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 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.
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 times 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. 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.