Visual system Dr. Clery Flashcards
what 2 thalamic nuclei are involved in vision?
pulvinar, lateral geniculate nucleus
what do the cornea and the lens do? which one can accomodate?
refract light; lens can accomodate (move)
what controls the lens shape?
ciliary muscles
what is myopia?
nearsighted = far away images are blurry (eyeball is too long)
what is hypermetropia?
farsighted = close objects are blurry (eyeball too short / hyperopic)
what is astigmatism?
the lens or cornea are not spherical
what is presbyiopoa?
lens gets stiff and is unable to accomodate for near vision
what is cataract?
change in the lens color
LAYERS OF THE RETINA
- pigment epithelium
- photoreceptor outer segments
- outer nuclear layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer
- nerve fiber layer
are there more rods or cones photoreceptors?
20x more rods than cones
what is the pathway of light
light -> photoreceptors -> electrical signal -> bipolar cells -> ganglion cells -> brain
what causes blurry vision?
different points from a same object on the retina
what are rods vs cones sensitive to?
rods are highly sensitive to light
cones are sensitive to color and shapes
rods and cones: which on is for night vision?
rods because they are most sensitive to light
how are photoreceptors in the dark?
depolarized
how do photoreceptors act in the dark?
produce a constant flow of neurotransmitter release onto bipolar cells
what value is the resting potential
-40mv
how are photoreceptors cGMP levels at rest?
high
what happens when light shines on the photoreceptor
Na+ channels close, K+ channels stay open -> photoreceptor becomes hyperpolarized
name the order the the cells that light hits first
ganglion cell, amacrine cell, bipolar cell, horizontal cell, photoreceptors
rods or cones: which is used for peripheral vision vs foveal vision?
peripheral vision = rods
foveal vision = cones
rods or cones: which function in scotopic vs photopic vision?
scotopic = rods
photopic = cones
can rods see color?
no
are rods or cones saturated in daylight?
rods because they are highly sensitive to light
do rods or cones have a high temporal and spatial resolution?
cones
do rods or cones peak at the fovea?
cones
what wavelengths of light can we see?
400 to 800
what are rods vs cones threshold in photons?
rods = 1 photons
cones = 100 photons
what current is dark current?
about -40mV
what is the problem of limited dynamic range?
we can “adjust” the lumination to see darker or lighter details, but can hardly see both at the same time
light adaptation is a solution to what problem?
the fact that we can’t have 10 photoreceptors for each light intensity/color
what causes light adaptation?
chemical changes in photoreceptors
what happens to cGMP when luminance increases? why?
gradual increase in cGMP to restore the membrane potential
what are the 2 main consequences of the way our vision adapt to light?
- cells are unresponsive to uniform light
- brightness measurement are relative (its a comparison)
what cell is the output of the retina?
retinal ganglion cells
what cells do ganglion cells received input from?
bipolar cells
what is ganglion cell’s output?
brain (optic nerve)
how do ganglion cells communicate?
via APs
what type of RF can ganglion cells have?
achromatic or colour-opponent
what are the colors of colour-opponent RFs?
green/red and yellow/blue(purple)
explain and on-center off-surround ganglion cell?
center responds to a small spot of light if it is BRIGHTER than the background
surround responds to a small spot of light if it is DARKER than the background
what happens if light shines on the entire on-center off-surround RF of a ganglion cell?
no change in AP because the signals cancel eachother out
what size are the RFs in central fovea?
small
what is Hermann Grid Illusion?
illusion of grey at intersection of black squares because the on-center ganglion cell responses are a bit weaker at the intersections due to the off-surround
what is different between color opponent RFs and achromatic RFs in ganglion cells?
in achromatic RFs a bright surround will shut down signal in an on center off-surround.
in color opponent, a red surround won’t affect a red-on center, green-off surround RF.
what happens if you shine green light in the center of a red-on center, green-off surround?
nothing; APs fire normally
what happens if you shine red light on the entire RF of a red-on center, green-off surround?
increase AP
what % of males are color blind? why?
10%. cus the mutation is on the x chromosome
color blindness results from problem with what photoreceptors?
long or medium wavelength photoreceptors
The most common form of color blindness (deuteranomaly) involves a mutation that shifts ____-________ _____ towards the ___ end of the spectrum
medium- wavelength cones;
red
what is the colour-opponent theory?
Perception of colour is linked to neurons that measure the difference between activity in different cone types.
why do we still see a green on red cross after staring at a red on green cross?
because we fatigue the red cones in one part of the retina and green cone sin the other part (REDUCED INHIBITORY INFLUENCE)
What are some evidences that colour perception is dependent on measuring the difference between cones of different wavelenght (colour-opponent theory)?
