Lecture 5: Psychophysics – measuring perception

Question 1

What is Psychophysics

Answer 1

“The analysis of perceptual processes by studying the effect on a participant’s experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions.”

Psycho comes first in psychophysics.
But the physical properties of the stimulus are important.
This includes stimulus content (type), magnitude (size and strength) and timing.

Question 2

Why study perception

Answer 2

As psychologists we should be interested in the brain.
About 50% of the cortex is devoted to sensory perception.
Vision alone occupies 1/3 of the brain.
Everything we think, feel emotionally, or do starts with sensation.
Even imagination, dreams and hallucinations use sensory areas of brain.
We do not perceive physical reality!
Perception changes with short term experience – this alters our view of reality!
Perception varies with age, gender and disability. Does that change our view of reality?

Question 3

Four elements of a psychophysics procedure

Answer 3

Manipulation.
Masking.
Facilitation / Priming.
Contrast effects.
Adaptation.

Measure.
Percent correct
Thresholds
Point of subjective equality

Task.
Single Interval Yes/No
Two alternative forced choice.
Single Interval symmetric Choice.

Stimulus Selection Method.
Constant stimulus.
Adaptive.
Method of limits.
Adjustment

Question 4

Psychophysics vs Cognitive Psychology

Answer 4

Cognitive Psychology
Concerned with mental processes.
Manipulations aimed at modifying mental processes.

Stimuli usually very obvious and present for a long time.
Stimuli often complex and naturalistic.
More concerned about stimulus interpretation.
Primary measure is Reaction Time. Accuracy is a secondary measure.
Stimulus selection straightforward. All conditions included.

Psychophysics
Concerned with sensory processes.
Manipulations usually about stimulus detectability or appearance.
Stimuli usually very subtle and present for a short period of time.
Stimuli often very simple and stylistic (but not always).
Concerned about stimulus composition.
Primary measure Accuracy. Reaction times are ignored.

Stimulus selection can be adaptive. Different stimuli shown to different people

Question 5

Kinds of Stimuli

Answer 5

Vision
Intensity of light
Contrast
Colour
Orientation
Size
3D Depth
Motion
Textures
Objects / faces

Hearing
Intensity of sound
Pitch
Direction

Touch
Intensity of press
Position on body

Texture
Taste
Intensity
Flavour (bitter/sweet)

Texture
Smell
Intensity
Odour

Question 6

How NOT to measure a detection thresholdSingle Interval Yes-No: target always present

Answer 6

(Look at Powerpoint)

Question 7

Calculating ThresholdThe Psychometric Function (SI-TP)

Answer 7

(Look at Powerpoint)

Question 8

The problem of Bias

Answer 8

People may say yes more because they see the stimulus more: either because it’s stronger or they are better at seeing it.
People may also say yes more because they want to say yes – change in the criterion for saying yes.
This tendency may vary between conditions / groups = BIAS.
‘Test’ condition may be obvious - Experimenter effect.
‘Yes’ answer maybe more favourable – Desirability effect.
Groups of people may vary in ways other than sensitivity.

Question 9

Detection thresholds

Answer 9

Single interval task revisited
Target absent trials
Two alternative Forced Choice task
Spatial intervals
Temporal interval

Question 10

Single interval Yes / No task: with target absent trials (A slightly better way to measure thresholds)

Answer 10

(Look at Powerpoint)