2. Research methods Flashcards
classical psychophysics (CLPH)
the study of the relationship between physical quantities and the experience of them
can be used to understand detection, discrimination, and scaling of physical stimuli
detection CLPH
detecting stimuli
did you see that animal? yes
discrimination CLPH
being able to tell the difference between stimuli
cats and dogs
Scaling CLPH
being able to tell how two stimuli
dogs tend to be bigger, louder, and have more district snouts and variation in shape/size
cats all tend to have a more consistent shape with small snouts and paws.
Absolute threshold
minimal limit of a sense modality, what is the smallest stimuli you can become aware of?
the smallest amount of a stimulus necessary to allow an observer to detect its presence
minimum stimulus energy or chemicals required to be detected 50% of the time
the point at which physical stimuli enters consciousness (threshold theory)-present a stimulus to an observer and ask if they can detect it
also called a limen
Method of constant stimuli
stimulus intensity chosen at random from a predetermined (“constant”) set on each trial (observation)
Constant meaning predetermined
- select intensity range: 0 to 100 lumens
- determine set of values: 8, 97, 42, 8, 67, 4, 35, 51,…
- observer says “yes” when stimulus perceived
- record observations
- plot results:
Pros and cons of the method of constant stimuli
☑ gives good estimate of threshold
☒ time-consuming(must know proper range because it could be between 1-100 or 0-1 for example)
☒ cannot measure threshold changes over time (e.g., in dark adaptation)
Method of Limits
change stimulus intensity monotonically(increasing or decreasing)
- increase stimulus intensity until it is perceived (ascending series) or reduce stimulus intensity until it is not perceived (descending series)
pros and cons of the method of limits
Pros & Cons:
☑ can track threshold changes over time
☒ induces errors of habituation-participants giving the same answers: yes,yes,yes, no (repeat)
☒ induces errors of anticipation-saying yes forever
Adaptive testing variant
stimulus intensity changed over a continuous series
▸ e.g., staircase method: changes intensity from ascending to descending when “yes” encountered; vice-versa for “no” (not just continuously going up or down, but also switching directions)
▸ the point at which perception changes is called the turnaround
▸ choose different starting point for each series from trial to trial
▸ threshold may vary between trials: take the mean of the turnarounds
Method of adjustment
observer directly controls stimulus values until threshold reached
Pros & Cons:
☑ fast
☒ least accurate method
Difference threshold
minimum difference needed to discriminate between two stimuli, 50% of the time
- judgment made between standard and comparison stimuli (both are well above absolute threshold)
- a.k.a. “just noticeable difference,” or JND
e.g., present 100 g standard vs. multiple comparison stimuli; ask whether there is a difference (yes/no)
- 0.75 → upper difference threshold
- 0.25 → lower difference threshold
- difference threshold = (upper - lower) ÷ 2
0.50 → Point of subjective equality (PSE)
Point of subjective equality (PSE)
stimulus that is apparently most like the standard
(PSE may differ from standard stimulus)
Weber’s Law (1834):
Is the difference threshold the same for all standard stimuli, or does it vary somehow?
k = JND / S
k = constant (“Weber fraction”)
S = standard stimulus intensity
e.g., for a 100 g weight, JND = 3 g (difference = 3%)
for a 1,000 g weight, JND = 30 g (difference = 3%)
Weber fraction = 3/100 = 0.03
What is the problem with Weber’s Law?
“law” does not extend to extremes. not a law because a law is supposed to hold under all conditions
☒ cannot be applied to stimuli close to absolute threshold-such as 0,000001 grams of salt
☒ also breaks down at higher stimulus intensities-like a million pounds
Scaling
- what is the subjective magnitude of a stimulus?