- We never perceive colours that appear reddish-green or bluish-yellow.
- Adapting one colour leads to illusory perception of the opponent colour.
- Retinal ganglion cells have opponent responses to different wavelengths.
what kind of ganglion cells are achromatic?
magnocellular cells
what kind of ganglion cells are colour-opponent?
parvocellular cells
where on the retina are magnocellular ganglion cells located?
outside the fovea
how is the resolution of magnocellular vs parvocellular ganglion cells?
- magnocellular have low spatial resolution but high temporal resolution
- parvocellular have high spatial resolution but low temporal resolution
what kind of visual information are magno vs parvocellular ganglion cells concerned with?
- magnocellular are concerned with change over time (motion) (remember they are achromatic)
- parvocellular are concerned with fine spatial detail (form)
what size are magno vs parvocellular ganglion cells receptive field?
- magnocellular have large receptive fields
- parvocellular have small receptive fields
name the visual pathway
retina -> optic nerve -> optic chiasm -> optic tract -> LGN -> optic radiation
what happens if you lesion the right optic tract?
you loose left vision in both eyes
what happens if you lesion the middle of the optic chiasm?
loose the external/lateral vision in both eyes
what happens if you lesion the right optic radiation close to LGN?
loose the top left quadrant of both eyes
the left side of our visual field is in what brain hemisphere?
right (opposite hemisphere)
a partir de quelle partie du visual pathway will a lesion cause a homonymous deficit (field loss in the same halves of the visual field for both eyes)
optic tract lesion and after (optic radiations)
how are LGN RFs?
same as ganglion cells:
- magnocellular are color-blind
- parvocellular are color selective
in what layers are magno and parvocellular LGN cells located?
1,2 = magno
3, 4, 5, 6 = parvo
name each LGN layer and if they carry info from ipsi/contralateral eye?
1 contra
2 ipsi
3 ipsi
4 contra
5 ipsi
6 contra
remember, what do magno vs parvocellular “see”?
magno = motion
parvo = shape details, color
what is contrast sensitivity?
A = describing observer’s ability to see dim gratings in %
low % of contrast sensitivity means what?
that you can see grating even if the colors are very similar
what is spatial frequency sensitivity?
w = way of describing an observer’s ability to see gratings of different spatial frequencies
what is a high spatial frequency?
when you see bars even when they are super close to eachother
formula for sinewave grating?
stimulus = A * sin(wx)
where A controls the contrast
w controls the spatial frequency
what is temporal frequency sensitivity?
a way of describing an observer’s ability to see gratings that flicker at different rates
do dogs have high or low temporal frequency sensitivity?
low (can’t see fluidely)
how is the velocity of a grating calculated?
temporal frequency / spatial frequency
how will increase the spatial frequency or the temporal frequency affect velocity?
Increasing the temporal frequency increases the velocity. Increasing the spatial frequency decreases the velocity.
what type of LGN cells are responsible for seeing the contrast at high speed?
magnocellular
what happens when you lesion layer 1 and 2 (magno) of the LGN with a chemical? the perception of what kind of stimuli is impaired?
- monkey’s contrast sensitivity decreases as the velocity of the stimulus increases
- no effect on immobile object
- ->impairs perception of FAST-MOVING STIMULI
what happens to monkeys contrast sensitivity when you lesion layer 3 TO 6 (parvo) of the LGN?
- contrast sensitivity decreases as spatial frequency increases (reduced sensitivity to gratings)
- color perception is completely eliminated (no chromatic contrast sensitivity)
- no effect when changing temporal frequency (only decreases sensitivity for slow motion)
why is spatial frequency harder to see?
because the bars get closer and closer to each other
what is acuity? what LGN lesions affect it?
ability to perceive gratings at high spatial frequencies.
affected by parvo lesions
what % of retinal output goes to the superior colliculus?
10%
what is the superior colliculus?
medial brain region that orients you to things in the environment
from what type of ganglion cells does the superior colliculus get input from? why?
mostly magnocellular cells:
- superior colliculus have big RFs, can’t see details
what is homonymous hemianopia?
can’t see half the visual field from each eye
name characteristics of blindsight
- does not reach awareness
- gets activated by large stimuli
- Most effective for low spatial frequencies and high temporal frequencies
- Little sensitivity to colour
how does vision restoration therapy work?
stimulus of moving white dots can help recover part of the vision; also with auditory feedback
what type of neurons must be responsible for blindsight? why?
magnocellular neurons because they are color blind, encode motion, and respond to big things
what were the results of vision restoration therapy?
contrast sensitivity was increased! (lower CONTRAST THRESHOLD)
how many cells are in retina vs V1?
