PL & VEP Flashcards
Qualitative Technique to assess VF
Infants prefer to fixate faces or complex patterns than homogenous fields
Quantitative techniques for VF in infants
-Preferential looking
-Visual evoked potential
-Optokinetic nystagmus
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
what do infants preferentially fixate over?
all age groups, the grating that infants fixated preferentially over the homogeneous field
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
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
strengths of FPL
-large # of trials in short duration
-little observer bias
-allows acuity estimate for individual infant
limitations of FPL
-limited usefulness in clinic setting
-60 trials for acuity estimate w/n 1 octave
-single acuity estimate takes 15 min
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)
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
Normative Binocular PL Acuity Procedures
-method of constant stimuli
-staircase procedures
-acuity card procedure
-good agreement despite variation in PL procedure and stimuli
Normative PL data for mean interocular acuity differences for normal infants tested with PL procedures
IOD < 1 octave
when is PL acuity development in premature infants more predictable?
more in post term rather than post natal age
post natal
months after birthdate
post term
months after birthdate minus # months premature
(1 mo premature, 3 month postnatal = 2 month post term)
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
