3. psychophysics Flashcards
define threshold difference
the smallest change in a stimulus that can be detected
a.k.a. JND - Just Noticeable Difference
Define absolute threshold
The minimum intensity of a stimulus that can be detected
what did weber define in Weber’s law
that the size of the JND is a function of the magnitude of a reference stimulus
e.g. if a weight has to be 41g before it can be discriminated form a 40g reference weight (JND=1g)
> then the JND would be 10g for a 400g reference weight
what did fechner say in building upon webers findings
if a weber fraction is constant for a given stimulus dimension, then the mind might use the weber fraction as a unit for perceiving that stimulus dimension
What are the implications of fechners law
> relates to internal experiences (psyche) and physical environment (physics)
> Psyche + Physics = Psychophysics
> is about the absolute , not relative intensity o fa stimulus
> turning the focus of research from difference threshold to absolute threshold
> asserts that our psychological experience of the intensity of a stimulus tends to change less quickly that the actual change in stimulus intensity
what procedures have been developed to measure thresholds
method of constant stimuli
method of limits
Staircase procedures
describe the process of:
Method of constant stimuli
> Construct a set of stimuli with magnitudes ranging from above to below the presumed threshold value
> present these stimuli a number of times in a random order
> participants respond whether or not they detect the stimulus on each trial
> plot the proportions of detections occurring at each stimulus magnitude
> the threshold is said to be at a magnitude at which the stimulus is detected at a certain proportion of the time (e.g. 50% of times difference is reported present)
advantages of
Method of constant stimuli
allows the shape of the psychometric function to be established
provides an accurate estimate of threshold
disadvantages of
Method of constant stimuli
requires pretesting to roughy estimate the threshold
wastes a lot of trials which lie jar from the threshold
> time consuming
is difficult to measure changes in threshold over brief time periods
describe the method of limits
measures threshold without determining the shape of the psychometric function
uses ascending and descending series of trials
threshold is the average of the limits form each of the ascending and descending runs
what is a descending series in method of limits
resent the stimulus as a suprathreshold level
decreases stimulus intensity in small steps until participant can no longer detect the stimulus
what is an ascending series in method of limits
presents stimulus at a substhreshold level
Increases stimulus intensity in small steps until participant can detect stimulus
advantages of
Method of limits
more efficient (quicker) than constant stimuli
still reasonably accurate in determining the threshold
disadvantages o f
method of limits
many trials wasted as they jre presented at intensities away from the threshold
Participants may habituate (get used to giving yes or no response) and over shoot the true threshold
Overall shape of psychometric function cannot be derived
how does staircase procedures work
- link series of ascending and descending runs so that successive runs are based on outcomes of previous run
- stimulus resented above or below threshold and intensity is changed until reversal (change in response) occurs
- direction of change is reversed when another reversal in response occurs
- procedure is terminated after a criterion number of reversals
- threshold is taken as the average of these reversal intensities
advantages of
Stair case procedures
even more efficient than method of limits
can be modified in a number of different ways of overcome other limitations
staircase procedure:
the standard procedure yields an estimate of __ threshold
production of 70% threshold by requiring 2 yes’s before reversal means:
50%
staircase procedure can be used to figure out the overall shape of the psychometric function
what is over come by the staircase procedure
issue of habituation by running multiple series of trials simultaneously
disadvantages of :
Staircase procedures
estimation of the threshold tends to require more complex calculations (especially when the procedure is modifies)
Making it less intuitive
signal detection
what criteria is used when we unconsciously decide that we have perceived the signal
noise Vs Signal+noise
> signal present, response yes = hit
> signal present, response no = miss
> signal absent, response yes = false alarm
> signal absent response no, correct rejection
what tells us how sensitive an observer is to a particular stimulus
the separation between the signal +noise and noise distribution
this measure of sensitivity called d-prime (d’)
How is d’ estimated in signal detection
the proportion of hits (or misses) tells us the location of the criterion relative to the signal+noise distribution
the proportion of false alarms (or correct rejection s) tells us the location of the criterion relative to the noise distribution
convert these proportions to z scores
> these scores tell is the distance from the criterion to each distribution mean
d’ is the sum of these distances
> d’ = Z (FA) - Z(HIT)
d’ is a measure of…
sensitivity which is independent of response bias in signal detection
> meaning it is possible to get the same d’ from a range of different response patterns
what is an ROC curve
received operating characteristic curve
this curve shows the range of hit and false alarm rates that yield the same sensitivity (d’)
