Lecture 7: Retina to primary visual cortex Flashcards
Define visual acuity
Smallest spatial detail that can be resolved
Seems to depends on frequency and contrast
Higher in central vision than peripheral
Level of detail we see
Describe visual acuity graphs
Acuity decreases for higher frequency bc lose lateral inhibition = works for mostly small areas
Somethings we cannot see
Upper limit tot amount of detail = due to amount of photoreceptors in retina
Describe how optometrist characterize visual acuity
Distance
Characterize visual acuity = 20/20 vision
Your distance/normal vision distance
Patient typically placed at 20 feet from letters
20/20 vision
When placed at 20 feet from letters
The smallest letter you can read is the smallest letter avg person can read at 20 feet
20/200= placed at 20 feet,200 = largest distance avg ppl can read
20/15 vision
When placed at 20 feet from letters, smallest letter you can read is teh smallest letter that avg person can read at 15 feet
Higher visual acuity than avg
20/40 vision
When placed at 20 feet from letters
Smallest letter you can read is smallest letter avg person can read at 40 feet
How do vision scientists measure visual acuity
Smallest angle of a cycle of grating
Describe visual angle
Vision scientists measure size of visual stimuli by how large image appears on retina (not by how large object is)
Visual angle = function of actual size of object and distance from observer
Corresponds to size of object on retina
Measure is independent of distance
Describe cycle
Consider a grating of alternating black and white stripes
Cycle = one black and one white strip
Basically a pattern
What is visual acuity for most people
Smallest cycle we can see- measure in terms of visual angle
Visual acuity corresponds to one minute of arc = 1/60 of one degree of visual angle = hard limit
Better vision =
Smaller visual angle required to identify a grating cycle
What is visual angle independent of
Actual size of object in world
Large object in distance may take up same degree of visual angle as a small object close by
= appear same size in visual angle
What is visual angle
Unit used to measure size of object in visual field
Regardless of its absolute distance or size
Direct 1:1 correspondence btweeen size of object in visual field and on retina = can say visual angle = unit used to measure size of object on your retina
What is visual acuity determined by
Spacing of photoreceptors in retina
How many cones do we need to distinguish objects
Cones in fovea = have avg center to center separation of 0.5 arc minute or 1/120 degree
Need 3 cones to see cycle of grating
Distance between leftmost and rightmost cones = 1 arc minute, 1/60 degree
cone —0.5—cone —0.5—cone
Why would we see gray
If frequency of grating smaller than this - 1 arc minute =
See gray
Because each cone receives a mixture of black and white
Frequency smaller than degree of separation bewteen photoreceptors = cannot resolve details
Describe visual acuity in low contrast environments
Visual acuity strongly dependent on spatial frequency = optimal at 7 cycles/degree
Number of cycles with one degree of visual angle
What is frequency of a certain pattern defined by
Number of cycles within one degree of visual angel
Describe sine waves
Represent simplest pattern that can repeat
Square waves = can be decomposed into sine waves (more gradually up and down)
Combine sine waves
High amplitude and low freq and add on top = higher frequency and lower amplitude sine wave = repeat and eventually can get square wave
What do low frequencies allow us to see
General picture
Visual system must combine = low and high frequency sine waves to give us whole image
What do high frequencies allow us to see
Perception of details
Visual system must combine = low and high frequency sine waves to give us whole image
Describe visual acuity -car and kid ex
Only see small portion of visual field wirh a high detail
Car and kid take up Same visual angle
But tree = takes up larger depress of visual angle
Describe visual acuity - Hermann grid
Foveal vision = smaller receptive fields
What causes increased visual acuity in Centrall vision
Retinal ganglion cells in fovea = smaller bc have low convergence
And higher acuity