Visual perception Pyschophysical Methodoly

what is psychophysics and its importance?

• A scientific study that investigates the relationship between physical stimuli and the sensations & perceptions they affect many of the routine procedures used in EYE CARE are psychophysical in
  nature.

what is the role of Psychophysics in the Clinical examination of the Visual System (aka what can it be used in the determination of)

• VA

• refractic status

• visual fields threshold (most common psychophysical measurement)

• color vision status

what does the knowledge of psychophysical theory and methodolgy enable you to do?

• enables u to properly perform these procedures

• interpret their results in meaningful manner

• use in research, analytical purposes and to follow the course of treatment

when considering the relation between the physical world and our perception of
it, what are the fundamental aspects of perception must be investigated?

incluedes but not limited too:

1. detection: what properties must a stimulus posses for us to be aware of its presence?

2. identification: how do we know what the stimulus is

3. discrimintion: how different do stimuli need to be for us to distinguish b/w them

4. scaling: how do we judge the magnitude of a given stimulus parameter? How do we
   judge degree of similarity or difference for discriminable stimuli?
   

what do psychophysical experiments (clinical procedures) frequently involve?

determination of a threshold

define threshold

minimum quantity of stimulus that can be detected or in psychophysics its’s the lowest minimum intensity at which a stimulus can be detected 50% of the time aka absolute threshold

In VA testing, what is the threshold?

MAR (minimum angle of resolution)

in VF testing, what is the threshold?

minimum light intensity that can be detected

What are 2 examples of a colour vision test and what is its threshold?

2 examples: FM 100 Hue Test, Dichotomous D-15 test

Threshold: just noticable hue (colour) that can be discriminated

why is the determination of a threshold complicated?

cuz humasn are not perfect observers

what answers would a perfect observer give when measuring threshold?

• would give the same threshold each time its measuresd

In practice, the threshold varies on repetition of its measurement

Example of results of a psychophyicsal experiment conducted with a hypothetical (1) ideal perfect observer vs (2) real observer for the following task:

Task: observer required to detect a light (stimulus)

• It’s flashed on the surrounding background (increment threshold)

• The task is repeated for a range of stimulus intensities, from dim to intense

What would the ideal perfect observer see?

Ideal Perfect Observer:

• manifests as an unambiguous threshold

• below threshold intensity: never sees the stimulus

• above threshold intensity: always sees the stimulus

Example of results of a psychophyicsal experiment conducted with a hypothetical (1) ideal perfect observer vs (2) real observer for the following task:

Task: observer required to detect a light (stimulus)

• It’s flashed on the surrounding background (increment threshold)

• The task is repeated for a range of stimulus intensities, from dim to intense

What would the real observer see and how does it differ from the ideal observer?

• as intensity of the stimulus ↑, the probability of seeing the stimulus ↑

Unlike an ideal observer:

• There is NO clearly defined intensity

• below which a stimulus is never seen

• above which it is always seen

What do you see on the x and y-axis of the frequency of seeing (FOS) curve, and what is this curve also referred to as?

• percentage of stimuli detected (y-axis) is plotted as a function of stimulus intensity (x-axis) to produce an FOS curve

• also referred to as psychometric function

since there are no perfect observers what is threshold based on?

on theoretical considerations

What is a threshold usually defined as? Where does this value come from?

• defined as the intensity that results in the detection of the stimulus on one-half of the presentations

• The value is read off the FOS curve

why are humans not perfect observers?

cuz they are complex biological systems, not simple mechanical devices

what are the factors affecting threshold?

1. Random neural noise

2. Fluctuations of the Stimulus

3. Attention

4. Motivation

5. Fatigue

6. Psychological bias

7. Diseased Visual System (more neural noise)

how is random neural noise and how does it affect threshold?

• A stimulus results in neural activity, and if this neural activity is sufficiently strong (strong enough) = stimulus is seen

• Random neural noise is inherent within the visual system; therefore, the signal produced by the stimulus must be perceived as different than this neural noise

• neural noise can vary over time

• At any time, the amount of noise is unpredictable; therefore, the threshold is variable

What is the difference between a diseased visual system and a healthy one, and how does the FOS curve change and what happens as a result?