1 million cells in retina
200 millions in V1
what does it mean that the projection from the LGN to V1 is retinotopic?
neurons that are physically near each other respond to similar parts of visual space
the mapping of space in V1 is _________ of eye of origin
independent
what measure on the retina described the size of an object?
the angle it covers on the retina
what is the cortical magnification factor?
Mc = the amount of space in the cortex(mm) occupied by an object of a given size (angle)
what is in this formula?
Mc = A / (E + k)
Mc = magnification factor
E = position on the retina relative to the fovea
A, k = constants
bigger E means what for the magnification factor? (E = position on the retina relative to the fovea)
bigger E = smaller Mc
input from the LGN goes to what layer of the visual cortex?
layer 4C
what are ocular dominance columns?
compartments from layer 4C that divide info from right vs left eyes (only one eye is represented in each column)
what differentiates the layer 4C sublayers?
4Ca is all magno cells.
4Cb is all parvo cells
where does layer 4C project to?
to the other V1 layers
where does layer 2/3 of V1 project to?
extrastriate cortex
where does layer 5 of V1 project to?
subcortex (ex superior collliculus)
where does layer 6 of V1 project to?
feedback projections to the LGN
what is V1 (area 17 / striate cortex) responsible for?
conscious vision
do LGN neurons respond to different bar orientations?
no
do V1 neurons respond to different bar orientations?
yes
When probed with a small spot, a V1 cell behaves somewhat like an ___ cell
LGN (on center off surround)
V1 orientation selectivity is thought to emerge from what?
from the spatial arrangement of ON and OFF-center LGN inputs
how are V1 RFs compared to LGN?
- larger
- more sophisticatedly tuned
- organized into columns
what aspect of vision is represented by V1 neurons?
every aspect including shape, motion, colour, depth
what do ‘simple cells’ in the visual cortex respond to?
light and dark stimuli
what can you predict by mapping subregions of visual cortex simple cells?
can predict the response to a more complicated stimulus than dark/light
what would you “see” in the V1?
weird distorted upside down picture of our visual field
the fact that LGN only have circular RF keeps them from seeing what?
LGN neurons can’t “see” edges / bars
in V1, there is an alternation of __ _____ __ ______ stripes of LGN cells
on-center and off-center
what is different from complex cells vs simple cells?
- complex cells have no discernable subregions and respond to an orientation no matter where it is
- their orientation CAN NOT be predicted from its response to a small spot of light
how do complex cells respond to light and dark stimuli in their receptive field?
respond equally to light and dark stimuli
simple vs complex cells convey information about what?
- Simple cells convey information about orientation and local
contrast - Complex cells convey information about orientation, but
NOT local contrast
complex cell’s RF are built from what?
from simple cell RFs
how are the simple cells that construct the receptive field of complex cell?
they have the same orientation but opposite arrangements of ON and OFF regions
from what we know about cortical layers, in what layers would simple vs complex cells be?
simple cells in layer 4 (get LGN input), complex cells in other layers
what were they able to show via anatomical labelling after injecting die in only 1 eye?
ocular dominance column in 4C (separation of ipsilateral and contralateral input)
one ocular dominance column has many what?
many orientation column
whats a hypercolumn?
a column in which all the orientations are represented from both eyes (right and left ocular dominance columns together)
the orientation selectivity are dependent on what?
depend on the development: what stimulus you are exposed to as you grow
how did they show that orientation selectivity depends on development?
exposed cats to different orientations: the cat has more cortical area dedicated to the orientation they were most exposed to
what is amblyopia?
imbalance between left and right eye
what did the post-mortem picture of a human brain from a patient who had lost an eye show? with cytochrome oxidase staining
he still has ocular dominance column even after not using one eye for years
why is it important to act early on amblyopia?
formation of ocular dominance columns happens early and stays for the rest of your life
what V1 cell don’t respond to orientation?
blobs
in what V1 layers are blobs found?
upper layers (2/3)
what do blobs respond to?
colour
what is different between color opponent LGN vs V1 (blobs) cells?
blobs are DOUBLE-OPPONENT cells.
Not just red-on-center, green-off-surround, but red-on-center green-off-center and red-off-surround green-on-surround
what is simultaneous color contrast?
the tendency of the visual system to perceive colors in a way that depends on the colors that surround
them
why do some people see the dress blue/black vs yellow/white?
because we don’t have a lot of information about the background in the picture - brain makes assumptions
what is colour perception related to?
differences between cone outputs along with more abstract assumptions about the environment
what are orientation columns?
columns in which all neurons are tuned to the same orientation
how do ocular dominance columns respond in layer 4C vs in other layers?
