Psychophysics

Sensations and Perceptions

Psychophysics• Psychophysics

– Study of how physical stimuli are translated into psychological experiences

• Sensation– Raw inputs through our senses to our brain

• Perception– How our brain organizes and makes sense of

the inputs

Sensations

• Most inputs never register…why?– What don’t we notice?

• Absolute threshold– Point where something becomes

noticeable to our senses– If less than this, we do not notice

Sensation

• Absolute threshold experiments:– hearing test

– Odor test

Sensations• Absolute thresholds for 5 senses:

– Hearing: the tick of a watch in quiet conditions at 20 feet

– Vision: the flame of a single candle at 30 miles distance on a dark night

– Smell: one drop of perfume in a 6 room apartment

– Taste: one teaspoon of sugar in 2 gallons of water

– Touch: the wing of a fly falling on cheek from 1 cm

Sensation• Difference Threshold

– The minimum stimulus change from a given level that is reliably detectable

– Just Noticeable Difference– Proportional

• Weber’s Law - logarithmic• JND = change in I/I

– Weight experiment

Sensations• Difference Threshold – Weber’s Law

– Pitch: 1/333 – Brightness: 1/60 – Lifted weights: 1/50 – Loudness: 1/10 – Pressure on skin: 1/7 – Taste: 1/5

• Which of our senses is the most sensitive to change?

Sensation

• Signal - Detection Theory– Sensory and decision-making

processes that determine what info to process and what to filter

– Influenced by distraction level (noise)– Criterion – based on expectations and

consequences– Ordering pizza at a party

Sensations

• Sensory Adaptation– The process of becoming less

sensitive to unchanging stimulus– Stimuli are still there, we just don’t

notice– Restaurant, Clock, Glasses, Old

people

Perception• Gestalt Principle

– Means “unified whole”– Interpret info by grouping to avoid

needless repetition• Similarity• Proximity• Continuity• Closure

Perception

• G.P. Similarity– What do you

see?– Why doesn’t

your brain

Compute each individual part?

Perception

• G.P. Proximity– What do you see?

Perception

• G.P. Continuity– What do you

see?

Perception

• G.P. Closure– What do you

see?

Perception

• Gestalt Principle– Phi Phenomenon Examples– Illusion of movement

Perception

• Illusions– Muller-Luyer

Illusion

Perception

• Illusion– Reversible

Perception• Illusions

– Impossible

Perception

• Perceptual Constancy– Ability to see things differently without

having to reinterpret the objects properties

– Size– Shape– Brightness

Perception

• Distance is perceived by:– Monocular cues

– Binocular cues

Perception

• Monocular Cues– Size– Texture– Overlap– Shadowing– Height– Clarity

Perception

• Binocular cues– Both eyes needed

– Convergence

– Retinal disparity