EP - S3 Psychophysics

What is psychophysics?

The scientific study of the relationship between physical stimuli and the sensations/perceptions they produce.

What is the cold-pressor test example in psychophysics?

Subjects immersed their hand in cold water at 5 degrees Celsius for up to 4 minutes. Those who breathed a sweet-smelling odor kept their hand in almost three times as long as those in a control condition. Demonstrates psychophysical measurement.

What did Edwin Boring claim about psychophysics?

The introduction of techniques to measure the relation between internal impressions (the psycho) and the external world (the physics) marked the onset of scientific psychology.

Who formalized psychophysical methods?

Gustav Fechner formalized psychophysical methods in 1860/1866.

What is Fechner's Law?

Subjective sensation (S) grows as a logarithmic function of physical stimulus intensity (I). Formula: S = k log I.

What is the absolute threshold?

The minimum stimulus intensity that can be detected 50% of the time.

What is the method of limits?

A psychophysical method for determining thresholds by presenting stimuli in ascending and descending series and noting the transition point between "yes/no" responses.

How does the method of limits work?

Start with a clearly audible tone and lower intensity until the observer says "no"; repeat across multiple blocks starting at different intensities. The threshold is the stimulus value detected 50% of the time.

What is the difference threshold (Just Noticeable Difference - JND)?

The minimum change in stimulus intensity needed for a person to detect a difference 50% of the time.

What is Weber's Law?

The difference threshold (ΔI) is a constant proportion (k) of the original stimulus intensity (I). Formula: ΔI / I = k.

What is the Weber fraction?

The constant k in Weber's Law; for weight discrimination it is approximately 1/30 meaning a 300g weight requires a 10g change to be noticeable.

What is an example of Weber's Law in action?

For a 300g standard weight the difference threshold is 10g; for a 600g standard it is 20g; for a 900g standard it is 30g; for a 1200g standard it is 40g. The ratio (10/300 = 20/600 = 30/900 = 40/1200) remains constant.

What is Signal Detection Theory (SDT)?

A framework that separates perceptual sensitivity (ability to detect a signal from noise) from response bias (the observer's tendency to say "yes" or "no" based on expectations and motivation).

What is the key insight of Signal Detection Theory?

Perception is controlled by both evidence and decision processes. Any decision depends on the costs and benefits associated with it.

What is a hit in Signal Detection Theory?

Correctly detecting that a signal is present when it actually is present.

What is a miss in Signal Detection Theory?

Failing to detect a signal that is actually present.

What is a false alarm in Signal Detection Theory?

Responding that a signal is present when it is actually absent (noise alone).

What is a correct rejection in Signal Detection Theory?

Correctly responding that no signal is present when indeed no signal is present.

What is response bias (criterion) in Signal Detection Theory?

The observer's tendency to say "yes" or "no" based on motivations

How does the blind date example illustrate Signal Detection Theory?

Deciding to go on a blind date involves weighing costs (wasted evening) and benefits (exciting evening). Your decision criterion is influenced by these costs and benefits just like perceptual decisions.

What is the relationship between motivation and signal detection?

A mother is awakened by a quiet murmur from her baby but not by other louder sounds because motivation lowers her response criterion for baby-related signals.

What is the difference between sensitivity and bias in SDT?

Sensitivity (d') is the ability to distinguish signal from noise regardless of bias. Bias (criterion) is the threshold for saying "yes" independent of sensitivity.

What is the method of constant stimuli?

A psychophysical method where stimuli of varying intensities are presented in random order and the observer judges each; threshold is the intensity detected 50% of the time.

How did Fechner contribute to psychology?

He formalized psychophysical methods and established mathematical relationships between physical stimuli and psychological sensations.

What is the significance of the 50% detection point in threshold measurement?

Because detection fluctuates due to noise and variability the threshold is defined statistically as the stimulus value detected half the time not an absolute biological switch.

What does Weber's Law tell us about sensory systems?

Our sensory systems detect relative changes not absolute changes which is adaptive across a vast range of stimulus intensities (e.g. vision in dim starlight and bright sunlight).

What is the difference between Fechner's Law and Weber's Law?

Weber's Law describes the relationship between stimulus intensity and the just noticeable difference (ΔI/I = k). Fechner's Law uses Weber's Law to derive the logarithmic relationship between stimulus intensity and subjective sensation (S = k log I).

What is an example of signal detection theory in everyday life?

A friend has set up a blind date for you. You decide whether to go based on costs (a wasted evening) and benefits (an exciting evening now and more in the future). Your decision criterion reflects your personal risk tolerance.

What is the relationship between stimulus intensity and psychological judgment in Fechner's Law?

As physical stimulus intensity increases arithmetically psychological sensation increases logarithmically. Large increases in physical intensity produce smaller increases in sensation at higher levels.

What is the "no threshold" position in Signal Detection Theory?

Traditional thresholds assume a fixed detection point. SDT argues there is no fixed threshold; detection depends on both sensory evidence and decision criterion which varies with context.

What is a receiver operating characteristic (ROC) curve?

A graph plotting hit rate against false alarm rate for different criterion levels; used to measure sensitivity independent of bias in Signal Detection Theory. (Not explicitly in slides but core SDT concept - included for completeness)

What is the difference between the method of limits and the method of constant stimuli?

Method of limits presents stimuli in ascending/descending series; method of constant stimuli presents stimuli in random order. Both determine the 50% detection threshold.

Why did Fechner's work mark the onset of scientific psychology?

It provided the first quantitative techniques to measure the relationship between internal mental experience and external physical reality transforming psychology into a measurable science.