Testing Infants and Toddlers

7/29/2019 Testing Infants and Toddlers

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Psychology 4051

Assessing Vision in

Infants and Toddlers

http://picasaweb.google.com/anngwinn/ZoeMonth14/photo



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 voluntarybehavioral 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.




• 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.




• Method of Constant Stimuli

• Variable stimuli are presented in random

order.




• 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.




• 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 suprathresholdintensities.




• The subject must detect the stimulus twice on at

each level.

• 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 .




• Stimulus level is then increased until the subject

is correct twice.

• Stimulus level is then decreased.

• Stimulus level is clustered around the subject’sthreshold.

– Should ensure accuracy and brevity.

– May lead to boredom.



The Problem with Infants

• Nonverbal

• Behavioral techniques rely on the finding thatinfants 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.




• 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).




• 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 locationbased 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.




• Size is relative, andone’s distance fromthe target must betaken into account.

• Spatial frequency: thenumber of time thepattern repeats in 1degree visual space.

• Measured in cyclesper degree (cpd).




• Low spatial frequencies

(2 cpd) correspond to

thick stripewidths.• High spatial

frequencies correspond

to thin stripewidths.

– 30 cpd = 20/20




• 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 fromlow to high.

• Each card contains a

3 mm peephole.

Teller Acuity Cards (TAC)



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.

Documents

Testing Infants and Toddlers