4.2 Classical Psychophysical Methods (Fechner)

what is the method of limits

Stimuli are presented in ordered, step-by-step sequences of either ascending (subthreshold → suprathreshold) or descending (suprathreshold → subthreshold) series until the observer's response shifts at a transition point

subthreshold

stimulus below threshold

suprathreshold

stimulus above threshold

how do we measure absolute threshold in classical psychophysical methods

the stimulus is presented in isolation as we establish what is the minimum light intensity for observer to detect the stimulus

how do we measure difference threshold in classical psychophysical methods

stimulus is presented against a background and we find the minimum difference in light intensity between the stimulus and the background so that the observer is able to detect the difference

describe descending method of limit

start with stimuli well above threshold and decrease light intensity as experiment progress until observer unable to detect stimulus

describe ascending method of limits

stimulus presented initially at low intensity below the observer’s threshold, and gradually the intensity of the stimulus is increased

why do we need several trials to calculate an average threshold for method of limit

because observer’s threshold fluctuates from time to time

what is interval of uncertainty (IU)

stimulus range over which the subject does not notice a difference between the reference and comparison stimulus

formula for IU

IU = upper limen – lower limen

formula for difference threshold for the method of limit

DL = IU / 2 = (LU - LL)/2

define upper limen

Point where judgment shifts between "Equal" and "Greater"

define lower limen

Point where judgment shifts between "Less" and "Equal"

formula for Point of Subjective Equality (PSE)

PSE = (LU+LL)/2)

define point of subjective equality (PSE)

point where the reference and comparison stimulus are perceived as equal (midpoint between LU and LL)

define point of objective equality (POE)

point where the reference and the comparison stimulus are measured as equal using an objective technique

formula for constant error

CE = POE - PSE

what is constant error

the difference between POE and PSE

what happens when we combine ascending and descending limits

the habituation and anticipation are cancelled out because the patient is not habituated in one direction of responses and they don’t anticipate a certain response

advantages of method of ascending limits

does not dirupt adaption of subject especially dark adaptation

disadvantages of method of ascending limits

patient’s anticipation

habituation

practice

fatigue

not knowing what the stimulus is before experience starts

precautions for method of ascending limits

ensure observer understands the test

test multiple times for reliability

vary the starting point to prevent habituation

example of method of ascending limits

visual fields (perimetry)

advantages of method of descending limits

patient familiarized with stimulus

obtains practice before reaches threshold

disadvantages of method of ascending limits

disrupts adaptation especially dark

patient’s anticipation

habituation

fatigue

precautions for descending method of limits

ensure observer understands the test

test multiple times for reliability

vary the starting point to prevent habituation

example of method of descending limits

snellen visual acuity

how do we determine the difference threshold using method of limits

the observer needs to make a comparison between a standard (or reference) stimulus, and a comparison (or test) stimulus

what is the adaptive method of method of limits

staircase method

how does the staircase method work

The stimulus intensity automatically reverses direction (ascending <–> descending) whenever the observer's response changes

what is the transition point in the staircase method called

reversal

how do we determine the observer’s threshold with the staircase method

using the average of the last three or four reversals

the direction of the stimulus of a staircase method depends on what

the observer’s responses

advantages of the staircase method

keeps stimulus close at threshold always and won’t disrupt observer’s adaptation

disadvantages of the staircase method

boredom

learning effect

how to prevent disadvantages in staircase method

use the double random staircase method

define double random staircase method

The two staircase stimuli are randomly presented to the observer so that they don’t know what to expect. Each set of reversals will be analyzed independently to measure their thresholds.

what do we do if it takes too long to reach the first reversal in the staircase method

use the Parameter Estimation by Sequential Testing (PEST)

define Parameter Estimation by Sequential Testing (PEST)

it uses a different approach to achieving reversals without causing observers to experience fatigue or boredom

how does PEST differ from regular staircase method

The step of decreasing or increasing the stimulus’ characteristic is not constant throughout the test, but changes depending on the observer’s response

how does PEST work

• Stimulus is first presented as either subthreshold or suprathreshold

• Depending on the observer’s response, the next stimulus’s value will change by a large step, eliciting the opposite response from the observer

advantages of Parameter Estimation by Sequential Testing (PEST)

