Lecture 4 Flashcards
definition of psychophysics
psychophysics:
behavioral study of relations between peoples perceptual experiences and correspondig physical properties
what is the absolute threshold?
absolute threshold:
what is the weakest stimulus that can be detected?
what is the difference threshold?
difference threshold:
what is the just noticable difference (JND) that can be deteted?
what are 3 classic methods to determine the absolute threshold?
- method of adjustment
- method of limits
- methods of constant stimuli
how to use the method of adjustment?
method of adjustment:
> use a range of stimulus intensities (typically continous)
> participants adjusts stimulus to their threshold
> starting point is chosen randomly
> absolute threshold = average of final settings
how to use the method of limits?
method of limits:
> set of stimulus intensities chosen by experimenter
> presentation of stimuli from lowest to highest (or reversed)
> after each stimulus report whether it was seen
> point where response turns is reported (also called turning point)
> absolute threshold: average of turning points
what is more efficient than method of limits?
adaptive testing: staircase method
what are limitations of the method if limits?
- hysteresis - ascending threshold is typically higher than descending
- not sure if the threshold. stimulus is always there
how to use: method of constant stimuli
method of constant stimuli
> number of fixed levels randomly presented across trials
> participant responds “yes” or “no”
> draw psychometric function
what is usually defined as the threshold in a psychometric function?
threshold: the x value at 50% correct detections
is the psychometric function consistent with the theory of an absolute threshold?
what is a problem, what is not?
not a problem:
- smooth transition instead of drastic change
- subliminal perception (e.g. blindsight)
is a problem:
> response bias: not only perception, but also strategy and motivation are measured
what is a possible solution to the problem of response bias in assessing a psychometric function?
forced choice: 2 methods
- temporal order judgement
> 2 stimuli, participant must pick 1
- equality judgement
> 2 stimuli, participant must decide if they are equal or not
what is one formula for the difference threshold?
difference threshold
=
(lower threshold + upper threshold)/2
lower threshold at 25%
upper threshold at 75%
what is the formula of webers law?
what does it mean?
webers law:
difference threshold/ intensity of standard stimulus = constant
> the JND is different in total size/intensity dependend of the absolute size/intensity of the stimuli compared
> the JND is constant as a proportion (described in the formula above)
what is important when considering the constant in webers law?
c is specific to a physical dimension
> comparing size has specific c
> comparing intensity has a different c
