11 Light Sensitivity & Spatial Resolution Flashcards
Why do we have two photoreceptors?
Our species lives on a planet that has day and night
Slide 1
If more than one photons was needed to activate one receptor they would have to be ________
synchronous
Slide 2
For a rod that needed two photons to respond, light intensity has to increase
102,000
Slide 2
Chances of two photons captured by the same rod at the same time ( within 100 ms) ?
once every 16 years
Slide 2
Dim light is _____
NOISY. We can very reliably distinguish objects from one from the other.
Slide 3
Objects (at day and night) are seen against a _______ background
Noisy
Slide 4
What stops noisy background noise?
All the physiology we have talked about (horizontal cells, etc)
Slide 4
Rods are _____ but they are _______
noisy, reliable
Slide 5
Why are rods so noisy?
They fire spontaneously, even when they do not have a photon to activate them
Slide 5
Rods fire spontaneously every _____ minute(s).
2
Slide 5
what does “eigengrau” mean?
Spontaneous activity (in German)
Slide 5
In the picture in Slide 5 there are 3 colors. What does each mean?
Black =
Red =
Green =
Photon catch on rod of:
black = star red = sky green = eigengrau, rod pretends to catch photon
Slide 5
***Picture in slide 6, important concept.
Rods will _________
saturate
Slide 6
Rods will capture to 1, 2, 3 photons and _____ their amplitude of response.
At some point rods will _______ and stop increasing the amplitude of response.
increase, saturate
The rod cannot keep increasing their respond because what if 100 photons hit it? No….
Slide 6
If rods saturate, what does this mean for day light?
They are at their max response rate
Dynamic range of rods is ____ log units?
6
Summing responses of photons in rods is ___ log units?
2
A single rod can pick up a single photon within ___ seconds
30
In what conditions do rods do their best?
Dim light
Rods help detect slight gradations in what color?
Black/Grey
True/False: Rod bipolar cells receive the response from a single photon from a single rod (or from upwards of 200 photons from 10-15 different rods) and scales it before relaying the signal
True
Cones can respond to ________ photons/s catch rate
1,000000
Cones respond in ____ms and sum responses above rate of ____photons/s
50, 20
Cones have a dynamic range of ______
50,000
True/False: Rods are less sensitive to a single photon than cones
False
How reliable is a rod’s response as compared to a cone’s response?
With rods, a single rod can receive a signal from a single cone. A cone may respond to a single photon about 5% of the time.
True/False: There is no overlap when cones pick up where rods begin to fail
False
What conditions are photopic, mesopic, and scotopic?
Daylight (no rods), intermediate (between thresholds), and very dim light (only rods)
Are rods or cones more reliable?
Rods (saturate more quickly)
What will saturate both rods and cones?
A flash of light
What will allow cones to operate at any light level?
Gradual increase in light
Why is photoisomerization rate is limited to 1,000,000/s?
Regeneration of visual pigment molecules
True/False: Like humans, all vertebrate cones do not saturate with steady light.
True
True/False: All vertebrate rods detect single photons and can saturate
False
Slide 14
MAR
minimum angle of resolution (1 degree = 60 minutes)
Slide 14
Human acuity threshold
about 30 seconds (0.5 minutes of arc); note: 360 deg, 60 min per deg, etc.
Slide 14
0.5 minimum angle corresponds
2.5 micrometers on the retina
Slide 14
Max VA of most humans
20/15 or 20/10 because of the size of receptive fields
Slide 14
Visual angles
Smaller when you are farther away from target; Bigger when you are closer to target
Slide 15
Contrast Sensitivity Stimuli
Contrast sensitivity and spacial frequency are closely related
Slide 16
Spacial frequency – Humans VS Cats
Humans are good at seeing spacial frequency grading in 2-6 cycle per degree range. Our VA is better than that of a cat
Slide 16
Spacial frequency 2
Cat Spacial frequency max out at 5-10 c/degree; humans max out at 60 cycles/degree
Slide 16
VA of a cat
Cats are myopic compared to humans; they can’t see far
Slide 16
logMAR
A logMAR of 1 equals 0 (equiv. of 20/20 Snellen); logMAR of 0 equals a NEGATIVE number (i.e 20/10)
Slide 17
Vernier Acuity
Very different from MAR. Requires good binocular vision; tests to see how well the 2 eyes are coordinating to line things up and organize perception in space of information coming in both eyes
Slide 17
Ambloypia
Can be improved with practice and training
Slide 19
Low Vision Chart
Letters are getting bigger as you deviate from the center. The chart is normalizing VA based on visual angle displacement; VA is much worse in the peripheral field
Slide 20
What is the target/goal behind viewing desaturated color dots with one’s peripheral vision?
It get to be a very difficult task for the person to do so and very inaccurate because of the desatutarion of the object color and because of the lack of focus
Slide 21
Can we detect a deficiency of cone density on the retina?
yes, through a model presented on this slide, because of the patient’s lack of detecting desaturated colors.
Slide 21.
what does a simple behavioral test do?
detect cone damage.
Slide 22
How are acuities measured as?
angular resolution.
Slide 22
What is angular resolution?
it’s the size of the gap (of the C).
Slide 22
What is a movement of a target detected by?
motion of the target within a certain range.
slide 22.
motion detection is detected by what?
parasol cells
Slide 23.
does spatial resolution decreases with increased visual eccentricity?
yes.
Slide 23.
what decreases minimum angle of resolution?
visual eccentricity
Slide 24.
does color resolution decreases with visual eccentricity?
Yes
Slide 25.
what does receptive fields increase with?
visual eccentricity
Slide 25.
what drives visual acuity?
the size of receptive fields
