VF - Absolute Threshold - Week 6 Flashcards
Consider an experiment to measure absolute threshold. Where on the retina should the stimulus be presented?
The location with the highest density of rods.
Consider an experiment to measure absolute threshold. What wavelength should be used?
How is this determined?
The wavelength to which the rods are most sensitibe, ~500nm.
Determined via the peak of the spectral sensitivity curve.
Is luminance a parameter that affects the absolute threshold?
No, because it is the threshold.
When measuring absolute theshold, in what condition should the observer be?
Dark adapted.
Define spatial summation. When considering the size of the stimulus, relate it in terms of spatial summation.
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.
Describe Ricco’s law, and the associated formula. Define the log format as well.
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
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?
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.
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?
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.
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?
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.
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?
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.
Consider an experiment to measure absolute threshold. What duration of the stimulus should be used?
Less than the limit of the temporal summation at the eccentricity used.
Describe summation according to Ricco’s law, and what happens to stimulus threshold luminance with increasing stimulus area.
Complete summation occurs, and as stimulus area increases, threshold luminance decreases linearly up to a critical area.
Describe summation according to Piper’s law, and what it says about transitions.
Partial summation occurs.
Transitions from under the critical to over the critical is smooth.
Define the critical duration.
Complete temporal summation occurs for brief stimuli up to a critical duration.
Define Bloch’s law and its formula.
Threshold luminance is inversely proportional to duration.
L ∝ 1 / D
L is luminance
D is duration