- e.g., are two 60 W lights twice as bright as one 60 W light? (no)
only concerned with if there is a difference or not, not with the degree/amount of difference
Indirect scaling
magnitude derived from multiple difference judgments
Fechner’s Law (1860)
derived a scale based on two assumptions:
* Weber’s Law is valid
* basic perceptual unit is the JND (there is no half JND)
S = (1/k) loge (I )
k = Weber fraction
I = stimulus intensity(physical magnitude)
S = sensory experience(perceived magnitude)
e.g., what if intensity is doubled? Let k = 1
if I = 100, S = 4.61
if I = 200, S = 5.30
- doubling intensity does NOT make stimulus seem twice as big
e.g., a light 20 JNDs above threshold is twice as bright as a light 10 JNDs above threshold
Problems with Fechner’s Law
☒ Weber’s Law breaks down at extremes
☒ research showed Fechner’s formulation was insufficient
Direct Scaling
observers assign values to stimulus intensities
Magnitude estimation
- observer presented with a reference stimulus (modulus); and a certain value (say, 10)
- other stimuli are presented; observer assigns values to them (if half as bright, give it a 5; if twice as bright, 20, etc.)
- values of stimulus magnitude provided directly by observer
- results conflicted with Fechner’s Law; instead, a “power law” held (power function)
Steven’s Law 1957 (SL)
P = K S^n
P = perceived magnitude
K = constant
S = stimulus intensity
n = exponent
response expansion SL
n > 1
e.g., doubling voltage more than doubles sensation of electric shock
linear relation SL
n = 1
magnitude of response matches changes in physical quantity
e.g., estimates of line length, distance
response compression SL
n < 1
e.g., doubling intensity of light produces only a small change in perceived brightness
Criticism of Steven’s Law
all the line describing it all look very different, the solution was to draw a line graph
perceived magnitude in descending order of stimulus is: electric shock, apparent length, brightness
Why are electric shocks perceived higher than apparent length and brightness?
Answer: evolution, aka, internal nervous system thinks pain is bad, distance gauge good, seeing in different light conditions good
- pain very quickly becomes very aversive
- estimating distances is very accurate-being able to tell how far prey or predators were compared to yourself
- allows us to handle a large range of light intensities, and not be blinded either in a dim room or by sunlight
Criticism: magnitude estimation may tell us how people “use numbers,” instead of how they judge stimuli
e.g., Which is the “bigger” difference: 1-2? or 2-3?
what are some problems and solutions?
Problem: How to estimate magnitude without using verbal reports of numbers?
Solution: cross-modality matching: one sense used to provide a measure of intensity in another sense
- typically uses a hand dynamometer
- grip squeezed to indicate magnitude of stimulus-squeeze harder if the light is brighter
- Kinesthesia (sense of movement of the parts of the body) used to estimate quantity in other modality
- results match magnitude estimation-bro finally got the numbers to prove his shit
Theory of Signal Detection (TSD)
(Tanner & Swets, 1954)-were developing radar in WWII
Problem: observer says “yes” a lot
- response bias confounds attempts to measure sensitivity-people just wanted to be helpful but instead muddied results
Solution: catch trials: no stimulus presented in half the trials-so they could reliably detect if the radar wasn’t detecting a plane
- help determine whether observer has a tendency to respond “yes” (or “no”) more frequently, aka if the person has a response bias
How can the probabilities for people’s responses be changed?
by adding payoffs for certain conditions
eg: by making hits pay $10, correct rejections +$1, and misses or false alarms -$1, you increase the rates of people responding with hits and false alarms
same can be done for any desired response
what is an ROC curve?
receiver operating characteristic curve?
-everyone’s will look different
-each point on the curve represents a different payoff
-the “bowing” of the curve is affected by:
1. the oberverst sensitivity
2. the intensity of the stimulus
Why do people respond when there is no stimulus present?
because background “noise” exists in the sensory system
the optic nerve in our eyes has a base level of activity, even with our eyes closed, this applies to all of our senses
sensitivity or discriminability, d’ (the difference in the means of the 2 distributions)
- is the distance between the means of the noise and signal + noise distributions
*is independent of response criterion (β) - the concept of threshold is not used in TSD
TSD and discriminating between stimuli:
- measured as discriminability (d’ ) between two signal + noise distributions (S1+N vs. S2+N)
- both must be detectable
