Karteikarten
Absolute Threshold
Karten in diesem Stapel (9)
- 1List and justify the three stimulus parameters and two observer characteristics specified in Hecht, Schlaer and Pirenne studyThree stimulus parameters are:
* retinal location - rods have highest density at about 15 degrees from fovea
* colour stimulus - wavelength should be one where rod are most sensitive to (~500nm)
* stimulus size - less than the limit of spatial summation at eccentricity used
* Duration of stimulus
Observer characteristics
* have to be dark adapted - minimise threshold and increase sensitivity - 2Explain spatial/temporal summationTemporal summation refers to the eye’s ability to sum the effects of individual quanta of light over time. (duration)
Spatial summation - Region of the retina where all the quanta in that region are summated to influence response of cell later in the pathway (area) - 3Define complete spatial/temporal summation for a critical area/durationComplete spatial summation - occurs for small stimuli up to a critical area
* Following Ricco's Law
* same number of quanta to reach threshold as long as it falls in the critical area
* As stimulus area increase, threshold luminance decrease linearly to critical area
Complete temporal summation - occurs for brief stimuli up to a critical duration
* follows Bloch's Law - 4Write Ricco's/Bloch's LawRicco's Law:
* Threshold luminance (L) is inversely proportional to area (A)
* L = Ka/A, where Ka is a constant
* Threshold quanta (Q) is constant for a given duration (D)
* L=Q/(D x A) = Ka/A
Bloch's Law
* Threshold luminance (L) is inversely proportional to duration (D)
* L = Kd/D, where Kd is constant
* Threshold quanta (Q) is constant for a given area (A)
* L = Q/(D x A) = Kd/D - 5Explain why the threshold number of quanta is constant for areas less than the critical area/duration, whereas the threshold luminance drops with increase area/durationThreshold number of quanta is constant if within critical area/duration as all quanta falling onto the area over a period will be summated
For a given increase in area - luminance will decrease as luminance is the number of quanta per unit area, since quanta is the same but area has increase, luminance would decrease
For a given increase in duration, the luminance will decrease as amount of quanta will impinge on surface will be the same but since there is a increase in time, you can have less intensity falling over a longer period of time - 6Describe quantal fluctuations and how they can be used to model psychometric functionsEvery time a flash is given - exact number of quanta varies in a range - known as Poisson distribution
These Poisson distributions can be used to model the psychometric function - converted the distributions to inverse cumulative poisson distribution - then produce psychometric function from these distribution for a given threshold
* the wider the poisson distribution, the flatter the psychometric function
* set threshold as a particular value - graph shape changes
* gives theoretical psychometric function - only if there is background noise
* for subject noise - would want a steeper theoretical psychometric function - 7Explain how we know that one quantum of light can excite a rod photoreceptorSince 6-10 active quanta is required for threshold and the amount of quanta falls on an area of 300-500 rods - suggest that one quantum can excite a rod
- 8Explain how we know that seeing at absolute threshold requires summation across several rodsRequire several individual rod response to occur in the same time for visual percept of light
* there is 50% chance of seeing the light at absolute threshold - therefore summation of several rods are required to occur at the same time - 9Absolute threshold of visionMinimum amount of light required to produce a perceptual response of seeing light
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- Postgraduate Visual Functions - Optometry (Visual Functions - OD1) Karteikarten am Absolute Threshold , erstellt von Kim Vu am 29/05/2018.
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- Kim Vu
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