visual Flashcards
where vs. what areas regarding vision?
where
- dorsal visual cortex areas
what
- ventral visual cortex areas
what is the cornea?
glassy transparent surface of the eye
-> it is continuous with the sclera
(white of the eye/wall of the eyeball)
what is the pupil?
the opening that lets light into the eye surrounded by pigmented iris
- iris = what gives eye its color
what is the lens?
located behind the iris and suspended by ligaments called zonule fibers (connected ciliary muscles)
how does the lens function in regarding to liquid in eye?
the lens divide liquid in eye into aqueous humor and vitreous
where is aqueous humor located?
behind cornea/in fornt of the lens
where is vitreous humor located
behind lens, keeps eye ball spherical
what is the retina?
area in the back of the eye that contains photo receptors
what is the optic disk?
where the retinal vessels originate and where optic nerve fibers exit
- there are no photoreceptors
- blind spot
what is the macula?
what does it contain?
- yellowish region responsible for central vision
- relatively lacks large blood vessels
- contains the fovea
what is the fovea?
what are the distinctions of the fovea?
def: center of the retina
distinctions
nasal
- retinal surface closer to the nose/medial relative to the fovea
temporal
- retinal surface closer to temple/lateral relative to fovea
what is the specialization of the fovea?
where does this specialization come from?
specialized for high acuity vision
due to
- highest density of cone photoreceptors
- no blood vessels obscuring retinal surface
- inner retinal layers pushed aside
what is refraction in the eye?
- what are the index’s of refractions present in the eye
cornea
-> aqueous vitrus and humor
-> lens
what is the amount that light bends in the eye due to?
amount of bending is proportional to the index of refraction
- higher index = greater bending
what is unique about the cornea and eye refraction?
cornea contributes most to light refraction because it is the first part of the eye that the light hits
sharpest change in index refraction
what is the purpose of the lens in regards to refraction?
the purpose of the lens is to accommodate via modulation of the shape of the lens
what happens to light when the lens is fattened?
the light bends more
what are zonule fibers and ciliary muscles?
zonule
- the tiny thread-like fibers that hold the eye’s lens firmly in place
ciliary
- changes the shape of the lens when your eyes focus on a near object
lens and fibers: far object?
less bending -> flattens
- because object is already parallel
zonule = tight
ciliary = relaxed
lens and fibers: close object?
more bending-> fattens
- because object is not as parralell
zonule = loose
ciliary = tight
what are the 2 refractive errors?
myopia
- cant see far objects which require less refraction
myopia cause
- too much refraction
- eye is too long horizontally
solution
- concave lens
hyperopia
- cant see near objects which require less refraction
hyperopia cause
- too little refraction
- eye is too short horizontally
solution
- convex lens
what are the 5 classes of retinal neurons? (bottom to top)
1) photo receptors (rods, cones)
2) bi-ploar cells
3) horizontal cells
4) amacrine cells
5) ganglion cells
how many retinal layers are there? what are the types?
5
- 3 nuclear layers (cell bodies)
- 2 plexiform layers (synapses)
plexi = network of neurons
how do horizontal cells function?
- they get input from photo receptors
- influence photoreceptors and bipolar cells
how do amacrine cells function?
- get input from bipolar cells
- influence bipolar and ganglion cells
what does the light hit first?
the ganglion cells
what do photoreceptors do? why are they unique?
they transduce light into a change in membrane potential
they are the only light-sensitive cells in the retina
how do non photoreceptor retinal cells fire in response to light?
via direct or indirect synaptic connections
what is a unique property of disks that help with them absorb light?
light-sensitive photoreceptors help absorb light to trigger Vm changes
what is melanopsin, where is it located?
ipRGCs contain melanopsin, large RFs allow humans to synchronize behaviour to changes in light (circadian rhythm)
why are retinal ganglion cells unique?
1) because they are the only retinal cells to fire action potentials
2) they send axons out of the optic nerve
where do ganglion cells project to?
Thalamus - LGN
(lateral geniculate nucleus)
rods. vs cones
- appearance
- light sensitivity
- mid-light
- concentrated in
- types
- color reception
what is rhodopsin?
rod pigment and functions like receptor for signal cascade
when is the acuity of rods best vs. cones?
rods
- best in periphery in scotopic conditions
cones
- best in fovea in photopic conditions
what is visual acuity?
they eyes ability to distinguish between 2 points (spatial sensitivity of the eye)
what are the electrical currents within the photoreceptors?
- outward K+
- inward Na+
what is the inward current of the photoreceptors called, what is the current gated by?