In layer 4C cells respond to input from only one eye (as in the LGN).
In other layers, cells
are binocular but still respond more to one
eye than to the other.
within a single hypercolumn, neurons have RFs that represent similar what?
similar locations in space
name 4 things for which the visual cortex can be selective?
Orientation
Motion direction
3D depth (binocular disparity)
Stimulus length
how can we have motion direction selectivity in the visual cortex?
via LGN inputs that are shifted in space and delayed in time and when they arrive synchronously to V1, allow APs to go above threshold only when stimuli goes in the preferred direction
what is the aperture problem?
Local measurements of edge motion are one-dimensional, so V1 neurons see the motion as perpendicular to the orientation of an edge and give misleading info on direction of movement
the aperture problem means that V1 neurons don’t have the ability to communicate information about what?
velocity
The size of an object is typically described by the size of the angle it covers on the retina, and therefore the visual system confuses size with what?
confuses size with depth
how does binocular disparity work?
disparity-selective neurons in V1 fire at specific horizontal disparity measurements
Selectivity for binocular disparity can be found in simple cells whose LGN
inputs have slightly different what?
different RF positions in the two eyes
Binocular disparity depends on the depth of the object relative to what?
relative to the plane of fixation
what is endstopping?
reduced response to long edges
how does endstopping work?
long bars go in the RFs of cells beside the RF of interest and the neighboring neurons inhibit the middle one via inhibitory horizontal connections
why could endstopping be super useful?
to detect curvature
why are curvatures so important to see?
is is easier to differentiate an object when you have just the curves vs just the straight lines
what cells are horizontally connected to eachother?
- cells from different hypercolumns in upper layers of V1 that have the same orientation or color specificity
- also Blobs are interconnected
what is contour integration?
Lines of similar orientation stand out against a background of random orientations
how does contour integration work? what kind of connections does it involve?
neighboring cells are excited by the same stimulus (opposite of endstopping)
- involved excitatory horizontal connections
the ventral pathway connects V1 to what cortex?
inferotemporal cortex
the ventral pathway is responsible for what part of vision?
shapes
lesions of the ventral pathway causes what?
normal vision, but loose the ability to see certain things ex to recognize faces
what cells are mostly involved in the ventral pathway?
parvocellular cells
what cells are mostly involved in the dorsal pathway?
magnocellular cells
what is the dorsal pathway mostly concerned with?
motion
what disease can come from ventral pathway lesion?
Prosopagnosia: inability to recognize faces
what disease can come from dorsal pathway lesion?
Akinetopsia: motion blindness; no smoothness in motion ex can’t pour coffee in mug
what are the 3 roles for extrastriate visual areas that we went over?
- more directly involved in guiding visual perception and behavior
- useful to measure quantities that can’t be measured with small RF
- interact with our cognitive state
remember: parvocellular vs magnocellular input go to what 4C sublayer in V1?
magnocellular goes to 4Calpha
parvo goes to 4Cbeta
where do blob cells project to in V2?
thin stripes in V2
where do the rest (non-blob) cells project to in V2?
pale stripes and thick stripes
what stripes project to the ventral pathway? to what area?
thin and pale stripes project to V4
what stripes project to the dorsal pathway? to what area?
thick stripes project to MT
what information is contained in each type of stripe?
Thin stripes: colour
Pale stripes: orientation
Thick stripes: orientation, direction
what visual pathway area can see illusory contours?
V1 neurons can’t see it,
V2 neurons can
give an example of why illusory contour is important?
used to see texture boundaries and estimate the depth of things
other than illusory contour how else does V2 contribute to perception of shape and depth?
processing of angles
how do V2 lesions affect orientation, color, motion direction, and texture discrimination
- no effect on orientation, color, motion direction discrimination
- impairment of texture discrimination composed of multiple orientations (seeing the bars in different orientation in a pattern)
basically what are the 2 things encoded in V2 that aren’t encoded in V1?
illusory contours and angles
what seems to be V3 role?
V3 appears to contain columns for processing retinal disparity (near/far objects)
what is MT?
middle temporal area (but it’s not in temporal lobe) (also called V5)
in what visual pathway is MT?
it’s the beginning of the dorsal pathway
where does MT get its input from?
- from the thick stripe in V2, that get their input from layer 4B of V1 that get input from 4Ca
- directly from V1
MT gets info for what type of stimuli? because it gets input from what layer?
motion direction because it gets input from V1 layer 4B
what are MT output involved in?
motion perception and eye movement control and self-motion
where do MT outputs go?
