Sensation & Perception

Psychophysics

Review of Mind-Brain Theories

• Epiphenomenalism: Conscious feelings and thoughts are produced by the brain but cannot affect the brain.
• Interactionism: Mind and brain are different but can affect each other.
• Monism: One kind of "stuff"; mind & brain differ in arrangement of stuff or perspective from which it is viewed.

Material that Won't be Covered on Exam

• Brain size & experience;
• Brain evolution.

Sensation vs. Perception

• Thomas Reid (1785)
• Sensation: Refers to experiences within a person; "meaningless."
• Perception: Refers to objects in the world; "meaningful."

Sensation vs. Perception

• Contemporary View
• Sensation: Relatively basic; focus of research is on physiology and sensitivity of sensory systems.
• Perception: Relatively more complex; focus of research is on how meaningful experiences are created from basic information provided by sensory systems.

Sensation vs. Perception

• Examples
• Necker cube
• Vase-faces

Psychophysics

• Attempts to measure the relation between physical stimulation and psychological sensations.
• Applications: Examinations of sight and hearing; design of instrument panels (e.g., cars); blending of wine; strength of perfume; design of web sites (?).

Methods

• Detection: How much sensory stimulation is needed in order for our senses to detect it?
• Discrimination: How small of a difference between two stimuli can be detected?

Detection

• Absolute Threshold: Intensity is increased or decreased until observer detects stimulus half of the time. This is the sensory threshold.

Absolute Threshold

• Why are absolute thresholds statistical?
• Examples:
• Absolute thresholds for various senses.

Absolute Sensory Thresholds

• Vision: Candle flame at 30 miles.
• Hearing: Tick of a watch at 20 ft.
• Taste: 1 tsp sugar in 2 gal of water.
• Smell: 1 drop of perfume diffused in 6 rooms.
• Touch: Wing of a fly falling on cheek from 1 cm.

Discrimination

• Ability to determine the difference between one stimulus and another.
• Minimum difference that can be detected is called the just noticeable difference, or JND.

Weber's Law

• The greater the magnitude of the stimulus, the larger the difference you need to detect a change.
• Examples
• Weights
• Housing prices

Weber's Law

• First noted by Ernst Weber (1795-1878)
• Equation: JND = KI or K = JND/I
• The Weber fraction, K, varies for different sensory stimuli.

Weber Fractions for Various Stimuli

Problems

• Weber's law holds for some stimuli (e.g., lights) better than others (e.g., electrical shock).
• The law works well for medium intensities, but less well for very low and very high intensities.
• General principle valid.

Stevens's Power Law

• S. S. Stevens asked people to judge intensity of stimuli directly, using magnitude estimation.
• Intensity of a sensation is directly proportional to the intensity of the physical stimulus raised to a constant power.
• S = cM^p (M raised to the power p)

Power Law Exponents

• Brightness of light in dark .33
• Loundness of tone .67
• Taste of saccharine .80
• Length of line 1.00
• Pain of shock on finger 3.50

 

Why a power law?

• Intensity of stimuli is constantly changing.
• One thing that tends not to change is the ratio of the magnitudes of stimulus elements with respect to each other.
• Only the power law preserves constant sensory ratios as overall intensity increases and decreases.

Example of failing to apply psychophysical laws

• Susan B. Anthony Silver Dollar
• SBA 26 mm (530.9)
• Quarter 24 mm (452.4)
• Nickel 21 mm (346.4)

Signal-Detection Theory

• Attempts to account for sensitivity and the decision process.
• Can be used to separate sensory threshold (sensitivity) from the decision to respond one way vs. another (response bias).

Signal-Detection Analysis

Factors that affect response bias:

• Motivation;
• Expectations;
• Relative frequencies of stimuli;
• Prior knowledge;
• Relative costs of false alarms and misses or of hits and correct rejections;
• etc.

 

Summary

• Psychophysics is the study of the relation between physical stimulation and psychological experience.
• Detection vs. discrimination.
• Absolute threshold.
• Weber's Law: JND = KI
• Stevens's Power Law: S = cM^p
• Signal Detection Theory.