Psychophysics

What is psychophysics?

The measurement of the relationship between physical stimuli and perceptual experience.

What is the core psychophysical process?

Physical stimulus → visual system → decision → behavioral response.

What can be measured directly in psychophysics?

Physical properties of a stimulus.

What must be inferred in psychophysics?

Perception.

Why is psychophysics important in eye care?

Every subjective clinical test is a psychophysical procedure.

What psychophysical question is asked during perimetry?

What is the dimmest light detected at a specific retinal location?

What psychophysical question is asked during subjective refraction?

Which lens produces the clearer image?

What psychophysical question is asked during visual acuity testing?

What is the smallest detail identifiable?

What psychophysical question is asked during color testing?

Can the distinct colored stimuli be discriminated or arranged?

What psychophysical question is asked during contrast sensitivity testing?

What is the lowest contrast detectable?

What psychophysical question is asked during dark adaptometry?

How much light is required for detection over time?

What is the first component of the psychophysical framework?

Stimulus.

What is a stimulus in psychophysics?

The physical presentation, such as a spot of light.

What is the second component of the psychophysical framework?

Variable.

What is a variable in psychophysics?

The manipulated property of the stimulus.

What is an example of a psychophysical variable?

Luminance intensity.

What is the third component of the psychophysical framework?

Task.

What is a task in psychophysics?

The observer's objective.

What is an example of a psychophysical task?

"Did you see it?"

What is the fourth component of the psychophysical framework?

Outcome.

What is an outcome in psychophysics?

The measured estimate produced by the experiment.

What is an example of a psychophysical outcome?

Detection threshold.

Before interpreting a psychophysical result, what should be identified?

The conditions and the rule.

What is a threshold?

The stimulus level associated with a defined level of detection or correct performance.

Is there a sharp biological boundary between "seen" and "not seen"?

No.

What can happen when the same near-threshold stimulus is presented repeatedly?

It may be detected on one trial and missed on another.

Are thresholds absolute or probabilistic?

Probabilistic.

Why are thresholds considered probabilistic?

Detection near threshold varies from trial to trial.

What internal factor can influence threshold performance?

Neural noise.

What internal factor related to effort can influence thresholds?

Fatigue and motivation.

What attentional factor can influence thresholds?

Attention.

What experience-related factor can influence thresholds?

Practice.

What adaptation-related factor affects threshold measurements?

Adaptation level.

What retinal variable influences threshold performance?

Retinal location.

What stimulus characteristics affect thresholds?

Stimulus size and duration.

What environmental brightness factor affects thresholds?

Background luminance.

What is an absolute threshold?

The minimum stimulus magnitude that can be detected under specified conditions.

What is an example of an absolute threshold?

The dimmest visible light.

What is another example of an absolute threshold?

The lowest detectable contrast.

What is a difference threshold?

The smallest detectable difference between two stimuli.

What does JND stand for?

Just-Noticeable Difference.

What is a JND?

The smallest detectable difference between two stimuli.

What is an example of a difference threshold involving luminance?

A barely detectable luminance increment.

What is an example of a difference threshold involving refraction?

A small lens-power change that alters clarity.

What is a psychometric function?

A function that maps physical stimulus intensity to perceptual probability.

What is plotted on the Y-axis of a psychometric function?

Probability of a specific response.

What response is commonly plotted on the Y-axis?

"Seen."

What is plotted on the X-axis of a psychometric function?

Stimulus magnitude.

What is an example of stimulus magnitude?

Light intensity.

How is threshold often defined on a psychometric function?

The intensity detected 50% or 75% of the time.

What does the psychometric function relate?

Physical intensity and probability of perception.

What is the method of limits?

A threshold technique using an ordered, systematic series of stimuli.

What occurs in an ascending series?

Stimulus intensity increases from below threshold until detected.

Where does an ascending series begin?

Below threshold.

What bias is associated with ascending series testing?

Anticipation bias.

What occurs in a descending series?

Stimulus intensity decreases from above threshold until no longer detected.

Where does a descending series begin?

Above threshold.

What bias is associated with descending series testing?

Habituation bias.

What is another name for habituation bias in descending series?

Perseverance bias.

What is the clinical advantage of the method of limits?

It is quick, intuitive, and practical for routine exams.

What is an adaptive staircase procedure?

A dynamic testing method in which each stimulus depends on the previous response.

How is the next stimulus determined in an adaptive staircase procedure?

By the observer's previous response.

What happens after a correct or detected response in a staircase procedure?

The task becomes harder and intensity decreases.

What happens after an incorrect or missed response in a staircase procedure?

The task becomes easier and intensity increases.

How is threshold estimated in an adaptive staircase procedure?

By averaging the reversal points.

What is a reversal point in a staircase procedure?

A point where stimulus direction changes from increasing to decreasing or vice versa.

What is the method of constant stimuli?

A threshold method using predetermined stimulus levels presented in randomized order.

Why is the method of constant stimuli considered the gold standard?

It characterizes the full psychometric curve.

What is the first step in the method of constant stimuli?

Select several fixed stimulus levels in advance.

What is the second step in the method of constant stimuli?

Present each level repeatedly in randomized order.

What is the third step in the method of constant stimuli?

Calculate the proportion detected at each level.

What is a major advantage of the method of constant stimuli?

High precision.

What order effect is eliminated by constant stimuli?

Predictable order effects.

What is a major limitation of the method of constant stimuli?

It requires many trials.

Why is the method of constant stimuli uncommon in routine clinical practice?

It is too time-consuming.

What is the method of adjustment?

Observer-controlled threshold estimation.

Who controls the stimulus in the method of adjustment?

The observer.

How is threshold determined in the method of adjustment?

The observer adjusts the stimulus until it is barely visible or barely invisible.

What matching task can be performed using the method of adjustment?

Perceptually matching a stimulus to a reference stimulus.

What is a clinical advantage of the method of adjustment?

It is extremely fast.

What is another advantage of the method of adjustment?

It is intuitive.

What type of judgments are well suited to the method of adjustment?

Matching tasks and appearance judgments.

What is the primary limitation of the method of adjustment?

Observer criterion bias.

What is the defining feature of the method of limits?

Ordered ascending or descending stimulus series.

What is the primary limitation of the method of limits?

Predictability.

What is the defining feature of the method of constant stimuli?

Randomized, pre-selected fixed stimulus levels.

What is the primary limitation of the method of constant stimuli?

Time-consuming and requires many trials.

What is the defining feature of the method of adjustment?

The observer directly controls the stimulus.

What is the primary limitation of the method of adjustment?

Heavy reliance on observer bias.

What happens in a yes/no procedure?

The observer reports whether the stimulus was seen.

Why is the yes/no procedure susceptible to bias?

It depends on the observer's internal criterion.

What type of criterion shifts can affect yes/no responses?

Strict versus lax criteria.

What happens in a forced-choice procedure?

The observer identifies a characteristic of the stimulus, even if guessing.

What stimulus characteristics may be identified in forced-choice testing?

Location, orientation, or interval.

What is the major advantage of forced-choice procedures?

They bypass the observer's subjective seen/unseen criterion.

How is threshold typically defined in a two-alternative forced-choice procedure?

75% correct responses.

Why is chance performance considered in forced-choice testing?

Observers are required to guess when uncertain.

What is Weber's Law?

The Just-Noticeable Difference is proportional to the starting intensity.

What is the formula for Weber's Law?

ΔI / I = k.

What does ΔI represent in Weber's Law?

The Just-Noticeable Difference.