Duplex Retina 2 Flashcards
A patient whose retina contains only rods views the following four patches of monochromatic light: 440, 510, 555, and 565nm. Each patch emits 20 quantal absorptions. Which is brightest
510nm
Your patient, a rod monochromat, has asked for advice regarding tinted lenses. You should advise a darkly tinted lens of which color?
Red
PI=0 at 650nm. Want the smallest PI
A patient has degenerative disease that has destroyed her cones. The expected VA for this patient is closest to
20/200
Under daylight conditions, a red and green surface looks equally bright. Compared to the green surface, the red surface most likely emits
More energy
Which of the following classes of cones contributes least to the photopic spectral sensitivity curve
S cones
Assume that the absorption of 10 quanta of 510nm by an eye that contains only rods results in vision. If the wavelength were 450nm, how many quanta would need to be absorbed to produce vision
> 10 quanta
Under scotopic conditions, a blue and yellow flower appear equally bright. Under photopic conditions, which flower will most likely appear brightest
Yellow flower
When foveal fixated by a person with trichromatic vision, a small target (o.5 degrees) of which of the following wavelengths will be least visible
430nm
No blue cones from 0.3-0.4 degrees, no rods
Photopic and scotopic thresholds are most similar to each other for which wavelength
650nm
Visual systems ability to adapt to the changes in the illumination levels
Light adaptation
Light adaptation may be studied with an ________procedure
Increment threshold
Stimulus only seen when brighter than background
How is threshold determined for light adaptation
Determined for a flash of light-an increment-that is presented on a background of a given intensity
After the threshold has been determined for the light adaptation
The background intensity is increased and the threshold measurement is repeated
Weber fraction for scotopic vision
0.14
M=0 on the scotopic light adaptation curve
Dark light, neural noise internal to retina
M=1/2 on the scotopic light adaptation curve
Square root law: this region is limited only quantal fluctuation of the background, and has a slope of 0.5
M=1 on the scotopic light adaptation curve
Weber law: the slope of the curve is equal to about 1. The ratio of increment intensity to background intentist remains constant, and covers ~4 log units range
M=infinity on the scoptopic light adaptation curve
Rod saturation: the slope increases rapidly becomes infinity. The rod system starts to beceom unable to detect the stimulus with the high background luminance
In terms of contrast, which is more sensitive, scotopic system or photopic system
Photopic system
Weber fraction for scotopic vision
0.14
Weber fraction for photopic vision
0.015
The smaller then Weber fraction
The more sensitive it is
Although the photopic systme is more sensitive (10x) to CONTRAST than the scoptopic system…
It’s absolute sensitivity is less
Ability to resolve: photopic vision
Superior
1 log unit difference
20/20
Ability to resolve: scotopic vision
Inferior 20/200
Contrast sensitivity: photopic
10X better
Weber=0.015
Contrast sensitivity: scotopic
Poor
Weber=0.14
Ability to detect: photopic
Inferior
Ability to detect: scoptopic
Superior
3 log units photochromatic interval
Why the difference between visual resolution an visual sensitivity for photopic and scotopic vision
Scotopic is absolute more sensitive.
Photopic is more sensitive for contrast (threshold). Resolution
Difference between visual resolution and visual sensitivity
Due to the manner in which the rods and cones are connected to the post-receptoral elements of the retina
Rods are connected in such a manner as to _______ information over space. This produces great sensitivity, but poor resolution
Sum up
Cones manifest connections that _____________ at the expense of sensitivity
Maximize visual resolution
How many ganglion per rod
10-15
How many ganglion per cone
1:5, or 1:1 in the fovea
Many more rods communicate with a ganglion cell than is the case for cones. This illustrates that the scotopic system ______ information _______ to a greater extent than the photopic system, it manifests greater ___________
Sums up
Over space
Spatial summation
When two spots of light are shown on rods, it does not signal the presence of two lights, this information is lost because of
Spatial summation
This system has excellent sensitivity, yet poor spatial resolution
Scotopic system
Under ______ conditions, the two ganglion cells each reach threshold and signal the presence of a stimulus; sonsequntly, two stimuli are seen
Photopic
Excellent spatial summation
Scotopic system
Excellent spatial resolution
Photopic system
Scotopic and photopic vision also demonstrate significant difference in their
Temporal (time-related) properties
Scotopic conditions:two lights below threshold, separates by an interval greater than 100ms
No stimulus seen
Scotopic vision: 2 lights shown under threshold, but within 100ms
They will be summed up to reach threshold, stimulus seen, but only one flash of light
Scotopic vision: two lights shown that go beyond threshold and within the interval period
Still just one flash seen. The scotopic systems high degree of temporal summation limits its availability to resolve distinct temporal events
Scotopic vision: when two suprathreshold pulses are separated by greater than the interval period (greater than 100mn)
Two flashes seen
Which has a shorter period of temporal summation
Photopic, 10ms
Photopic: when a light shown at a longer interval than 10s and neither reach threshold
No flash seen
Photopic: two lights shown within the 10s, but do not reach threshold
One flash seen
Photopic: When two lights are shown that both reach above the threshold, but in two different time intervals
See two flashes
Threshold contrast, under photopic conditions, is closest to
0.010
To optimize the rod-cone break of the dark adaptation curve for clinical testing, the stimulus should be
5.0 degrees in diameter and green
Consider the region of the photopic light adaptation curve that follows Weber law. As the background intensity decreases, contrast
Remains constant
Ratio must remains close to one
The percentage of rhodopsin that is bleached at rod saturation is closest to
15%
The photochromatic interval is expected to be smalles for a
Centrally fixed 0.5 degree yellow target
An abnormal dark adaptation curve may be indicative of
ARMD and RP
