• The relationship between the stimulus (what is actually in the environment) and perception (what we experience)
• Quantifying the mind

Absolute Threshold

• Minimum amount of energy necessary to detect a stimulus 50% of the time
• This is the lower limit of an organism’s sensitivity, the point at which the organism is just noticing something

Difference Threshold

• Minimum amount of energy necessary to detect a change in a stimulus 50% of the time

• This is when an organism can recognize that something is different from something else.

Sensitivity

• Inverse of threshold – (1/threshold)
• If a person is very sensitive to something, they have a low threshold. If a person has a low sensitivity, they have a high threshold.

Why measure threshold? – Threshold is not rigid – it varies

• See differences within a sense
• For example, we are more sensitive to yellow light than we are to blue light. If you ask someone to make a yellow light and a blue light equal in perceived brightness (perception), you would find that the actual intensity of the yellow light is less than the actual intensity of the blue light (stimulus).
• We are also more sensitive to salt than we are to sugar.
• See differences between senses
• For example, we are more sensitive to changes in pitch and light than we are to changes in smell and taste.
• See differences between individuals
• For example, individuals have different preferences for spicy foods. Some people can eat very hot foods and not even blink an eye and other individuals would be in agony. Supertasters are more sensitive to bitter
• See differences within an individual
• For example, our sensitivity can change. We are more sensitive to sounds in the morning than in the evening.
• We can adapt to odors. If you put cologne or perfume on, you will smell it at first, but over time, we no longer can smell it.

How do we measure threshold? 

o The Classic Methods (Fechner)

• Participants are given stimuli both just below and just above threshold
• Look for crossover point – when person says yes instead of no (when using ascending trials) OR when person says no instead of yes (when using descending trials)

• The participant adjust the intensity of the stimulus until they can either just detect or just not detect a stimulus
• Least accurate, but the fastest
• Example – Person turns a knob until they can just sense tone or just not sense a tone
• Example – using the contrast button or knob on your computer

2. Method of Limits

• The participant is given discrete steps in order controlled by the experimenter
• Example – eye test
• Example, have 8 glasses of water, each with 2 cups of water and different levels of sugar. Glass 1 has no sugar, glass 2 has ¼ teaspoon (t) of sugar, glass 3 has ½ t of sugar, glass 4 has 1 t of sugar, etc. and glass 8 has 2 t of sugar.
• Present glasses to individual, starting at different points going from until participant can just taste or not taste sugar. Y means can taste, N means can’t taste.

Glass 2NNN

Glass 3NNNN

Glass 4YYNY

Glass 5YYY

Glass 6YY

Glass 7Y

Glass 8Y

• Why go in both directions (ascending and descending)? To prevent and cancel out the following errors
• Error of habituation – individuals who keep responding the same way, work on principle that the stimulus is going to be the same as the last stimulus
• want to be absolutely sure tasted sugar before responding yes
• Error of anticipation – individuals who jump the gun and change their responses quickly, work on principle that the stimulus is going to be different from the last stimulus
• as soon as possibility that might has tasted sugar, say yes

 

3. Method of Constant Stimuli

• The stimuli are randomly presented a number of times
• Slowest, but most accurate
• Example, instead of giving above glasses of water in order, present them to the participant randomly.
• Hearing test

Signal Detection theory

• Are we measuring absolute threshold? Instead of a single number threshold, move to a curve giving range of performance levels for part under different conditions – individuals sensitivity w/o bias

• What if participant thinks she might have detected a stimulus? What if a subject thinks she should have detected a stimulus? SDT deals with uncertainty

• Similar to a t-test

 

• Say yes if hear tone, no if don’t hear tone

• Laurie – supersensitive, responses every time faintest possibility she hears tone – liberal responder (more likely to say yes than no)

• Chris – wants to be totally sure hears tone before responding yes – conservative responder – not willing to report hearing tone unless very sure

• Results (100 trials)

– Laurie – 90% correct 10% wrong

– Chris – 60% correct 40% wrong

• Need trials that have NO tone

– Laurie 60% correct 40% wrong

– Chris 90% correct 10% wrong

• Tone given & state hear tone – hit

• Tone given & state don’t hear tone – miss (type 2)

• No tone given & state don’t hear tone – correct rejection

• No tone given & state hear tone – false alarm (type 1)

 

• We can detect the signal (the stimulus presented).

• But we also detect noise (all other stimuli in the environ). Level can vary

• A detection experiment is really a discrimination between the signal and noise. The signal has to be intense enough to exceed the subjective decision threshold.

• Talk about N or S+N (noise always present – sometimes add a signal).

 

Noise Distribution (can vary)

Signal + Noise Distribution

d’ – difference between the 2 curves, measure of strength of signal or sensitivity of person (similar to t in t-test)

Criterion (?) – person must decide if what sensed part of N or S+N

Conservative – fewer hits, but fewer false alarms

Liberal Criterion – greater number of hits but greater number of false alarms

 

ROC curve

• Receiver operating characteristic curve

• Plot Hits by False Alarms

• When d’ is straight line – chance level

• Farther from the straight line, greater than chance performance (greater sensitivity)

• Each point represents criterion

Weber’s Law (just noticeable difference)

• Relates size of JND to size of the standard. 

• Difference threshold gets larger as standard gets larger but ratio remains the same

• ?I/I = k 

– ?I = difference

– I = magnitude of standard stimulus

– k = Weber fraction

• Smaller the fraction, better can discriminate

 

Common Weber Fractions (?I/I)

• Light intensity 0.079, Sound Intensity 0.048, Lifted Weight 0.022, Line Length 0.029, Taste 0.083, Electric Shock 0.013

 

Fechner’s formula

• Assume all JND are psychologically equal (60-61 change is same as 300-305 change)

• P=k log I

– Perception, constant, intensity

– Log – the power to which 10 must be raised to equal that number (log 10 = 1, log 100 = 2)

 

• Say have k = 1

• P = 1 * 10

• P = 1

• What if want P = 2 (perc as 2x as powerful)?

• I = 100

• Works at midrange. See in use calculating decibels

Magnitude Estimation (Stevens)

• Gave a standard and assigned a number, then had participants assign a number directly to a test stimulus as it relates to the standard (so if __________ is a 10, then what is _____ ?)

• Found the relationship was usually described by a power function:

^• P = k * I^m

– k is a constant, I is physical intensity, m is the exponent (determined by calculating slope on a log/log scale) P is perception

Interpreting m

• If m = 1.0 then doubling physical intensity doubles perceived intensity.

– Line length

• If m < 1.0 then doubling physical intensity less than doubles perceived intensity.

– Brightness – response compression

• If m > 1.0 then doubling physical intensity more than doubles perceived intensity.

– Electric shock – response expansion

 

Common Exponents

• Loudness = 0.6, Brightness = 0.33, Taste = 0.8-1.3, Vibration = 0.95, Force of handgrip = 1.7, Electric shock = 3.5