Na+ current is called “dark current”, because it is not based on light stimulation
how does light activate photopigment?
- photopigment is located in discs of the outer segments
- light changes the confirmation (shape) of the photopigment
what is the Vm of photoreceptors in dark conditions?
Vm = -30mV
what are the 2 reasons why the photoreceptors have fundamentally different electrical activity?
1) they have more Na+ leak channels
2) Photoreceptors hyper-polarize to light
what keeps Na+ channels open?
secondary messenger cGMP keep Na+ channels open
what is opsin?
what is retinal?
what is bleaching, what does bleaching cause?
opsin
- receptor protein (has 7 transmembrane regions like G protein-couples receptors)
retinal
- is pre-bound against opsin
bleaching
- light changes conformation of retinal -> it activates opsin
- activates G-protein transducin
- transducin activates PDE
- reduces levels of cGMP
- Na+ channels close, hyper polarization
what are different photoreceptors hyperpolarized by?
different wavelengths of light
what is the color we perceive based on?
it is based on the relative activation of R, G and B cones
is color a physical property?
no, color is a function of the mind
- different species presumably experience color differently
where are each RGB pigment located?
Red and green = X chromosome
Blue = chromosome 7
how do our eyes deal with sustained changes in light?
adaptation
what is adaptation?
changes in visual sensitivity when there is a sustained change in light level
what are the 2 kinds of adaptation?
1) dark adaptation
(adapting to darkness)
2) light adaptation
(adapting to light)
what are the mechanisms for adaptation?
1) pupil constriction or dilation
2) switch between rods and cones
3) Ca++ feedback in photoreceptors
how do pupils adapt to light?
high intensity of light
-> constriction of pupils
low intensity of light
-> dilation of pupils
details
constriction
- increases depth of focus
reflex is consensual
- shining light on one eye while other is covered will cause both to constrict
what is the range that you can see when adapting?
the more light, the photoreceptors will hyperpolarize
- cones can only change over a 10^3 range of light intensity
how is cGMP synthesized?
cGMP is synthesized by guanylyl cyclase from GTP
how does each photoreceptor adapt? (group or independent)
- each photoreceptor adapts independently
what is an afterimage?
the image seen after adaptation
rod and cone adaptations
low light vs. high light
low light
- rods become higher input for ganglion cells
high light
- cones become higher input for ganglion cells
what do the 3 different types of opsin in the cones represent?
RGB
red=long
green=medium
blue=short
what does right temporal see?
what does right nasal see?
right temporal
- left central
right nasal
- right peripheral
- right central
what does left temporal see?
what does left nasal see?
left temporal
- right central field
left nasal
- left peripheral
- left central
where do nasal portions decussate?
at optic chiasm
what do the optic tracts carry?
information from the contralateral visual field
what is the fovea specialized for?
high acuity vision
- fovea’s have the highest cone density
- no overlaying blood vessels
- inner retinal layers pushed aside
describe rod and cone distribution?
92 million rods
5 million cones
1.5 million axons leave the eye -> much signal processing in the eye
what is the result of the distribution of cones and rods in the eye?
blind spot
color vision
- best in fovea
- poor in periphery
- none at night
acuity
- best in fovea with photopic light
- best in periphery with scotopic light
(fins a star on a moonless night)
what is the receptive field of a neuron?
the field of stimulus to which a cell responds
what is the best or optimal stimuli for a cell?
the one that causes the maximal response
photoreceptor receptive field?
spot of light on retina
bipolar cell, ganglion cell, thalamus
receptive field?
have center/surround antagonistic RFs
ON-center/OFF-surround
or
OFF-center/ON-surround
V1 - receptive field?
Bar of light with a particular orientation
dorsal secondary visual area - receptive field?
spots of light moving in particular direction -> motion processing
ventral secondary visual area - receptive field?
biologically significant objects -> form/color processing
- hand or a particular person
what do photoreceptors release when depolarized?
what happens when there is more light?
glutamate
more light = greater hyperpolarization
-> less glutamate is released
what are the two types of bipolar cells?
On and Off
what is the purpose of lateral inhibition?
to enhance visual contrast
what are the cells that fire action potentials in the visual system?
retinal ganglion cells
what are the different types of retinal ganglion cells?
magno, parvo, nonM-nonP
describe magno RGCs
- 5% of RGCs
- Large RFs
- Color insensitive
- Transient burst of APs
describe parvo cells
- 90% of RGCs
- Small RFs
- Color sensitive
- Sustained discharge of APs as long as stimulus is present
describe nonM-nonP RGCs
- 5% of RGCs
- small RFs
- most are color sensitive
- Poorly charachterized
what is the LNG?
a thalamic nucleus that relays information from the retina to the primary visual cortex.
where is the LGN located?
why are there 2?