MST -> Parietal cortex
what is also sometimes called the dorsal pathway?
“where” pathway
MT neurons show specificity for what?
for motion direction
MT is organized in ?
columns for different direction preference
how did they test for the MT role in motion direction selectivity?
microstimulation of single MT direction columns while the monkey was doing a motion direction task.
It biased the monkey’s perception.
what were the results of the MT microstimulation experiment?
Microstimulating a column that preferred rightward motion biased the animal’s percept toward rightward motion
can monkeys already have innate biased for motion direction?
yes
what happens to ppl with MT lesions/MT damage?
can not see if the dots are going a certain way unless there is 100% coherence (loose perception of motion)
as you go higher in the hierarchy of the visual system, you get ________ ___
larger RFs
the first activity we observe in MT when a bar moves is based on what?
on the motion preference for the orientation
what cells can solve the aperture problem?
“later” MT cells that act after about 60ms and understand the correct direction of motion
how does MT solve the aperture problem?
MT has “smart” cells that can select which input is important to solve the problem
what input are most important for MT to solve the aperture problem?
the corners of the moving object are most important. MT can ignore the edges
how big are MT’s RFs compared to V1?
10x bigger
dorsal visual pathway neurons respond to moving stimuli, with little dependence on what?
shape, color, texture
what type of information do you loose in MST?
information about the precise location of things
where does the retinotopic map appear in the visual pathway
V1
what’s the difference between MT and MST tuning?
MT are tuned for translation, MST are tuned for more complex motion like optic flow
what is the definition of optic flow?
combinations of translation, rotation and expansion
the complex motion patterns in MST are bigger than what?
bigger than the MT receptive fields
MST is organized in?
in columns clustered based on selectivity for complex motion stimuli (rotations, expansions, contractions)
for what process must MST neurons be most useful for?
navigation (because they see optic flow)
individual MST neurons can be tuned for what?
tuned to the direction of heading in large optic flow
how did microstimulation of a leftward MST column affect monkeys?
in he case we saw it undid the animal’s rightward bias of heading direction
what is different about MST eye of visual field?
Unlike in V1, V2, or MT, MST receptive fields often
include parts of the ipsilateral visual field
what is the ventral pathway specialized in?
shapes
Where does V4 get the input from?
V1 upper layers and thin and pale stripes of V2
what are the structures of the ventral pathway?
LGN -> V1 -> V4 -> IT
does V4 have info about orientation?
yes but it is not its main job
how does attention influences your perception of the world?
it increases neuronal and perceptual sensitivity
how does attention affect the activity of neurons?
it multiplies their activity, increasing the amplitude of the tuning curve of the normalized response without changing the shape
in what visual pathway area does attention have a bigger impact on neuronal response?
stronger effect in V4 compared to V1
in what visual pathway is attention important? how does it change through the pathway
attentional enhancement is present in dorsal and ventral pathway and increase a you ascend in the hierarchy (MST/IT > V1)
what is the behavioural measure for tilted grating?
How far does the grating have to be rotated before the subject gets the right answer 75% of the time?
in humans and monkeys, how far does the grating have to be rotated before the subject gets the right answer 75% of the time?
around 5 degrees
how did V4 9and TEO) lesions affect attention?
moneys performed poorly in the titled grating experiment in the presence of distracters, which didn’t affect monkey with intact V4.
describe V4 receptive fields?
10x larger than V1 RFs, they respond to complex 3D shapes defined by curvature and/or slant
what is IT in ventral pathway crucial for?
representation of complex shapes.
TO RECOGNISE OTHER PEOPLE
how do IT neurons respond to smiley face
activated by face-like features
some IT neurons respond specifically to what feature of a face?
profile (or side views of other objects)
there appears to be clustering for what features in the IT?
clustering of columns selective for complex shapes
what can they establish by adding noise to an image?
a continuum from stimuli that are just faces to stimuli that are other objects
using the face stimuli continuum what did microstimulation of IT face-selective clusters cause in monkeys?
biases the mokey’s perception toward faces
what about when they microstimulate IT face-selective cluster in humans?
same; patient imagined faces
what is the middle face patch? where is it?
area in macaque cortex in the floor of the superior temporal sulcus that responds strongly to faces
Perception of biological motion requires a conjunction of what?
conjunction of form and motion
where did they find cells that respond to conjunction of form and motion (biological motion)
anterior superior temporal polysensory area STPa
the activity in the anterior superior temporal polysensory area STPa correlated from what?
withthe coherence of the biological motion
