VF - Visual Psychophysics - Week 1

Question 1

How can the mechanisms of a black box be inferred?

Answer 1

Using an input, and observing the output.

Question 2

Define threshold.

Answer 2

Smallest stimulus intensity required for detection or discrimination.

Question 3

What is the threshold for wavelength discrimination?

Answer 3

> 2nm.

Question 4

How are threshold and sensitivity related? Describe the formula.

Answer 4

They are inversely related

Threshold = 1 / Sensitivity

Question 5

Why are threshold vs sensitivity curves often logged?

Answer 5

Taking the log allows representation of a large range of data in a compact form, allowing more practical representation, as well as more accurate interpolation.

Question 6

Name the four methods of measuring threshold.

Answer 6

Method of adjustment
Method of limits
Method of constant stimuli
Staircase method (modified method of limits)

Question 7

What are the two main errors when measuring threshold?

Answer 7

Error of habituation

Error of anticipation

Question 8

Describe the method of constant stimuli.

Answer 8

Subject is randomly presented stimuli at different intensity levels. The data is collated and a psychometric function is constructed.

Question 9

When measuring threshold, is there a steep step in which the subject never sees the stimulus to where they always see it?

Answer 9

No, it is instead a gradual change from consistently no, to consistently yes, and has an sigmoid curve.
The steepness at the inflection point depends on the sensitivity of the individual, with higher sensitivities resulting in a greater degree of steepness and vice versa.

Question 10

On a psychometric curve, what is defined as the threshold?

Answer 10

The intensity at which 50% yes is reported.

Question 11

How does modern threshold measurement compare to classical threshold theory?

Answer 11

Classical threshold theory attempted to deduce the neural threshold itself, and had steep steps from no to yes, where modern theory doesnt.

Question 12

When presenting a subject with stimuli repeatedly, what is the classical threshold theory’s take on this vs modern theory? Explain why.

Answer 12

Classical theory asserts that repeated stimulation would always produce the same response.
This is false in modern theory due to the presence of noise - quantum fluctuations that mean the stimulus may not always be the same.
Similar noise will exist in the visual system as well, such as spontaneous chemical activity even in the absence of stimuli.

Question 13

Is noise detected by the brain, or can it be ignored entirely?

Answer 13

No, signals are always detected alongside noise, and the brain must distinguish what is noise, and what is signal + noise.

Question 14

Describe in detail the process by which the brain my interpret noise vs signal + noise. Explain the importance of the overlap, and the need for compromise. Mention why it isn’t feasible to have a 100% detection rate with no false positives, or why the intensity level should/shouldn’t be clear of the noise.

Answer 14

Consider noise to have a normal curve, with the mode at a given intensity. Signal + noise will have a similar normal curve, but with a mode higher than the noise normal curve. This results in an overlap between the two. The brain must select an intensity such that it recognises the signal, but with a low enough detection of noise.
Selecting an intensity clear of the noise may result in 0% false positives (ie. the mode), but doing so will mean a great portion of the curve is missed. Selecting an intensity that covers the entire signal + noise curve may result in a 100% detection rate, but will also mean a high false positive rate.
The brain compromises, and instead selects between the two, at around 85%.

Question 15

Concerning noise vs signal + noise, what would the curve of hit rate vs false positive be called, and what does it look like? Explain what distance to the diagonal means, and whether a 100% hit rate and 0% false positive rate is possible.

Answer 15

It is hyperbolic, rapid increase, rapid plateau, called a receiver-operator curve. The steepness of the curve is related to the overlap between the normal curves of noise and signal + noise curves.
Higher degree of distance from the diagonal indicates greater separation of the normal curves.

Question 16

Describe the method of adjustment.

Answer 16

Give the subject a scaling slide, and allow them to adjust until they see (or don’t see) the stimulus.
Similar to gamma settings in video games.

Question 17

Can the method of adjustment still have false positives? Why/why not?

Answer 17

Yes it can, it is not criterion free.

Question 18

What are some problems with the method of adjustment?

Answer 18

The subject may memorise the position of the instrument that results in the treshold, allowing them to respond with their eyes closed.

Question 19

Describe the method of limits, and why it is done alternatingly. What error does this cancel out?

Answer 19

The subject is given a series of stimuli with an increasing/decreasing gradient of intensity such as fading letters, alternating either ascending or descending.
The alternation cancels out error of anticipation.

Question 20

Describe the staircase method.

Answer 20

Subject begins with the lowest intensity, below threshold. It is incrementally increased until the subjects observation changes. Once so, it is incrementally decreased until the observation changes. Stimuli is then incrementally increased, and so on, resulting in a wave-like staircase.

Question 21

What happens to subject responses with liberal criterion vs strict criterion?

Answer 21

Liberal - percentage seen almost never reaches 0%

Strict - percentage seen is much lower for the corresponding intensity compared to liberal criterion.

Question 22

What happens to subject responses with forced choice criterion? Give an example with a tumbling E chart.

Answer 22

Subjects asked the orientation of the E (up or down only) will eventually guess when they can no longer see it.
With two options, this means a 50% correct guess rate at the lowest intensity. Threshold is between 50 and 100% at 75%.
With more options this changes, for example 4 options means 25% correct rate, and a 62.5% threshold.

Question 23

How can visual acuity in babies be measured before they can read or recognise shapes?

Answer 23

A board with two coverable grills on either side, with a pinhole in the middle is used.
An optometrist looks through the pinhole and alternates which grill is uncovered. If the baby’s eyes change in correspondance, indicates visual acuity.

Question 24

Define suprathreshold.

Answer 24

Stimuli above threshold level, used for relative stimulation.

Question 25

Define Fechner and Stephen’s law, and the associated equation. How is the curve of this equation linearised? Define both equations.

Answer 25

The law predicts the magnitude of estimation of intensity.
Small changes in intensity at low levels result in large changes to magnitude of estimation, and vice versa.
The equation is S = I^c
S = magnitude of estimation
I = Intensity
c = constant
It is logged to linearise, resulting in:
log (S) = log (I^c) or
log (S) = c log (I)