- It is located in the thalamus
- There are 2 because the thalamus is a bilateral structure
What is the distribution of input in the LGN?
20% = retina
80% = V1
how many layers are in the LGN?
6
what layers of the LGN receive signals from the ipsilateral eye?
2, 3, 5
what layers of the LGN receive input from the contralateral eye?
1, 4, 6
what layers of the LGN receive input from the different types of RGCs?
1 & 2 = Magno
3, 4, 5 and 6 = Parvo
Koniocellular = nonM-nonP input
LGN functions?
- thalamic relay between optic tract and V1
- dreams in burst mode
- visual attention
what is the dorsal stream of the LGN?
Magno LGN -> 4C alpha-> 4B -> Occipital/Parietal
what is the ventral stream of the LGN?
Parvo LGN -> 4C beta -> 2/3 -> Occipital/Temporal
what is the occipital and temporal cortex related to?
identification and recognition
what is the occipital and parietal cortex related to?
motor, visual motion, visual control of action
what are retinotopic projections an example of?
topography/topographic maps
what is the significance of layer 4C?
1) inputs from the L and R eye remain segregated
2) Layer 4C is monocular, the last time in the visual pathway where input from the two eyes is separated
3) the receptive field properties are center/surround antagonistic RF
describe how V1 layers outside of 4C function?
what do the V1 layers contain, and produce?
they are binocular but are still dominated by one eye depending on their proximity to left eye input in layer 4C or right eye input in layer 4C
contain
- cortical modules
produce
- ocular dominance columns
in what direction do ocular dominance columns run?
perpendicular to V1 layers
describe orientation selectivity?
certain orientation gives highest response and form orientation columns
what is orientation selectivity used for?
form perception
direction selectivity?
- makes up a subset of neurons that are orientation selective
- response based on orientation and direction of movement
- response is also grade
what is direction selectivity used for?
motion perception
what are the characteristics of a simple cell?
1) orientation selective
2) distinct on/off areas
3) center-surround RF inputs
what are the characteristics of a complex cell?
1) orientation selective
2) on/off areas overlap
3) presumably constructed from the summation inputs from multiple simple cells preferring the same orientation
parvo and konio factors?
- ventral and cortical areas
- form and color
- “what”
magno factors?
- dorsal cortical areas
- motion
- “where”
5 factors of the V1 cortical modules?
1) each cortical module is necessary and sufficient to analyze a particular point
2) removal -> blind spot in visual area
3) 2 pairs od ocular dominance columns
4) 2 complete samplings of all 180 of possible orientation columns
5) also contain 16 cytochrome oxidase blobs
what is achromatopsia?
cortical color blindness
akinetopsia
motion blindness
what is the purpose of the dorsal stream?
for analysis of visual motion and visual control of action
“where”
describe area V5/MT?
- nearly all cells are direction selective
- unlike V1 has direction columns
- large RFs, responsive to stimulus motion direction
describe area MST?
- cells selective for types of motion
- proposed purposes: navigation, directing eye movements, motion perception
what could damage in the dorsal stream cause?
akinetopsia
what is the purpose of the ventral stream?
what could damage to this area cause?
for perception of visual world and recognition of objects “what”
- prosopagnosia, inability to recognize faces
describe area V4?
what could damage to this area cause?
- very large RFs with color and orientation selectivity
- achromatopsia
what is area IT?
farthest extent of visual processing
- faces, objects with biological significance
- large RFs
- may be important for recognition and visual memory
what is the area associated with face recognition?
fusiform face area
what 2 areas are in the dorsal stream?
area V5/MT
area MST
what 3 areas are in the ventral stream?
area v4
area IT
fusiform face area
in what layer of the V1 does LGN feedback occur?
layer 6 of V1
what layer of the V1 controls eye movements
layer 5
what happens during spiking mode?
LGN firing follows ganglion cell input
what happens during intrinsic burst mode?
what role does it play?
LGN cut off from sensory input
- dreams
- visual attention
what is the function of orientation selectivity?
- form orientation columns
- used for edge detection
and form perception
what is the function of direction selectivity?
- used for motion perception
what do cortical modules contain that are important?
cytochrome oxidase blobs
what are the cytochrome oxidase blobs used for?
color perception/processing
what does the v1 contain? what are the functions of the v1?
contains
4C, 2, 3, 4A, 4B, 5, 6
- center and surround antagonistic RF similar to LGN
- orientation selectivity
- direction selectivity