Concentrates testing right around the threshold, dramatically shortening test duration and minimizing patient fatigue

example of PEST

Automated Visual Field testing (Perimetry) in glaucoma screening

golden standard of psychometric test

method of constant stimuli

define method of constant stimuli

Predefined stimuli shown in random order and the observer is asked if they see the stimulus by responding with YES or NO

why is the observer exposed to each value of stimuli multiple times during the experiment

for higher accuracy

how is the Psychometric Function or the Frequency of Seeing (FOS) Curve plotted

number of YES responses as a function of stimulus intensity

how do we determine the stimuli used in the experiment for the method of constant stimuli

we need to run preliminary experiments to establish the range within which the threshold for our observer population might be

advantages of method of constant stimuli

no habituation

no anticipation

disadvantages of method of constant stimuli

time consuming

why is the method of constant stimuli the golden standard

it yields highly accurate estimates of absolute threshold

what can FOS curve reveal even if the threshold is calculated to be within normal limits

abnormalities on the detection of a stimulus

what is the S-shaped function that reveals abnormalities on the detection of a stimulus in the FOS curve

ogive

how to measure difference threshold for the method of constant stimuli

• a reference stimulus and a comparison stimulus are shown to the subject

• the value of the comparison stimulus is changed from trial to trial and the subject is required to judge whether this value is higher or lower than the value of reference stimulus

what is the LL in method of constant stimuli for difference threshold

intensity value at 25% higher

what is the LU in method of constant stimuli for difference threshold

intensity value at 75% higher

x and y axis for ideal/real observer

intensity, % detected

x and y axis for x and y axis for the method of limits

presentation sequence and stimulus intensity

x and y axis for the method of constant stimuli

stimulus intensity and % YES

x and y axis for the method of constant stimuli - difference threshold

stimulus intensity and % higher

what is PSE on the method of constant stimuli for difference threshold

intensity value at 50% higher responses

in method of constant stimuli, what does a steep psychometric function mean?

small difference limen (DL) and high sensitivity. Observer responses changes decisively from not greater to greater over a very narrow range of stimulus values

in method of constant stimuli, what does a shallow psychometric function mean?

large difference limen (DL) and low sensitivity (large change in stimulus intensity is needed before the observer ‘s judgement changes

define method of adjustment

Observer adjusts the stimulus using the criterion given by the experimenter and each trial begins with a subthreshold or suprathreshold stimulus

how to measure the absolute threshold in the method of adjustment

average of multiple thresholds defined during multiple trials

advantages of the method of adjustment

natural and straightforward to observer, observer won’t lose interest if they in control of experiment

disadvantages of the method of adjustment

• experimenter does not have control of experiment

• takes time to complete test for inexperienced observers

• observers may use cues in experimental setup to set the threshold

If use method of adjustment to measure the difference threshold for an observer, what is it called

method of average error

how to obtain method of average error

it requires a reference stimulus and a test stimulus and the patient to adjust the test stimulus until it matches the reference stimulus

what is the matching point in the method of average error

point of subjective equality (PSE)

in the method of average error, what does the experiment do after knowing the point of objective equality (POE)

they can use it to calculate the constant error for each trial

what is the difference threshold in method of adjustment

it is a range of difference thresholds instead of a single value because constant errors are normally distributed

how is POE and CE distributed

normally distributed or bell shaped

when plotting PSE for difference threshold in the method of adjustment, what is the x and y axis

constant error, frequency with which L is seen as equal to POE

what does a small deviation mean in the difference threshold plot for method of adjustment

it means small DL (difference threshold), high precision, and the observer’s matches are very consistent

define alternative forced choice (AFC)

it  presents a set of stimuli and “forces” the observer to choose from these options

why is method of forced choice used

because observers have different criteria for setting their threshold and criteria for an observer change between trials from moment to moment

example of method of forced choice for optometric test

The Jackson Cross Cylinder

x and y axis for method of forced choice

intensity and % correct

starting point for 2AFC experiment

50%

threshold for 2AFC experiment

75%

how do we obtain threshold for method of forced choices

halfway between the starting point and 100%

starting point for 4AFC experiment

25%

threshold for 4AFC experiment

62.5%

Increasing the number of choices __ the probability of guessing correctly

decreases

how do we obtain the starting point for alternative forced choice

it is the % chance of the observer getting it correctly

how must stimuli be presented for alternative forced choice

either at the same time and in separate retinal locations or at the same retinal location and at different times

how to control space error for alternative forced choice

reference stimulus will be randomly presented half the time in one location, and half the time at the other location

how to control time error for alternative forced choice

reference stimulus must be presented as first in half of the trials and as second in the other half of the trials

Method of counterbalancing spatial location or temporal order

based on assumption that when the results from all the trials are combined, the effects of the time and space errors will be cancelled, providing an unbiased estimate of DL

what is Response bias

a strong preference that an observer shows toward one or another response when making a decision

what is the method to determine degree to which an observer is guessing (guessing rate)

catch trials

formula to control for YES bias

YES_corrected = (YES - G) / (1 - G) x 100%

formula to control for NO bias

YES_corrected = (YES) / (1 - G) x 100%

guesing rate formula for biases towards YES

G = false positives/all trials

guesing rate formula for biases towards NO

G = false negatives/all trials

when will the correction formulae for response bias work

only under the assumption that the guessing rate is constant throughout the range of stimuli (assume that the observer’s rate of bias does not change)

we do not need to use catch trials under which method

method of constant stimuli

ideal observer

real observer