Week 2 - Psychophysics and signal detection theory Flashcards
Thresholds
- difference threshold
- absolute threshold
Difference thresholds
The smallest change in a stimulus that can be detected (JND - just noticeable difference)
Absolute threshold
the minimum intensity of a stimulus that can be detected
Weber’s law
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 from a 40g reference weigh (JND = 1g), then the JND would be 10 g for a 400g reference weight
Weber’s law equation
ΔI/I=K
ΔI: size of detectable difference
I: physical intensity of a reference stimulus
K: constant
Fechner’s law
Fechner’s law asserts that our psychological experience of the intensity of a stimulus tends to change less quickly than the actual change in stimulus intensity
Fechner’s law equation
ΔS = k x ΔI/I
S: Intensity of sensation of a stimulus
k: Constant
I: Physical intensity of the stimulus
example thresholds
sight a candle flame: 48km on a clear dark night
hear mechanical watch ticking: 6m in noise free environment
taste teaspoon of sugar dissolved in 7.6L of water
smell one drop of perfume diffused through three rooms
feel the wing of a fly dropped on our cheek from a height of 7cm
How do we measure thresholds?
- method of constant stimuli
- method of limits
- staircase procedures
method of constant stimuli
- construct 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 proportion of detections occurring at each stimulus magnitude
- the threshold is taken as the magnitude at which the stimulus is detected a criterion proportion of the time (e.g. 50%)
psychometric function of method of constant stimuli
don’t observe a clear-cut discontinuity between detectable and undetectable stimuli in this psychometric function - usually curved, not two levels of can and cannot detect
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
- require pre-testing to roughly estimate the threshold
- wastes a lot of trials which lie far from the threshold (time consuming)
- difficult to measure changes in threshold over brief time periods
Method of limits
- measures the threshold without determining the shape of the psychometric function - average of limits from both runs
- uses ascending and descending series of trials
descending: present stimulus at a suprathreshold level and decrease stimulus intensity in small steps until participants can no longer detect the stimulus
ascending: present stimulus at a subthreshold level and increase the stimulus intensity in small steps until participants can detect the stimulus
advantages of method of limits
- more efficient than method of constant stimuli
- reasonably accurate in determining the threshold
disadvantages of method of limits
- many trials are still wasted as they are presented at intensities away from the threshold
- participant may habituate (get used to giving response) and overshoot the true threshold
- overall shape of psychometric function cannot be derived
Staircase procedures
designed to overcome problems of method of constant stimuli and method limits
- use ascending and descending runs and base successive runs on previous outcome
- stimulus presented either above or below threshold and intensity changed in small steps until change in response occurs
- direction of change is reversed when change in response occurs
- procedure is terminated after criterion number of reversals
- threshold is taken as the average of these reversal intensities
advantages of staircase procedures
- even more efficient than the method of limits
- can be modified in a number of different ways to overcome other limitations
psychometric function of staircase procedures
- standard procedure yields an estimate of 50% threshold
- by requiring two yes responses before the stimulus intensity is decreased, this procedure can estimate the 70% threshold
- can be used to figure out the overall shape of the psychometric function
- habituation can be addressed by running multiple series of trials simultaneously
disadvantages of staircase procedures
estimation of the threshold tends to require more complex calculations, making it less intuitive
Signal detection
We can never perceive stimuli under the perfect
condition
– There is always some noise, even when there are no
stimuli in the environment
* And we can never know whether we are
perceiving the true stimuli (signal) or the noise
* As a result, often, what we do is to use a certain
criterion with which we (unconsciously) decide
that we have perceived the signal
