Final Exam - Psychophysical Methods

Psychophysics

scientific study that investigates the relationship between physical stimuli and the sensations and perceptions they affect. Common for routine eye care procedures. Ie) visual acuity, visual field testing, color vision testing

performing tests

interpreting tests

treatment

the use of psychophysical clinical examination aids in these three tasks

Detection

aspect of perception which is the most basic perception of the stimulus. Ie) seeing a light as present

Identification

aspect of perception which is the identification of the stimulus. Ie) Detect a stimulus as being present AND identify it as a letter, number, picture, etc.

discrimination

aspect of perception which is the ability to distinguish two stimulus as different in some way. Ie) D15 discrimination of two color as different

scaling

aspect of perception which is the ability to grade what is being perceived. Ie) How does the patient rate their pain on a scale of 1-10?

Threshold

the minimum quantity of a stimulus that can be detected. Commonly measured by psychophysical clinical procedures. Varies on repetition due to human error by the observer or clinician. Ie) visual acuity testing indicates the minimal angle of resolution (MAR). Ie) visual field testing, D15 measuring just noticeable difference in hue that can be discriminated

Absolute threshold

the minimum quantity of a stimulus that can be detected at least 50% of the time.

Ideal observer

observer that manifest an unambiguous threshold. Will always see a stimulus that is above threshold, and will never see a stimulus that is below threshold.

Real observer

observe whose probability of seeing a stimulus increases as the intensity of the stimulus is increased. No clearly defined threshold as seen by an ideal observer.

Psychometric Function (Frequency of Seeing Curve)

curve where percentage of stimuli detected is plotted as a function of stimulus intensity.

Random Neural noise

factor affecting threshold. Inherent neural activity within the visual system present at all times. Stimulus must be perceived as different from this neural noise in order for it to be detected. Neural noise varies over time and therefor so does threshold.

peripherally

is neural noise greater peripherally or centrally?

Diseased visual system

Factor affecting threshold that increases neural noise. Results in a flatter FOS curve. More difficult to determine threshold. Broader range of values that could correspond to 50% detection.

Differential Threshold (just noticeable difference)

the minimum difference in stimulation for detecting that the two stimuli differ in some characteristic when the stimuli are physically separated in space. Ie) used of JCC in subjective refraction

Weber's law

law expresseing the constant relationship of just noticeable difference to the reference stimulus. Delta I is the JND, I is the reference stimulus, K is a constant. Applies for higher levels of background luminance where there are neural noise interactions. Visual system does not behave like an ideal detector.

Increment threshold

minimum difference in stimulation for detecting that the two stimuli differ in some characteristic when the stimuli are immediately adjacent or superimposed.

Intensity discrimination

the process of distinguishing one stimulus intensity from another. A fundamental importance for seeing and the basis for many clinical tests. Ie) spatial VA, detecting boundaries, VF testing.

Glare

affects intensity discrimination thresholds by reducing contrast. Can be introduced by adding a constant amount of light to both the background and test stimulus, or by the scattering of ocular media as seen in cataracts.

DeVries-Rose law

law stating that as background luminance level increases to a small range of scotopic luminance levels, the threshold change in luminance needed to detect the test stimulus from the background increases as the square root of the background luminance. Human visual system behaves like an ideal detector.

Method of ascending limits

classical psychophysical method where the stimulus is initially below threshold and is not visible. Stimulus intensity is gradually increased until it is visible. Several trials are performed to obtain an average. Serves an advantage in dark adaptometry because the stimulus minimally affects retinal adaptation because it is initially not visible.

Method of descending limits

classical psychophysical method where the stimulus is initially clearly visible and decreases in visibility gradually until it cannot be detected. Commonly used to determine visual acuity. Serves as an advantage because it is very patient friendly.

Observer anticipation

serves as a disadvantage for both ascending and descending method of limits. This can be minimized by beginning each trial at a different intensity Ie) a patient may stop reading letters before they are unable to read them during method of descending limits to legitimize their vision complaints.

Method of constant stimuli (yes no experiment)

classical psychophysical method where the visibility of the stimulus is varied randomly from presentation to presentation. Maintains the observer's expectations at the same level, patient cannot predict what will be coming next. Used to produce a FOS curve.

Catch trials

points in a method of constant stimuli testing that are well above threshold. Used to detect false negatives.

False negative

the number of times an observes misses a stimulus during a catch trial.

Blank trials

points in a method of constant stimuli testing that are well below threshold. Used to detect false positives.

False positive

the number of times an observer reports seeing the stimulus during a blank trial.

time consuming, subjective

the disadvantage of the method of constant stimuli is that is it ______. It is also highly ______. Ie) some patients are more relaxed or stricter than others.

Time consuming and patient bias

two disadvantages to the method of constant stimuli

Method of adjustments

classical psychophysical method where the subject adjusts the stimulus intensity until it is barely visible, or invisible. The most direct method of measuring threshold. More easily used when the stimulus can be changed in a continuous manner rather than in steps.

poor repeatability, anticipation, variations

three disadvantages of the forced choice method

Forced Choice Method

A refined method of classical psychophysical methods where the subject is forced to respond. This minimizes subject anticipation by forcing the subject to choose between several alternative choices, one of which contains the stimulus.

