What's the least we can see?
18 important questions on What's the least we can see?
What are the 2 ways to conceptualize light?
How does the visual system cope with the vast range of intensities we encounter? (5 points)
2. Rods and cones
3. Spatial summation
4. Temporal summation
5. Dark and light adaption
How do rods and cones help the visual system cope with the vast range of intensities we encounter?
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What is meant by spatial summation?
Ricco's law:
- greater sensitivity but poor spatial resolution
- trade off between the fovea and the peripheral retina
What is meant by temporal summation?
Bloch's law:
- greater sensitivity but poor temporal resolution
- depends on the state of adaptation
What is meant by dark and light adaptation?
- increase in temporal integrating time leads to poorer resolution
- in darkness, flickering lights less likely to be detected
---> rods become more sensitive to light when we are in the dark
---> at 25 minutes in the dark, rods are at their maximum sensitivity of light (this is when our eyes have fully adjusted to the dark)
---> within 5 minutes of being in the darkness, cones are no use
How does dark and light adaptation happen?
---> the role of light is to isomerise retinal, causing it to change shape. This induces the electrical signal
What can be said about rod vs. cone sensitivity?
---> many rods converge onto a single ganglion cell. This allows for efficient light gathering, but poor acuity
---> cones show much less convergence. This allows for finer acuity, but lowered light sensitivity
What is the minimal amount of light energy necessary to elicit a visual sensation?
What factors affect thresholds for seeing a dim flash in the dark adapted eye i.e. the absolute threshold for vision?
1. Retinal eccentricity
2. Size of test patch/(flash)
3. Flash duration
4. Wavelength of flash
What is the absolute threshold of retinal eccentricity?
i.e. the flash needs to be located 20 degrees to the side of the fixation point
- most densely packed rods with spatial summation properties here
What is the absolute threshold of the size of the test patch?
- this summates all the protons that hit it
Ricco's Law: what matters is the number of quanta and not the area up to a critical point of 10 arc min
What is the absolute threshold of the flash duration?
What is the absolute threshold of the wavelength of the flash?
(remember the eye dark adapted - meaning only rods are active)
In an area of 10 arc min, how many rods are there?
(less than 10 photons hit this whole area, so this makes it very unlikely that 1 rod will absorb more than 1 photon = A single quantum of light (photon) is sufficient to activate a rod receptor under normal conditions
What lighting conditions do rods and cones operate best under?
Cones - bright (photopic) illumination
---> having an area at the centre of the fovea with no rods means that, under dim illuminations, the central 1 degree or so around the fovea is effectively blind
What is age-related macular degeneration (AMD)?
---> the leading cause of visual loss among the elderly in the US
---> AMD gradually destroys sharp central vision
(p. 42 perceptual textbook)
What is retinitis pigmentosa (RP)
---> in most cases, the rods are affected more than the cones i.e. people with the disease first notice vision problems in their peripheral vision and under low light conditions
(p. 50 - 51 perceptual textbook)
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