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
What is convergence
How many photoreceptors connected to retinal ganglion cells
When high convergence
In periphery
Describe high convergence
Multiple rods synapse on diffuse bipoalr cells
Multiple bio polar cells synapse on parasol ganglion cells
Highe number of photoreceptors connected to ganglion cells
Large receptive field
When low convergence
Fovea
Describe low convergence
Single cone cells synapse on single midget bipolar cells —> synapse on midget ganglion cells
Small receptive field
More one to One organization
Higher acuity = see small things, associated with higher frequency
Parasol ganglion cells aka
Magnocellular ganglion cells
Midget ganglion cells aka
Parvocellular ganglion cells
Describe retinal ganglion cells - in periphery vs fovea
Rods = periphery, many photoreceptors - each —> more than one bipoalr cell —> one ganglion cell
Cones = fovea = one to one relationship of photoreceptors to bipoalr cells to ganglion cells
Describe retinal ganglion cell in periphery
More sensitive to light because will respond if 1 or 2, 3 or 4 bc each 2 synapse on same bipolar cell and will activate retinal ganglion cell
Reacts if light anywhere in receptive field, does not matter where it comes from
Describe retinal ganglion cell in fovea
If 1 activated = activates one retinal ganglion cell only
Cannot catch presence of light - need it in specific parts
Describe when light hits all photoreceptors - in periphery and fovea
Periphery = 1234 activated = ++++ on A RETINAL ganglion cell = responds more strong = high degree convergence = greater light sensitivy
Fovea = 1,2,3,4 activated = one plus on A B C D = ganglion cell will not repsond more if more ligjt = low convergence
Where is there less acuity and why - explain
Less acuity in periphery = cannot distance between 1 and 3 vs 2 and 4 = retinal ganglion still activated the same = general layout, more sensitive to movement
But in fovea = can distinguish which one = bc 1 = A activated and 3=C ACTIAVTED = low convergence but high acuity = more precise and clear
What forms optic nerve
Retinal ganglion cells = bundle together
Where is optic chiasm
Located in front of hypothalamus
What happens at optic chiasm
Site where half of the optic nerve fibres from each eye decussate to contra lateral brain hemisphere = left visual field processed by right =
Visual vortices process info from contra lateral visual field, not contra lateral eye
Describe temporal retina
Oriented towards contralateral visual field = NO decussation (no crossing to contralateral side)
Describe nasal retina
Oriented towards ipsilateral visual field
Decussates to transmit visual info to contralateral hemisphere
= cross to other side
Describe optic tract
Retinal projections beyond optic chiasm and bewteen thalamus= called optic tract
After optic chiasm
Where do most optic tract nerve fibres synapse
90% of optic tract nerve fibres synapse in lateral geniculate nucleus of thalamus- first relay
What is hemianopia
Left side - hemisphere lost for both eyes
= right optic tract lesion
What is left eye anopia
Nothing from left eye
Due to lesion of left optic nerve
What is tunnel vision
Lose left of left eye and right of rigth eye
= lesion at optic chiasm - issue with crossing over
What is lateral geniculate nucleus
Located in thalamus
Divided into 6 layers = retinotopic organization = each point on retina corresponds to point in lgn= preserves relative positions of things
Name layers of lgn
Type of visual info they receive = diff
Magnocellualr layers
Parvocellular layers
Konioclleualr layers
Describe magnocellular layers of lgn
1 and 2
Input from parasol ganglion cells
Process peripheral vision
More important for layout of scene and perceiving movement
Describe parvocellular layers of lgn
3-6
Input from midget retinal ganglion cells
Procesess central vision
Describe koniocellular layers of lgn
Bewteen layers =
Located in intralaminar regions fo lgn
Specialized for colour perception due to sensitivity to wavelength and low light sensitivity
Describe lgn axons
Form optic radiations that project to primary visual cortex in occipital lobe
(Axon projections - connection = called optic radiations)
Right visual cortex proceses stimuli from left visual field - both eyes and vice versa
Describe superior colliculus
10% of retinal outputs bypass lgn and project to superior Colliculus
Retinotopic layout of contralateral visual field = explains blind spot phenomenon
Describe blind spot phenomenon- what is it
Seen in patients wirh damage to primary visual cortex
Describe blind spot phenomenon- unconscious visual perception
Patients respond to visual stimuli without conscious awareness
Can accurately guess line orientation, object movement and spatial location
Know where, but say cannot see it
Reflexive behaviour almost = see that something is there but do not know it
Describe blind spot phenomenon- why does it happen
Superior colliculus projections bypassing primary visual cortex
Reinforces role of primary visual cortex in conscious perception
Need to have sensory cortex to consciously perceive things = need cortical processing
