Dark Adaptation Flashcards
(44 cards)
In the dark, our visual system must maximize the ability to capture photons AKA ____ sensitivity
Increase
Three things that occur to increase dark adaptation (which is the recovery of visual sensitivity that occurs in the dark)
- Pupils dilate
- Increased sensitivity of photoreceptors
- Increased dominance of rod activity with decreasing ambient light.
We are way more efficient at detecting light under (scotopic or photopic) conditions
Scotopic.
Photochromatic interval
The change in sensitivity to light of a specific wavelength as one adapts to light or darkness.
can be seen on a luminous efficiency graph
Spatial summation
The eyes ability to sum, or add, quanta over a given area. The consequences of summation is that we get an increase in light sensitivity as stimulus size increases (up to a limit).
Spatial summation is based upon the ____
Neural convergence of signals generated by the retinal photoreceptor, bipolar, and ganglion cell.
Critical angle and diameter of complete spatial summation
The critical area of complete spatial summation is the retinal area over which all light energy is summed to contribute to perception.
Limit of critical angle is dependent on retinal location.
Riccos law
Applies only to small images on the fovea at near threshold luminance. Tells us that the same total amount of light energy will have the same effect, no matter how it is distributed as long as its within the critical area of complete spatial summation.
Ex: two stimuli, one large and one small in diameter, will provide the same total energy. They just distribute it differently. As long as each is within the critical area, they will produce the same response change in the neuron.
Riccos law formula
LA= C
L- Stimulus luminance
A- Stimulus area
C- constant
Larger areas of spatial summation produces (2)
- Greater light sensitivity and brightness due to increased space over which photons can be captured.
- Poorer spatial resolution (VAs). If you use a bigger net, you can’t be asa discriminating about what you catch.
___ convergence determines the critical area for complete spatial summation.
Neural.
The critical area (diameter) is larger under scotopic than under photopic conditions and larger for peripheral than for foveal retinal stimuli.
Bc cones to bipolar cells are one to one whereas rods to bipolar cells are many to one. Convergence occurs.
When viewing a VF screening, what is the trend you should expect of the numbers in the center vs numbers in the periphery
Numbers in the center should be higher than numbers in the periphery. This is normal because we expect the periphery to not be as sensitive to light as the center.
Spatial antagonism
Under photopic conditions, if the stimuli is larger than the critical area. The energy falling just outside of the critical area actively decreases its perceived brightness.
Under scotopic conditions, what happens if energy falls outside of the critical area?
The energy falling outside of the area does not contribute to its brightness.
Given two otherwise identical stimuli with the same total energy, one smaller and the other one larger than the critical area , the smaller one would be brighter bc more photons are in the area that can be summed for the smaller target.
Temporal summation
The eyes ability to sum the effects of individual quanta of light over time.
Temporal summation operates across time like spatial summation operates across space.
For stimulus durations less than the critical duration, a decrease in the luminance requires that
the stimulus be shown for a longer time in order to be detected.
For stimulus durations longer than the critical duration, the threshold for detection depends only on
The luminance of the stimulus
Bloch’s Law
Stimulus luminance x stimulus duration= constant
The constant must reach a certain level in order to be seen.
Larger C value? That light will look brighter.
Same C value but different sizes? The same total amount of light will have the same effect, no matter how it is distributed across time as long as all is received within the critical duration for complete temporal summation.
Temporal summation is longer at low or high light levels?
Low. Scotopic vision, rods
Decreasing stimulus size results in a longer or shorter temporal summation?
Longer. Result is poorer temporal discrimination. Poor ability to discriminate flicker.
The goldmann-Weekers Dark Adaptometer Testing Procedure
- Light adapt for 5 minutes. Head in bowl. Light on.
- Turn off the background light in bowl, leaving the patient in total darkness.
- Over the next 40-45 mins, repeatedly measure the patient’s light detection threshold using the ascending method of limits. Use this in order to record the absolute lowest amount of illumination every time.
- Plot the measured threshold. Y axis- log of threshold, X axis- time in darkness in mins,
What should the graph look like for the goldmann-weekers dark adaptometer testing
Y axis- log of threshold. X axis- Time in darkness in minutes.
On the left side of the graph, the cones will capture light quicker. They were hardly bleached during the light adaption step, so they recovered quickly and were able to capture light. In the middle and right side of the graph, the rods will start to recover slower. They were heavily bleached during the light adaption step, so it took them longer before they were able to capture light again.
Results in a rod/cone break.
Photochromatic interval
The difference in sensitivity between the rod and cone systems for a given visual stimulus.
For then Goldman seekers dark adaption test, what happens when you dim the pre-adaption light?
Fewer rods and cones will be bleached. Therefore, both systems will recover faster when in darkness.
Results in a graph shifting down and to the left. May even eventually lose the rod/cone break.