• The diseased vis sys is noisier than a healthy one

• makes the FOS curve flatter (less steep)

• more difficult to accurately ascertain a threshold for a FLAT CURVE (because there is a broader range of values that could correspond to 50% detection)

• since there is a broader range of values corresponding to 50% detection (when FOS curve is flat), it complicated measurement of visual fields in diseased eyes

the visual system is noiser ___________ than ________. This makes the assessment of peripheral VF in diseased eyes more ________.

noiser peripherally than centrally

• makes assessment of peripheral VF is diseased eyes more challenging

define absolute threshold

minimum quantity of stimulus that can be detected (same as threshold)

what is differential threshold and what is it also known as? Give an example.

• as JND (just noticeable difference)

• minimum difference in stimulation for detecting that the two stimuli differ in some characteristics (such as intensity) when the stimuli are physically separated in space

ex. sub refraction: lens 1 is better, or lens 2 is better (JCC)

what is increment threshold and give an example?

• minimum difference in stimulation for detecting that the two stimuli differ in some characteristics (such as intensity) when the stimuli are immediately adjacent or superimposed

Ex: Subjective Refraction, Detecting letters in CS Chart, Colour Vision

Arrangement Tests (D15, FM100 etc.,)

what law expresses the constant relationship of increment threshold? What is the equation and what does each variable stand for?

Weber’s Law:

ΔI is the JND

I is the reference stimulus

K is a constant, the Weber fraction

what is intensity discrimination and what is it important for? What is it measured in?

• process of distinguishing one stimulus intensity

  from another

• important for seeing and is the basis for many tests, like spatial VA, for detecting boundaries of objects

• for the basis of VF testing

• Isopters define points in the visual field w/ similar increment thresholds and provide a topographic map of retinal sensitivity

what affects intensity discrimination?

glare- by reducing contrast

• Glare can be introduced by adding a constant amount of light to both background and test stimulus, or by scattering of light by ocular media (such as occurs in cataracts)

Descibe intensity discrimination at low background luminance levels and what law applies to them at this level:

• At low background levels, background luminance increased from 0 to a small range of scotopic luminance levels

• The human visual system acts like an ideal detector

• as background luminance level ↑- threshold change in luminance (ΔL) needed to detect that the test stimulus differs from background luminance, increases as √background luminance

• follows the deVries–Rose law

Descibe intensity discrimination at higher background luminance levels and what law applies to them at this level:

• The visual system does not behave like an ideal detector because of neural interactions

Result: threshold ΔL of the test spot ↑ in direct proportion to ↑in the background luminance

• Weber’s law applies

Descibe intensity discrimination at very high intensities background luminance levels and what law applies to them at this level:

• neither relationship holds because the rod and cone systems saturate

Result: the test spot luminance must differ greatly from the background luminance to be seen as differing in brightness.

• DeVries-Rose law and Weber’s law are useful in predicting visual performance

what does the most suitable method to measure threshold depend on? What is it also deteremined by?

• most suitable method depends on advantage and disadvantages

• determined by the nature of the experiment or clinical procedure

In order to determine the threshold, there are 3 classical psychophysical methods. What are they, and what is one method that is not part of the classical psychophysical method?

3 Classical Psychophysical Methods:

1. methods of limits (ascending and descending)

2. method of constant stimuli

3. method of adjustments

Forced choice procedure

what is a method that is often thought to be ineffficient and why? Give 2 examples of this?

• Adaptive psychophysical methods are often argued to be inefficient

• Adaptive procedures differ from the classical ones mainly in that they are designed to concentrate stimulus presentations at or near the presumed value of the threshold

ex. Staircase method and magnitude estimation

what are the 2 types of methods of limits

• Ascending Method of Limit

• Descending Method of Limit

Describe the method of ascending limits

• stimulus is initially below threshold (it is not visible)

• During a trial, the stimulus intensity is increased systematically until the observer reports that it is VISIBLE

• Several trials are performed (Non-seeing to Seeing) and the results are averaged to obtain a threshold

what are the advantages and distadvantages of the method of ascending limits?

Advantage

▪ advantageous in Dark Adaptometry (where it’s important that the state of retinal adaptation be minimally affected by the stimulus)

Disadvantage

▪ disadvantage is observer anticipation (may lead to an inaccurate results)

*Beginning each trial at a different intensity can mitigate this disadvantage

Descibe the method of descedning limits

• The reverse of the method of ascending limits.

