VF - Intensity Discrimination - Week 6

Question 1

Why is intensity discrimination important?

Answer 1

It is fundamental to seeing, and is needed to identify the shapes of objects.

Question 2

Define 4 means by which intensity discrimination is measured.

Answer 2

• Bipartite field - two halves of a circle with different intensities
• Superimposed bipartite field - bipartite field with a background
• Increment threshold - adjustable intensity circle with a background
• Two alternative forced choice

Question 3

In terms of a threshold vs. intensity (TVI) curve, define the following and in order:
Weber’s law
Saturation
deVries-Rose law
Absolute threshold
Define the symbols for threshold and intensity

Answer 3

-Absolute threshold - threshold is not affected by adapting intensity
-deVries-Rose law - threshold increases as the square root of the adapting intensity
-Weber’s law - threshold increases in direct proportion to the adapting intensity
Saturation - threshold increases sharply with only small increases in adapting intensity
Threshold = ΔI
Intensity - I

Question 4

What is the theoretical lower limit for intensity discrimination, and how does this apply in reality?
Are observers able to detect intensity differences if they vary by a small number of quanta?

Answer 4

Smallest detectable difference is theoretically one quanta.
In reality, the number of quanta in a stimulus is variable.
Impossible to always detect an intensity difference if they differ by a small number of quanta.

Question 5

Define the following on a Poisson distribution:
Variance
Standard deviation

Answer 5

Variance = mean

Standard deviation = √mean

Question 6

How do humans behave at low adapting intensities, and what law applies here?
What photoreceptor is responsible for this?

Answer 6

They behave as ideal detectors, and the deVries-Rose law applies.
Cone system acts as an ideal detector over this intensity range.

Question 7

How do humans behave at high adapting intensities, and what law applies here?
Does the cone system perform better, the same, or worse than an ideal detector over this range?

Answer 7

They perform worse than an ideal detector, and Weber’s law applies here.
Cone system performs worse than an ideal detector.

Question 8

Describe the formula for the deVries law, the variables included, and the log form of the equation.
Define the slope and the y intercept on the log form.

Answer 8

ΔI = k √I
or
log ΔI = 0.5 log I + log k
ΔI is the threshold
I is the intensity
k is a constant
The slope is 0.5
log k is the y intercept

Question 9

Describe the formula for Webers law, the variables included, and the log form of the equation.
Define the slope and the y intercept on the log form.

Answer 9

ΔI = kI
or
log ΔI = log I + log k
ΔI is the threshold
I is the intensity
k is a constant
The slope is 1
log k is the y intercept

Question 10

What does Weber’s law demonstrate about how the visual system transmits information?

Answer 10

It is designed to transmit information about relative luminance rather than absolute luminance.

Question 11

True or false

Saturation level intensities can be accounted for by Weber’s law or the deVries-Rose law.

Answer 11

False, neither law holds for saturation level intensities.

Question 12

True or false

The rod system cannot saturate, while the cone system does.

Answer 12

False
The rod system eventually saturates
The cone system will never saturate

Question 13

Consider saturation. What does the cone system protect normal observers from?
What would a rod monochromat see at saturation levels?

Answer 13

From rod saturation.
Monochromats would see only entirely white at saturation levels, though it is not painful or due to high intensity, but only due to saturation.

Question 14

What happens when you decrease test stimulus size?

Answer 14

Increase threshold intensity.

Question 15

What happens when you decrease test stimulus duration?

Answer 15

Increase threshold intensity.

Question 16

What two factors affect the Weber’s fraction?

Answer 16

Stimulus eccentricity

Stimulus wavelength

Question 17

What can be used to predict the effect of wavelength on intensity discrimination?

Answer 17

The duplicity model.

Question 18

What four assumptions are made in the duplicity model?

Answer 18

• There are two detection systems
• The two above systems have unique and invariant spectrial sensitivities
• The systems are independent
• The two systems adapt independently

Question 19

Explain in detail how the duplicity model can be used to predict the effect of changing wavelength. Describe both vertical and horizontal movements.

Answer 19

• Consider a TVS curve to a particular wavelength, including the threshold regions for both cones and rods.
• The specral sensitivity will also be given.
• Find the point on the spectral sensitivity curve corresponding with the new wavelength.
• Find the vertical difference between the two points on the spectral sensitivity curve.
• Use this measurement to shift the TVS curve either up or down, depending on whether the new wavelength point is above or below the given wavelength point respectively.
• Shift the rod and cone curves separately, and it is likely they will shift differently.
• For a horizontal shift, find the horizontal difference between the two wavelength points on the spectral sensitivity curve, and consider whether the new point is to the left or to the right of the given wavelength point.
• Shift the corresponding curve to the left or to the right respectively.

Question 20

When using the duplicity model to predict the effect of varying wavelengths, what happens to the saturation levels?

Answer 20

The two will eventually converge. This can be read off a relative adapting field intensity scale, which can usually be found above the TVS curve.

Question 21

Define field threshold intensity. What does this allow for?

Answer 21

The adapting intensity that increases the threshold by one log unit above threshold.
Done by selecting one log unit up on the y-axis, and reading off the corresponding x-axis value.
This allows differentiation of TVS curves shifted horizontally using the duplicity model.

Question 22

Does absolute threshold change when adapting wavelength values are changed? What value does it have, and why?

Answer 22

It does not change.

It will be 0 because there is no background for absolute threshold.

Question 23

What happens to the Weber’s fraction as stimulus eccentricity increases?

Answer 23

It also increases

Question 24

What happens to the Weber’s fraction as stimulus duration increases?

Answer 24

It decreases.