VF - Absolute Threshold - Week 6 Flashcards

1
Q

Consider an experiment to measure absolute threshold. Where on the retina should the stimulus be presented?

A

The location with the highest density of rods.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Consider an experiment to measure absolute threshold. What wavelength should be used?
How is this determined?

A

The wavelength to which the rods are most sensitibe, ~500nm.

Determined via the peak of the spectral sensitivity curve.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Is luminance a parameter that affects the absolute threshold?

A

No, because it is the threshold.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

When measuring absolute theshold, in what condition should the observer be?

A

Dark adapted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Define spatial summation. When considering the size of the stimulus, relate it in terms of spatial summation.

A

Because the photons are counted, the light area bust be less than the spatial summation, because as they count photons, they approach the threshold. Anything over is not counted, and therefore wasted.
Not wanted when measuring threshold.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe Ricco’s law, and the associated formula. Define the log format as well.

A
Threshold luminance is inversely proportional to area.
L ∝ 1 / A
log L = -log A + log kA
kA is one value, and constant.
L is luminance
A is area
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Consider a critical area, with a radius of 2.
Suppose the threshold quanta, Q, to be 100.
What intensity is needed to achieve this, if the light area is less than the critical area?

A

If the stimulus area is <2, then an intensity of 100 is needed to ensure 100 photons are within the critical area, and thus reach the threshold.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Consider a critical area, with a radius of 2.
Suppose the threshold quanta, Q, to be 100.
What intensity is needed to achieve this, if the light area is equal to the critical area?

A

If the stimulus area is equal to 2, then an intensity of 100 is needed to ensure 100 photons are within the critical area, and thus reach the threshold.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Consider a critical area, with a radius of 2.
Suppose the threshold quanta, Q, to be 100.
What intensity is needed to achieve this, if the stimulus area is 4?

A

Only the light entering the critical area counts.
Area of a circle = πr^2
Area of the stimulus is 50, and the critical area is 12.5
The area of the stimulus is 4 times larger than the critical area.
100 photons are needed for threshold, and so the stimulus area must have 4 times the intensity to ensure that the critical area will get 100 photons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Consider a critical area, with a radius of 2.
Suppose the threshold quanta, Q, to be 100.
Now consider what intensity is needed if two stimulus areas are present within the critical area, each with a radius of 1?

A

The two stimuli both have an area that are less than the critical area, and thus only 100 total photons are needed to reach threshold between the two beams. As there are two beams, they each need an intensity of 50 assuming they are equal, or any value summing to 100 if not.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Consider an experiment to measure absolute threshold. What duration of the stimulus should be used?

A

Less than the limit of the temporal summation at the eccentricity used.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Describe summation according to Ricco’s law, and what happens to stimulus threshold luminance with increasing stimulus area.

A

Complete summation occurs, and as stimulus area increases, threshold luminance decreases linearly up to a critical area.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe summation according to Piper’s law, and what it says about transitions.

A

Partial summation occurs.

Transitions from under the critical to over the critical is smooth.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Define the critical duration.

A

Complete temporal summation occurs for brief stimuli up to a critical duration.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Define Bloch’s law and its formula.

A

Threshold luminance is inversely proportional to duration.
L ∝ 1 / D
L is luminance
D is duration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Does summation occur for long duration stimuli?

A

No.

17
Q

By what method should the stimulus be presented to the observer when measuring absolute threshold?

A

Method of constant stimuli.

18
Q

Describe the following for ideal rod stimulus:
Three stimulus parameters and their ideal values
Two observer characteristics
The method for stimuli presentation

A
Parameters
-Wavelength - 500nm
-Diameter - 10 mins of arc
-Duration - 1msec
Observer characteristics
-Eccentricity - 20° in the temporal retina, max rod density
-Fully dark adapted

Method of constant stimuli

19
Q

How many quanta are required to reach the cornea for threshold under ideal rod stimulus conditions?
What percentage correct value does this correlate to?

A

54-148 quanta required.

Threshold is defined as 60% correct.

20
Q

Describe the area of the retina by its component parts, and the percentage each takes up.

A

Cone area taking 20% of retinal area
Inter-receptor space taking 10% of retinal space
Rods form 70% of the retinal area.

21
Q

Consider how much area of the retina rods form. How much light is absorbed by ocular media, and what percentage is absorbed by the rhodopsin?
What effect does this have on threshold?

A

Ocular media is assumed to be 50% transmittance.
Rod area is 70%
30% of quanta are absorbed by rhodopsin.
Resulting in 0.3 x 0.5 x 0.7
From 50-150 quanta entering the cornea, this is then reduced to 4-14 quanta.

22
Q

How many rods are needed to respond to reach absolute threshold? What does it depend on?

A

It follows a psychometric function, with a sigmoidal shape.

Shape depends on subject variability and stimulus variability.

23
Q

Explain the situation with the number of quanta at low light levels, and its predictability.

A

At low light levels, the number of quanta in a given flash is variable.
However, this randomness follows a Poisson normal distribution.

24
Q

Suppose a given observer is perfect. What happens to the distribution of the number of quanta seen?

A

It follows a Poisson normal distribution.

A perfect observer would always respond only when quanta are greater than or equal to the threshold.

25
Q

Consider a psychometric function for ideal rod stimulation.

What effect does subject variability have on the curve?

A

Flattens the curve.

26
Q

Consider a psychometric function for ideal rod stimulation.

What effect does increasing the threshold have on the curve?

A

Steepens the curve.

27
Q

True or false

One photon can excite a rod, but several rods must respond

A

True

28
Q

Define the threshold quanta for cones.

A

5 quanta per cone

Summation over 2 cones

29
Q

Which threshold is higher, cones or rods, and why is this so?

A

Cones higher, because they have higher levels of thermal noise than rods.

30
Q

How many activated rods are required for absolute threshold?

A

6-10

31
Q

What is needed to trigger a large enough response in the ganglion cell to elicit a response to absolute threshold?

A

Sufficient spatial and temporal summation.