• trial starts with a clearly visible stimulus

• i.e., the stimulus is above threshold and the visibility is decreased systematically until it can no longer be seen

clinical relevance of the method of descending limits

• used to determine VA commonly

• pt asked to read down the Snellen eye chart, and ptotypes that become progressively smaller from top to bottom (and more difficult to resolve)

• The threshold MAR is determined when the optotypes are too small to be resolved.

what are the disadvantages of descending limits

observer anticipation

▪ May lead to an inaccurate result

▪ Solution: Beginning each trial at a different intensity can mitigate this disadvantage

• The manner in which a clinical psychophysical test is performed can influence the result

Suppose you determine the Visual Acuity of a patient who is worried he may be losing his vision & who wishes to be sure that you take his symptoms seriously, what would happen when asked to read down eye chart?

If asked to read down an eye chart (method of descending limits)

▪ It is possible that the patient may stop reading letters when he notices blur and has uncertainty regarding what he is seeing.

How would the result be different if YOU set the criterion by instructing the patient to continue to read down the chart even if he says he cannot clearly see the letters (in method of descending limits)?

• You encourage the patient to guess at what he sees

• tell pt not to worry about being wrong.

Using this forced choice methodology, it would not be surprising to find

the acuity to be 20/20 rather than 20/30 (acuity better than measured)

how should VA be measured and who sets the threshold criterion?

Visual acuity should be measured in a standardized manner

• ensures that the practitioner, not the patient, sets the threshold criterion

what are some things to keep in mind when performing method of descending limits?

Not only will different patients have different criteria

▪ Some will be adventurous guessers

▪ Others will be cautious nonguessers

▪ The criterion that a given patient uses may vary from visit to visit (depending on his mood or other factors)

How are treatment desicions made and what is important to know?

Since treatment decisions are often made based on Visual Acuity and/or changes in visual acuity over time,

▪ It is important to know that

▪ Our measurements are reliable and

▪ Not unnecessarily confounded by issues of threshold criteria.

in method of descending limits, what termination rules should be employed when measuring Visual acuity?

• Asking the patient to continue to guess at optotypes once it is obvious that they are not visible

▪ Can be frustrating for the patient and time-consuming.

what does the study by carkeet (2001) suggest regarding bailey lovie chart?

t when using Bailey-Lovie chart, the test be terminated after the patient makes four or more mistakes on a line.

what is one way u can improperly administer a psychophysical test?

by allowing the patient to set the threshold criteria

during stereopsis testing, if the pt says i cant tell, what must practioner do

ask the patient to “guess which image is elevated”

what is the method of constant stimuli?

• In this method, Stimulus is varied every presentation

• stimulus visibility is varied randomly from presentation to presentation, and the FOS curve is plotted based on the subject’s responses

• since subject is often asked whether or not a stimulus is seen this method is sometimes referred as “Yes–No” procedure

what are blank/catch trials?

blank trials- in which no stimulus is presented

catch trials- stimulus well below or above threshold are presented

In the method of constant stimuli, what response is recorded?

• number of times that the observer reports seeing the stimulus during a blank trial (FALSE POSITIVE or FALSE ALARM responses) is recorded

Example 1: YES-NO EXPERIMENT

▪ Total: 220 Trials, 20 Blanks

▪ Stimulus seen by subject 145 Trials; Stimulus present but not seen= 60 trials

▪ We can now calculate Hits, False Positives, and Correct Rejects

stopped at pg 51

starting pg 59

what is the forced choice method

procedure is essentially a modified version of method of limits/constant stimuli, where subjects are forced to response

in forced choiced emethodoly, what happens to the effects of the subjects criteria

the effects of the subjects criteria (anticipation) are minimized (among all psychophysical methods) by forcing them to choose b/w several alternative choices, one of which contains the stimullus

The threshold value is typically taken as the point midway between _____________- and ______________ performances.

b/w chance performance and perfect performance

what is the threshold for a 2AFC(alternative forced choice)experiment?

75% (b/w 50 and 100%)

what is the threshold for a 4AFC experiment?

62.5% (b/w 25 and 100%)