporally = esodeviationCorneal reflections displaced nasally = exodeviation

Convergence

• Use an interesting detailed target

Motility

• Use toy of some kind • Get parent to hold the child’s head

Presence of Binocular Vision

• 20 prism dioptre test– Infants

– Young, unco-operative children

– Mentally handicapped

Cover Test

• Use an interesting detailed target• Change target frequently• Use hand as occluder

Stereopsis

• Lang test

– Dissociates images from the right and left eyes with the use of tiny cylinders on surface

– Binocularly 3D figure “pop out” of the background

– In 1year onwards children will enjoy touching (or naming) the pictures

– Less than 1 year you will have to watch the child’s eye movements

– Check if movements are consistent with the position of the test figures

Stereopsis

• Lang Test

– Stereotargets become invisible when presented vertically - useful for comparison

– 1200 - 200 seconds of arc

• Frisby Stereotest

– Uses real depth cues

– With minor modifications can be used to test infants from 7 months

Retinoscopy• Be quick

– Looking for gross differences between the eyes or large refractive errors

• Near retinoscopy• Cycloplegia

– Usually cyclopentolate will do– Use when non-cycloplegia failed– Fixation/accm unsteady– Convergent strabismus– Esophoria (significant or symptom producing)– Significant difference between ret and subjective

Examination of a 1-2 year old• Family history from parent• Cover test• Motility• Retinoscopy• Ophthalmoscopy• Visual acuity• Stereopsis

– Get better results with this than VA

Visual Acuity• Cardiff Acuity Test

– 1-3 years old

– Preferential looking

– Picture instead of gratings

– 50cm or 1m viewing

– 3 cards at each acuity level

– Keep toddlers motivated

– Must assess only the eye movements

Visual Acuity• Kay Picture Test

– Single black pictures on a white background

– 8 different pictures used

– Snellen progressionsome experience required by the child

– Examiner must assess what is correct

Examination of a child greater than 3 years old

• History and symptoms• Visual acuity (67% compliance with letter testing

at 3 years old)• Motility• Stereopsis (TNO, Frisby, Titmus)• Retinoscopy - cycloplegic• Ophthalmoscopy

Visual Acuity

• Sheridan Gardner– Single letter presentation in Snellen progression

– Has a keycard

– No crowding present

Visual Acuity

• Cambridge Crowing Cards– Aimed especially at the detection of amblyopia

– Can be performed on children as young as 2 years

– Snellen letter progression

– Greater reduction than one line between singles and crowded card would indicate amblyopia

– Control of contour interaction

Visual Acuity

• Logmar Crowded test– 3m test distance

– Equal number of letters per line letters of approxiamtely equal legibility

– Letters symmetrical about vertical midline

– Acuity scale 3/19 to 3/1.5

– Geometric progression of letter size

– Screening cards

– Control of contour interaction