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215 > PL & VEP > Flashcards

PL & VEP Flashcards

1
Q

Qualitative Technique to assess VF

A

Infants prefer to fixate faces or complex patterns than homogenous fields

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2
Q

Quantitative techniques for VF in infants

A

-Preferential looking
-Visual evoked potential
-Optokinetic nystagmus

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3
Q

PL procedure for acuity assessment

A

-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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4
Q

what do infants preferentially fixate over?

A

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

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5
Q

limitations of early PL procedure

A

-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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6
Q

forced choice PL (FPL) trial results

A

-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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7
Q

strengths of FPL

A

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

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8
Q

limitations of FPL

A

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

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9
Q

FPL and operant PL in humans and monkeys

A

-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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10
Q

FPL Teller Acuity Card Procedure

A

-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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11
Q

Normative Binocular PL Acuity Procedures

A

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

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12
Q

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

A

IOD < 1 octave

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13
Q

when is PL acuity development in premature infants more predictable?

A

more in post term rather than post natal age

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14
Q

post natal

A

months after birthdate

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15
Q

post term

A

months after birthdate minus # months premature

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

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16
Q

based on postnatal age VA in premies show ____

A

delayed acuity development

17
Q

clinical applications of acuity card procedures

A
  • 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
18
Q

Strengths of PL testing

A
  • Non-invasive, easy to perform and inexpensive
  • Applicable to several clinic populations
    –Present choices up-down to patients with nystagmus
19
Q

Procedural Limitations of PL testing

A
  • 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
20
Q

what is VEP

A
  • Electrical signal generated in the occipital region of the cortex in response to visual stimulation
  • Reflects activity of post-synaptic potentials
21
Q

VEP procedure

A

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

22
Q

how can VEP vary

A

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)

23
Q

VEP amplitude:
Estimating Visual Acuity

A
  • 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
24
Q

VEP amplitude: Fellow and Amblyopic Eye

A
  • 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
25
Q

Spatial Frequency
Sweep VEP Procedure

A
  • 10 seconds per trial
  • Present 19 different spatial frequencies
  • Linear extrapolation of VEP amplitude to zero mv for acuity estimate
26
Q

Spatial Frequency
Sweep VEP Advantages

A
  • Increased sampling of VEP amplitude versus spatial frequency function
  • Repeated measures
  • Clinical applicability
27
Q

Sweep Visual Evoked Potential Normal Visual Acuity Development

A

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

28
Q

Sweep Visual Evoked Potential Clinical Applications

A
  • Amblyopia
  • Media opacities
  • Cortical visual impairment
    – Recordable flash VEP in behaviorally blind
29
Q

does visual experience improve acuity?

A
  • 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
30
Q

VEP testing strengths

A
  • 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
31
Q

VEP testing Limitations

A
  • Expense, personnel expertise
  • Does it tell you what the patient can see?
  • Grating acuity underestimates vision loss compared to Snellen acuity
32
Q

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

A

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

33
Q

Difference in scoring
techniques for FPL and VEP grating acuity?

A

– 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

34
Q

differences between FPL and VEP acuity

A
  • 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
35
Q

advantages and disadvantages of PL

A

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

36
Q

adv and dis of VEP

A

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

37
Q

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

A
  • 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

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