Psychophysics Flashcards
What is Weber’s law?
Weber found that the Just Noticeable Difference (JND) is a function of the magnitude of a reference stimulus. For each stimulus dimension, there is a constant value. Therefore, the difference threshold is represented as a fraction of the JND/Stimulus magnitude.
What is Fechner’s law?
Fechner’s law proposes that weber fractions are the units of measurement for the human mind. There needs to be a constant unit if these values exist, what this suggests is that the human mind is using constant values as units of measurement.
What is Fechners formulae?
S = k log I
S: Intensity of sensation of a stimulus
K: Constant
I: Physical intensity of the stimulus
If you graphically represent this formula, it will draw a curve.
How can you explain Fechners curve?
As the physical intensity increases the subjective experience of the stimulus intensity doesn’t increase much- it depends on how the stimulus changes over time and the value of its intensity compared to your original perception of its presence.
What are the implications of Fechner’s law?
It relates to internal experience (psyche) and physical environment (physics).
It is about the absolute, not relative, intensity of a stimulus.
Our psychological experiences of the intensity of a stimulus tend to change less quickly than the actual change in stimulus intensity.
What are three methods used to measure thresholds?
Method of constant stimuli
Method of limits
Staircase procedures
What is sensation?
How our sensory organs (eyes, ears, etc.) convert physical/chemical information into signals that our nervous system can understand.
What is perception?
How we acquire information from the environment
How we process that information to form internal representations of the environment
What is cognition?
How we use the internal representations to do more “complex” things.
What is the difference threshold?
The smallest change in a stimulus that can be detected (a.k.a. JND “Just Noticeable Difference”)
What is the absolute threshold?
The minimum intensity of a stimulus that can be detected.
What is the 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 random order
Participants respond whether or not they detect the stimulus in 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
What is a psychometric function?
An inferential psychometric model applied in detection and discrimination tasks. In psychophysics, the ability to determine the threshold of stimuli.
What are the advantages of the method of constant stimuli?
Allows the shape of the psychometric function to be established.
What are the disadvantages of the method of constant stimuli?
Requires pre-testing to roughly estimate the threshold
Wastes a lot of trials that lie far from the threshold
It is difficult to measure changes in threshold overtime
It is not useful for measuring changes that happen quickly
What is the method of limits?
It measures the threshold without determining the shape of the psychometric function. It uses descending and ascending trials. The threshold is the average of the limits from each of the ascending and descending runs.
What are the advantages of the method of limits?
More efficient than the method of constant stimuli
Still reasonably accurate in determining the threshold
What are the disadvantages of the method of limits?
Many trials are “wasted” as they are presented at intensities away from the threshold
Participants may habituate and thus overshoot the true threshold
The overall shape of the psychometric function cannot be derived
What is the staircase procedure?
It is designed to overcome the issues in the method of constant stimuli and the method of limits.
It is a linked series of ascending and descending runs with each successive run being based on the outcome of the preceding run
The stimulus is presented either above or below the threshold and the intensity is changed in small steps until a reversal (change in response) occurs
The direction of change is then reversed when another reversal in response occurs
The procedure is terminated after a criterion number of reversals
The threshold is taken as the average of these reversal intensities
What are the advantages of the staircase method?
More efficient than the method of limits
Can be modified in a number of different ways to overcome other limitations i.e. habituation
Trials can be tweaked to determine the psychometric function at different percentage intervals
Trials can be run simultaneously to avoid response habituation and response overshoot
What are the disadvantages of the staircase procedure?
Estimation of the threshold tends to require more complex calculations (especially when the procedure is modified), making it less intuitive
What are some other explanations outside of the stimulus, which determine a yes response in threshold research?
Signal noise.
Why must signal noise be considered as a criterion in threshold theory?
You will never be able to create the perfect situation in stimulus detection. Noise is present and causes sensory activity in the brain, therefore noise must be plotted alongside sensory criteria to determine the effect in your data.
When understanding responses in the noise and signal+noise distributions in signal detection, there are a few responses that can be made once your criterion has been determined from both distributions. What are the four responses present in the trials?
On some signal trials, the level of activity will be above the criterion, leading to a correct “yes” response (hit)
On other signal trails, the level of activity may be below the criterion, leading to an incorrect “no” response (miss)
On some catch trials, the level of activity may be above the criterion, leading to an incorrect “yes” response (false alarm)
On other catch trials, the level of activity will be below the criterion, leading to a correct “no” response (correct rejection)
How do you estimate the sensitivity deprive measure d’?
Firstly, you plot the hits in the signal+noise and noise distributions. The average of these scores is then the criterion cut-off. You find the proportion of data from the centre of both distributions to the cut-off criterion and convert them to Z scores. d’ is the sum of these distances.
d = Z(FA) - Z(HIT)
When understanding d, what is a rule that will exist for the signal+noise distribution?
The Z score will always be negative.
What is d’ a measure of?
The sensitivity, which is independent of response bias. Therefore, it is possible to have the same d’ from a range of different response patterns.
What is the receiver operating characteristic curve?
The curve shows the range of possible hit and false alarms. This is because the same d value can have different response patterns.
