Psychophysics

Question 1

What is Weber’s law?

Answer 1

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.

Question 2

What is Fechner’s law?

Answer 2

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.

Question 3

What is Fechners formulae?

Answer 3

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.

Question 4

How can you explain Fechners curve?

Answer 4

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.

Question 5

What are the implications of Fechner’s law?

Answer 5

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.

Question 6

What are three methods used to measure thresholds?

Answer 6

Method of constant stimuli
Method of limits
Staircase procedures

Question 7

What is sensation?

Answer 7

How our sensory organs (eyes, ears, etc.) convert physical/chemical information into signals that our nervous system can understand.

Question 8

What is perception?

Answer 8

How we acquire information from the environment
How we process that information to form internal representations of the environment

Question 9

What is cognition?

Answer 9

How we use the internal representations to do more “complex” things.

Question 10

What is the difference threshold?

Answer 10

The smallest change in a stimulus that can be detected (a.k.a. JND “Just Noticeable Difference”)

Question 11

What is the absolute threshold?

Answer 11

The minimum intensity of a stimulus that can be detected.

Question 12

What is the method of constant stimuli?

Answer 12

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

Question 13

What is a psychometric function?

Answer 13

An inferential psychometric model applied in detection and discrimination tasks. In psychophysics, the ability to determine the threshold of stimuli.

Question 14

What are the advantages of the method of constant stimuli?

Answer 14

Allows the shape of the psychometric function to be established.

Question 15

What are the disadvantages of the method of constant stimuli?

Answer 15

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

Question 16

What is the method of limits?

Answer 16

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.

Question 17

What are the advantages of the method of limits?

Answer 17

More efficient than the method of constant stimuli

Still reasonably accurate in determining the threshold

Question 18

What are the disadvantages of the method of limits?

Answer 18

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

Question 19

What is the staircase procedure?

Answer 19

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

Question 20

What are the advantages of the staircase method?

Answer 20

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

Question 21

What are the disadvantages of the staircase procedure?

Answer 21

Estimation of the threshold tends to require more complex calculations (especially when the procedure is modified), making it less intuitive

Question 22

What are some other explanations outside of the stimulus, which determine a yes response in threshold research?

Answer 22

Signal noise.

Question 23

Why must signal noise be considered as a criterion in threshold theory?

Answer 23

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.

Question 24

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?

Answer 24

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)

Question 25

How do you estimate the sensitivity deprive measure d’?

Answer 25

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)

Question 26

When understanding d, what is a rule that will exist for the signal+noise distribution?

Answer 26

The Z score will always be negative.

Question 27

What is d’ a measure of?

Answer 27

The sensitivity, which is independent of response bias. Therefore, it is possible to have the same d’ from a range of different response patterns.

Question 28

What is the receiver operating characteristic curve?

Answer 28

The curve shows the range of possible hit and false alarms. This is because the same d value can have different response patterns.