75%

what is the threshold for a forced choice method having two choices? Hint: the midpoint between the chance performance and the perfect performance

62.5%

what is the threshold for a forced choice method having four choices?

reliability, steeper

Increasing the number of choices in a forced choice procedure increases the _____ of the test, but also increases the complexity and time it takes to perform it. This will make the psychometric function ______, with a more clearly defined threshold point.

lower thresholds

do forced choice methods result in lower or higher thresholds? The change in threshold occurs because the criterion of the subject is minimized.

Adaptive psychophysical methods

psychophysical method that differs from classical psychophysical methods because stimulus presentations are at or near the presumed value of the threshold and is then adjusted depending on the patient's response. Threshold is determined by the average of the reversal points. Ie) staircase method, magnitude adjustment.

Weber's law

law stating that as illumination of a target increases, contrast remains constant because the amount of light reflected from both the dark and grey surfaces increases at the same rate.

Sensitivity regulation

process resulting in a constant contrast threshold regardless of background brightness. Explained via Weber's law

Lightness constancy

phenomenon where the appearance of a stimuli remains the same under different lighting conditions because the contrast remains constant.

14%

what is the scotopic contrast threshold?

1.5%

what is the photopic contrast threshold?

Scaling

observers are asked to judge the sensation magnitude of a scale base on their own perception. Subjective, but quantitative.

Direct scaling

scaling method where stimuli of various intensities are presented several times in a randomized order. First stimulus is given an arbitrary number by the observer and the observer must then assign numbers to the following stimuli relative to the first stimulus. Has no limit.

Indirect scaling

scaling method where the observer is presented with a reference stimulus is told that it has a certain value. The observer must then assign a number to indicate their perceived magnitudes relative to the reference stimulus.

Nominal

scale where the values are used to distinguish one object from another. Ie) employee id number

Ordinal

scale where the values provide enough information to order objects. Ie) good, better, best. Moderate, severe, etc.

Interval

scale where the differences between values are meaningful. Ie) temperature

Ratio

scale where both differences and ratios are meaningful. Ie) age, mass, length. Ie) AC/A ratio, cup to disc ratio, etc.

Fechner's Law (indirect method)

law describing the log relationship between intensity and sensation. Ie) if a light were to increase by 5 JNDs it would appear 5 units brighter. Assumes that all JNDs produce an equal difference in the magnitude of sensation. This was disproven by the Steven's method.

Steven's Method (direct method)

law revealing the power relationship between stimulus magnitude and stimulus intensity. Obtained by asking an observer to assign a number to the perceived brightness of a set of stimuli. Shows that there is a compression of sensation as stimulus intensity increases.

Signal detection theory

a statistical model that explains how we detect signals when there is variability and uncertainty in different variables such as characteristics of a stimulus, the background stimulation, and the detector. Aids in threshold determination (detectability) in the presence of background noise

Neural noise

randomly fluctuating levels of background neural activity that are always present even in the absence of stimulus. Stimulus will superimpose its signal over this noise. In order for the stimulus to be perceived, it must have significantly more neural activity than this baseline noise level.

Detectability (d')

the difference between the mean of neural noise, and the sum of neural noise and stimulus. Represents threshold value in signal detection theory. Designed to overcome shortcomings of classical psychophysical theory. Increases as the signal of neural noise plus stimulus noise becomes larger.

Lax criterion

Increasing number of hits, increases the number of false positives. High sensitivity, lower threshold. The start of the FOS curve will depend on the false positive rate. Ie) referring out every single patient will result an increased detection of disease (hits), but will increase the number of patients referred even without eye disease (false positives)

Strict criterion

Decreasing number of hits, increases the number of false negatives. Low sensitivity, higher threshold. The FOS curve will never reach 100% because even stimuli at high intensities are sometimes not seen. Ie) a deer hunter with a limited number of bullets will only shoot when he is absolutely sure that he sees a deer. This results in an increase in deer missed even when seen (false negatives) and a decreased number of deer being hit

Receiver Operating characteristic curves

curve showing the probability of a hit as a function of the probability of a false positive. Allows us to predict the effect of the observer criteria for a given detectability.

strict

Rewards for hits and penalties for false positives will cause the subject to adopt a _____ criterion

lax

By minimizing the penalty for a false positive relative to the reward for a hit, this will cause the subject to adopt a _____ criterion

Sensitivity

ability of a test to correctly identify a patient as having the condition.

Specificity

ability of a test to correctly identify a patient who is normal, not having the condition being tested.

criterion

Sensitivity and specificity of a test can be altered by changing the ____ of the test

Positive predictive value

probability that subjects with a positive screening test truly have the disease. Equal to TP/TP+FP

Negative predictive value

probability that subjects with a negative screening test truly don't have the disease. Equal to TN/TN+FN