PL & VEP Flashcards by Athena Chen

Qualitative Technique to assess VF

Infants prefer to fixate faces or complex patterns than homogenous fields

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Quantitative techniques for VF in infants

-Preferential looking
-Visual evoked potential
-Optokinetic nystagmus

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PL procedure for acuity assessment

-infant presented with 10 pairings of a grating target and a homogenous field
-5 different spatial frequencies (sf) used
-each sf presented twice on left or right of peephole
-record duration of looking
-number of fixations of each stimulus

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what do infants preferentially fixate over?

all age groups, the grating that infants fixated preferentially over the homogeneous field

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limitations of early PL procedure

-acuity results based on group not individual infant
-subjective nature of observer’s task
>judge how long infant fixated
>how many times infant fixated on each stimulus
>which stimulus initially fixated and corneal reflection of grating apparent to observer

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forced choice PL (FPL) trial results

-correct ranges from near chance 50% for highest sf to 100% for lowest sf
-acuity is sf produced 75% correct by observer

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strengths of FPL

-large # of trials in short duration
-little observer bias
-allows acuity estimate for individual infant

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limitations of FPL

-limited usefulness in clinic setting
-60 trials for acuity estimate w/n 1 octave
-single acuity estimate takes 15 min

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FPL and operant PL in humans and monkeys

-both show similar acuity near birth
-increase with age but faster by a factor of 4 in monkeys
-acuity is roughly numerically equal to age (months for human infants, weeks for monkey infants)

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FPL Teller Acuity Card Procedure

-series of card @ 50cm
-rapid presentation of grating
-interact with infant b/w stimulus presentations
-judgement about grating location is not scored on trial by trial basis
-judge based on quality and consistency of looking behavior whether the infant can resolve that grating
-wide stripes, based on 2 presentations
-fine stripes, several presentations

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Normative Binocular PL Acuity Procedures

-method of constant stimuli
-staircase procedures
-acuity card procedure
-good agreement despite variation in PL procedure and stimuli

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Normative PL data for mean interocular acuity differences for normal infants tested with PL procedures

IOD < 1 octave

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when is PL acuity development in premature infants more predictable?

more in post term rather than post natal age

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post natal

months after birthdate

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post term

months after birthdate minus # months premature

(1 mo premature, 3 month postnatal = 2 month post term)

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based on postnatal age VA in premies show ____

delayed acuity development

clinical applications of acuity card procedures

Development of acuity in infants with delayed visual maturation who were tested with the acuity card procedure
Infants showed little visual attention in early infancy and poor VAs
Improvement in visual responsiveness & VAs in first months after birth

Strengths of PL testing

Non-invasive, easy to perform and inexpensive
Applicable to several clinic populations
–Present choices up-down to patients with nystagmus

Procedural Limitations of PL testing

Variability of PL results
–Accurate to only + or - 1 octave (+/- 3 lines in logMAR)
–Mean test-retest variability (0.5 octave not uncommon)
Potential for observer bias
Grating acuity is not the same as Snellen (or optotype) acuity

what is VEP

Electrical signal generated in the occipital region of the cortex in response to visual stimulation
Reflects activity of post-synaptic potentials

VEP procedure

-clean scalp to reduce impedance
-apply electrode paste
-apply recording elecrodes

how can VEP vary

waveforms vary by characteristics of stimuli
– Pattern v. luminance
– Stimulus presentation mode
» Onset/offset
» Reversal
– Stimulus temporal frequency
» Transient (<4 cycles/sec)
» Steady-state (>4 cycles/sec)

VEP amplitude:
Estimating Visual Acuity

Signal averaging required to reveal the 1 to 10 uv VEP response embedded within the 50 to 100 uv background EEG activity
VEP amplitude decreases linearly with increasing spatial frequency of the stimulus
Adult-like VEP waveform by 6 months of age
Visual acuity adult-like by 6 months of age

VEP amplitude: Fellow and Amblyopic Eye

VEP amplitudes to gratings reduced in the amblyopic eye compared to the fellow (normal) eye
VEP amplitude to luminance flashes are the same in amblyopic and fellow eyes
Cortical not retinal locus for amblyopia

Spatial Frequency Sweep VEP Procedure

* 10 seconds per trial * Present 19 different spatial frequencies * Linear extrapolation of VEP amplitude to zero mv for acuity estimate

Spatial Frequency Sweep VEP Advantages

* Increased sampling of VEP amplitude versus spatial frequency function * Repeated measures * Clinical applicability

Sweep Visual Evoked Potential Normal Visual Acuity Development

Visual acuity improves from 4.5 c/d (20/130) at birth to 20 c/d (20/30) by 8 months

Sweep Visual Evoked Potential Clinical Applications

* Amblyopia * Media opacities * Cortical visual impairment – Recordable flash VEP in behaviorally blind

does visual experience improve acuity?

* The “pre term” infants show significantly higher acuity than the “post term” infants during the first few months of life * Pre term – Born more than 1 week but less than 3 weeks before due date * Post term – Born late; after due date

VEP testing strengths

* Rapid test, does not require subjective response from subjects * Applicable to several clinic populations * Inter ocular differences and 99% confidence intervals are 2-4 times less than PL –Mean test-retest variability and IOD (< 0.25 octave) * Grating acuity norms are higher than PL

VEP testing Limitations

* Expense, personnel expertise * Does it tell you what the patient can see? * Grating acuity underestimates vision loss compared to Snellen acuity

Is there a Discrepancy between FPL and VEP grating acuity for visual stimuli?

probably not – VEP gratings modulated temporally » Pattern reversal or pattern onset-offset – PL static presentation of grating target – No difference between PL acuity with reversing checkerboards compared to static square wave gratings – Similar PL acuity in 1-3 mo olds using static and drifting gratings

Difference in scoring techniques for FPL and VEP grating acuity?

– PL strict criteria: spatial frequency required to elicit 75% correct responding – VEP criteria: spatial frequency for which VEP amplitude is 0 uV Does not account for all of the difference – If use 55% correct in PL scoring, would increase acuity estimates by 1 to 1.5 octaves » 55% is close to chance levels – Discrepancy between PL and VEP acuity varies across age

differences between FPL and VEP acuity

* VEP, PL and OKN yield different acuity estimates at some ages because they tap different visual mechanisms * VEPs measure visual responses in early cortical processing * FPL techniques rely on the whole infant. Later stages of visual processing, attention and motor control are needed to produce looking behavior * Information is available in the primary visual cortex but the infant is unable to use the information to guide his or her looking behavior

advantages and disadvantages of PL

A: * Portability * Ease of use * Accessibility D: * Requires cooperation * Observer bias * Limited durability of cards * Grating acuity measure

adv and dis of VEP

A: * Objective test * Expect better acuity than PL, smaller IOD and St. Dev * Rapid repeated measures D: * Requires technician * Cost of equipment * Grating acuity measure

what is VA based on? OKN, VEP and PL?

* Visual acuity based on the highest spatial frequency of the moving grating at which a reliable OKN response was obtained * Adult OKN acuity estimates do not agree with psychophysical estimates