week 4: psychophysics Flashcards
what is sensation
how our sensory organs (eyes, ears, etc) convert physical/chemical info into signals that our NS can understand
what is perception
how we acquire info from the environment
how we process that info to form internal representation of the environment
what is cognition
how we use the internal representation to do more complex things
examples of sensation
psychophysics
vision
hearing
examples of perception
pattern recognition
mental imagery
attention and performance
examples of cognition
memory
problem solving
reasoning and decision making
navigation
language
what is difference threshold
the smallest change in a stimulus that can be detected
what is absolute threshold
the minimum intensity of a stimulus that can be detected
what is webers law
weber found that the size of the JND (just noticeable difference) is a function of the magnitude of a reference stimulus
what was fechners idea
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
implication of fechners law
fechners law relates to internal experience (psyche) and physical environment (physics)
(psyche+physics=psychophysics)
what is fechners law about
the absolute. not relative, intensity of a stimulus
fechners law asserts that our:
psychological experience of the intensity of a stimulus tends to change less quickly than the actual change in stimulus intensity
what does fechners law relate to
fechners law relates internal experience (psyche) and physical environment (physics)
what is fechners law about
fechners law is about the absolute, not relative, intensity of a stimulus
fechners law asserts that:
our physical experience of the intesity of a stimulus tends to change less quickly than the actual change in stimulus
why is it hard to measure thresholds
because humans are very good at perceiving
methods to measure thresholds
methods of constant stimuli
methods of limits
staircase procedures
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 proportion of detections occurring at each stimulus magntitude
the threshold is taken as a magnitude at which the stimulus is detected a criterion proportion of the time (eg. 50%)
advantages to the method of constant stimuli
allows the shape of the psychometric function to be established
provides an accurate estimate of threshold
disadvantages to the method of constant stimuli
requires pre-testing to roughly estimate the threshold
wastes alot of trials which lie far from the threshold
it is difficult to measure changes in threshold over brief time periodss
methods of limits
measures the threshold without determining the shape of the psychometric function
types of series in method of limits
ascending and descending series in trials
what is descending series in method of limits
present the stimulus at a suprathreshold level
decrease stimulus intensity in small steps until participants can no longer detect the stimulus
what is ascending series in method of limits
present the stimulus at subthreshold level
increase stimulus intensity in small steps until participants can detect the stimulus
advantages to method of limits
more efficient than method of constant stimuli
still reasonably accurate in determining the threshold
disadvantages for method of limits
many trials are wasted as they are presented at intensities away from the threshold
participant may habituate (get used to giving a yes or no answer) and thus overshoot the true threshold
the overall shape of the psychometric function cannot be derived
staircase procedures
they involve linked series of ascending and descending runs with each successive run being based on the outcomes of the preceding run
the stimulus is presented either above or below threshold and the intensity is changed in small steps until a reversal (change response) occurs
the procedure is terminated after a criterion number of reversals
the threshold is taken as the average of these reversal intensities
why was staircase procedure designed
to overcome the problems surrounding the other methods
advantages to staircase procedure
even more efficient than the method of limits
can be modified in a number of different ways to overcome other limitations
disadvantages to staircase procedure
estimation of the threshold tends to require more complex calculations (especially when the procedure is modified), making it less intuitive
why can we never perceive stimuli under perfect conditions
because there is always noise and we never know whether we are perceiving the true stimuli (signal) or the noise
correct signal trials in signal detection
the level of activity will be above the criterion and lead to a correct yes response (hit)
signal trials for signal detection that are incorrect
will have activity below the criterion leading to an incorrect no response (miss)
what leads to a false alarm in signal detection
on some catch trials the level of activity may be above the criterion, leading to an incorrect yes response (false alarm)
what leads to correct rejection in signal detection
on the other catch trials the level of activity will be below the criterion, leading to a correct no response (correct rejection)
the separation between the signal+noise and noise distribution tells us:
how sensitive an observe is to that stimulus
what is the measure of sensitivity called
d-prime (d’)
how do we estimate d prime
the proportion of hits (or misses) tells us the location of the criterion relative to the signal+noise distribution
the proportion of flase alarms (or crrect rejections) tells us the location of the criterion relative to the noise distribution
convert these proportions to z scores
d’ is then the sum of these distances
d’=Z(FA)-Z(HIT) (=Z(FA)+{-Z(HIT)})
