Sensation & Perception : Psychophysics

Psychophysics

Study of relationship between stimuli from environment & people's perception of these stimuli

- Detection

- Discrimination

Detection

When the sensory system detects any energy changes

- Absolute Threshold

Absolute Threshold

Minimal energy needed to detect stimulus

- Below AT => Not detected

- Reverse relation with sensitivity

Method of Constant Stimuli

Stimulus is presented many times in a random order

- Respond "yes" or "no" for detection

Method of Limits

Estimate threshold instead of tracing out entire psychometric function

- Descending

- Ascending

Descending

Begins by presenting a stimulus at a high enough intensity to be heard

Ascending

Begins by presenting a stimulus at a low intensity

Method of Adaptive Testing

Keep test stimuli hovering around threshold

Method of Adaptive Testing

What is the method that is the least accurate & takes the shortest time?

Example of Absolute Threshold

Ticking of wristwatch : 20 feet (6 m)

Discrimination

Involves comparison

- "Is this stimulus different from that one?"

Difference Threshold

Threshold for the perception of a difference between standard and comparison stimuli

- JND

Point of Subjective Equality (PSE)

Stimulus that appears most like the standard

Interval of Uncertainty (IOU)

Interval between 25% & 75%

Just Noticeable Difference (JND)

Average of the threshold for "greater than" and threshold for "less than"

- JND = IOU / 2

Negative Time Error

Stimulus presented first is assumed to be less intense than the later stimuli

Positive Time Error

Stimulus presented first is assumed to be more intense than later stimuli

Weber's Law

About the difference threshold for the standard weights of different magnitude

False, JND is larger with larger standards

T/F : JND is larger with smaller standards

Steven's Law

Measuring magnitude

- "Magnitude estimation"

Magnitude Estimation

Observers were asked to assign numbers to stimuli on the basis of how intense they appear to be

Brightness

n < 1

- "Response compression"

Electrical Shock

n > 1

- "Response expansion"

Signal Detection Theory

Response Criteria

- Liberal & Conservative

Receiver Operating Characteristic Curve (ROC)

The graphical plot of the hit rate as a function of the false alarm rate

Probability Distribution Graph

Y Axis : Likelihood of occurrence

X Axis : Perceptual effect (loudness)

As β line moves left...

False Alarm & Hit Frequency both rise

- More Hits, but also more False Alarms

As β line moves right...

False Alarm rises, but Hit falls

- More False Alarms, less Hits

Receiver Operating Characteristic Curve (ROC) with Sensitivity Curve

SDT attempts to measure sensitivity independently of the response criteria

- L is more liberal than C

Noise

Sensory Noise

- Amount of noise varies

- Fluctuation in noise level in physiological perceptual and other variables

Sensitivity

Becomes greater the more the line reaches the upper left corner

- The more you are towards the corner, the more sensitive you are

Context & Bias

Contextual effects influence judgements of sensory magnitude