Psychology 4051
Assessing Vision in
Infants and Toddlers
Outline
• Psychophysical Testing
– Preferential looking
– Forced-choice preferential looking
– Habituation
• Electrophysiological Testing
– Visual evoked potential (Sweep VEP)
Psychophysics
• Measurement of thresholds.
– Absolute Threshold: The minimal (smallest,
dimmest, softest) stimulus that can be
detected.
– Difference Threshold: The minimal detectable
change between two stimuli.
• Relies on some sort of voluntary
behavioral response from the subjects.
– Referred to as behavioral testing.
Psychophysics
• Uses input mapping strategies to measure
thresholds.
– Different stimulus sizes/intensities are presented until
the threshold is found.
– But the use of different sizes/intensities takes time
and infants and toddlers may become fussy, bored,
and/or sleepy.
Threshold Measurement
• Method of Limits
• Method of Ascending Limits: Multiple stimulus
levels are presented.
• Subject provides a yes/no response.
• Stimulus level is low initially (subthreshold) but is
then presented in progressively increasing
values until threshold is reached.
Threshold Measurement
• Method of Descending Limits: Stimulus levels
are high initially (suprathreshold).
• Stimulus levels are presented in decreasing
order until threshold is reached.
• On each trial, the subject provides a yes/no
response.
Threshold Measurement
• Method of Constant Stimuli
• Variable stimuli are presented in random
order.
Threshold Measurement
• Method of Adjustment
• Subject controls the stimulus levels.
• Stimulus levels are increased or decreased
(adjusted) until threshold level is reached.
• In each of these procedures, multiple estimates
of threshold are taken.
Threshold Measurement
• Staircase Procedures
• Stimulus level on each trial depends on whether
the subject was correct or incorrect on the
previous trial.
• Most common procedure is the two-down one-
up procedure.
• Stimulus presentation begins at suprathreshold
intensities.
Threshold Measurement
• The subject must detect the stimulus twice on
each trial.
• If the subject is successful, stimulus level is
decreased.
• Once the subject makes one error, stimulus level
is increased.
– This change in direction is reversal.
Threshold Measurement
• Stimulus level is then increased until the subject
is correct twice.
• Stimulus level is then decreased.
• Stimulus level is clustered around the subject’s
threshold.
– Should ensure accuracy and brevity.
– May lead to boredom.
The Problem with Infants
• Nonverbal
• Behavioral techniques rely on the finding that
infants prefer a patterned stimulus over an
unpatterned stimulus (Fantz, 1958).
• Stimuli can be presented simultaneously and by
pairing a patterned stimulus with a blank field.
• Infants will prefer to look at the patterned
stimulus.
The Problem with Infants
• If the infant can detect the stimulus,
he/she will prefer to look at it.
• The infant’s direction of first fixation,
number of fixations, total fixation time on
each field can be measured.
– This is known as preferential looking (PL).
The Problem with Infants
• In a variant of this procedure, the two stimuli are
presented.
• An observer who is unaware of the location of
the patterned stimulus must judge its location
based on any aspect of the infant’s behavior.
– Forced-choice preferential looking (FPL)
Assessing Infants and Toddlers
• These techniques can be combined with
psychophysical techniques to measure visual
function in infants.
Visual Acuity Measurement
• Visual Acuity: the
smallest pattern that can
be resolved or
recognized.
• In infants, visual acuity
can be measured using a
square wave grating.
• Striped patterns that vary
in size.
Visual Acuity Measurement
• Size is relative, and
one’s distance from
the target must be
taken into account.
• Spatial frequency: the
number of time the
pattern repeats in 1
degree visual space.
• Measured in cycles
per degree (cpd).
Visual Acuity Measurement
• Low spatial frequencies
(2 cpd) correspond to
thick stripewidths.
• High spatial
frequencies correspond
to thin stripewidths.
– 30 cpd = 20/20
Visual Acuity Measurement
• Stripe size can be varied, and the thinnest
stripe size detected by the infant can be
taken as a measure of visual acuity.
– Resolution acuity
– Grating acuity
The Teller Acuity Cards
• A series of
rectangular cards.
• Each contains a
square wave grating
opposite a blank field
of equal average
luminance.
• Overall, spatial
frequency varies from
low to high. Teller Acuity Cards (TAC)
• Each card contains a
3 mm peephole.
The Teller Acuity Cards
• The cards can be presented through an
opening behind a backboard to reduce
distraction.
Habituation
• Infants habituate to stimuli that are
presented repeatedly.
• They dishabituate, or recover, when a
novel stimulus is presented.
– Infant can discriminate the two stimuli.
Habituation
• Thus, in a visual habituation study, a blank
pattern can be presented repeatedly.
– Fixation time is recorded.
• Presentations continue for a fixed number of
trials, or until a fixation time criteria is reached.
• A high frequency square wave grating is then
presented.
• If the infant dishabituates, he/she can detect the
grating.
Habituation
• The highest spatial frequency grating that
causes dishabituation can be taken as a
measure of visual acuity.