Vision Lectures Flashcards
Why study vision?
- humans are a highly visual species
- the visual system is a good model to understand sensory processing and cortical function
We see a limited spectrum of _____
wavelength
The color of visible light is specified by _____
wavelength
Objects both ____ and ___ light
absorb and reflect
What type of energy is light?
electromagnetic energy
Light colored objects reflect ____ of light
a lot of light
An object’s color is ____
the wavelengths of light it reflects
Function of the visual system
transforms patterns of reflected light as viewed by the eye into mental/cognitive image of the world
How many levels of processing are there in the visual system? What are they?
three; low, intermediate and high level processing
What is low level processing? Where does it occur?
break down of an image into simple elements, occurs in retina, but simple elements are maintained through LGN and V1
What visual info is extracted in low level processing?
orientation, color, contrast, disparity between eyes, and movement direction
What is intermediate processing? Where does it occur?
joins simple elements together to result in: contour integration, surface properties, shape discrimination, surface depth, surface segmentation, object motion/shape from kinematic cues; occurs in higher levels of cortex
What visual info results from intermediate-level processing?
contour integration, surface properties, shape discrimination, surface depth, surface segmentation, object motion/shape from kinematic cues
What is high-level orcessing? Where does it occur?
uses prior memories and semantic info to identify an object; occurs in frontal cortex and other higher cortical areas
What are the two ways the brain processes visual information?
hierarchical processing and parallel processing
What is hierarchical processing?
building more complex receptive field properties of neurons at higher stages due to converging inputs from neurons of lower level areas
What do neurons in the LGN respond to?
small dots and contrast
What do neurons in V1 respond to?
orientation, retinal disparity, some color
What do neurons in V4 respond to?
color, basic 2D and 3D shape, curvature
What do neurons in inferior temporal cortex respond to?
complex features and objects
Pathway for hierarchical processing:
LGN -> V1 -> V4 -> IT
What happens as you move from lower areas to higher areas?
- the receptive field becomes larger; LGN neurons only “see” a small portion of the visual world, IT neurons receptive field covers the entire visual field
- the receptive fields become tuned to progressively more complex visual features
What is parallel processing?
information in V1 is partitioned into two major pathways: dorsal/where/action pathway and ventral/what/perception pathway
dorsal pathway
- aka where/action pathway
- determining spatial relationships between objects to guide movement
ventral pathway
- aka what/perception pathway
- object recognition, so combines visual information with memories and semantic info for object recognition
The cornea and lens ____
focus an imge on the retina via refraction
What is refraction?
the change in direction of a wave due to a change in its speed
Cornea
on the outside of the eye, is large, has a fixed size
Lens
on the inside of the eye, is mall, but the curvature can change due to the muscles that innervate it
What are the variable aperatures of the eye?
the iris and pupil
What is the function of variable aperatures?
decrease size and so decrease the amount of light getting through, but increase the depth of the field
What are the similarities between the eye and a camera?
both invert the image and both have at least one variable aperature to change the amount of light getting through
Where are photoreceptors located in the retina?
the very back of the retina
Why are the layers of cells in the retina shifted away at the foveola?
allows a more direct pathway to the photoreceptors
What is the purpose of the pigment epithelium (melanin)? Where is it located?
reduces light scatter by absorbing light not captured by the retina; it is further back from the photoreceptors
What is the optic disk?
the retinal blind spot
Why do we have a blind spot?
here the axons of retinal ganglion cells form the optic nerve and axons and blood vessels exit and enter here
Are there photoreceptors or vision in the blind spot?
no
What is the purpose of eye movements?
bring images into the fovea
Why are we unaware of our blind spot?
our brain fills it in
How many cell types are in the retina? What are they?
five; rods, cones, bipolar cells, horizontal cells, amacrine cells, retinal gangilion cells
What are the different layers of the retina from back to front?
outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer
Outer nuclear layer contains which cells?
photoreceptors
Outer plexiform layer contains which cells?
synapse between photoreceptors and horizontal cells
Inner nuclear layer contains which cells?
bipolar cells and amacrine cells
Inner plexiform layer and outer plexiform layer are the layers where _____
presynaptic terminals synapse on post synaptic cells
Which photoreceptors are highly sensitive to light, and so are for night vision?
rods
Why are rods more sensitive to light than cones?
they have more photopigments that cones
Why can rods detect a single photon?
higher amplification of signal due to large number of disks in outer segment
Why are rods achromatic?
only have one type of pigment
Why do cones capture less light?
have less photopigment
Why are cones involved in high acquity vision?
in the fovea, there is 1 cone:1 bipolar cell:1 retinal ganglion cell (low convergence)
Why are cones chromatic?
Have three types of cones, each type has a different pigement that absorbs different wavelengths of light
Outersegment
contains the discs, where phototransduction occurs
Innersegment
contains the nucleus and organelle, responsible for biosynthesis
Synaptic terminal
where the photoreceptor synapses on a bipolar cell
Basics of phototransduction
- Activation of visual pigments by light
- Stimulation of cGMP phosphodiesterase (breaks down cGMP), thus decreasing cGMP concentration
- Closure of cGMP-gated ion channels leads to hyperpolarization; Na can no longer flow in so the membrane potential becomes more negative
Is there higher cGMP concentration in the dark or in the light?
in the dark
Phototransduction stage 1
- the photopigment rhodopsin (protein opsin + rential) absorbs light
- now rhodopsin undergoes a conformational change from 11-cis-retinal to all-trans conformation; all trans is unstable and so quickly converts to metarhodopsin I then to metarhodopsin II
- metarhodopsin II activates the G-protein transducin, this starts stage 2
Phototransduction stage 2
- transducin activates cGMP phosphodiesterase, which converts cGMP to 5’GMP
- this reduces the concentration of cGMP
- now w/o cGMP around, the cGMP-gated Na channels close and cell hyperpolarizes
1 photon leads to a XmV hyperolarization, due to ____
1mV, amplification
In the dark, there is a high concentration of ____, so ____ channels are open and the cell is ____
cGMP, Na, depolarized
Are K channels gated?
No
What maintains the ratio of Na to K inside the cell
the Na/K pump pumps K in and Na out
When light hits the eye, Na channels ____, but ____ remain open, so the cell ____ to ___mV
close, K channels, hyperpolarizes to -70mV
Transducin self-inactivates to terminate the light response
GTPase activity breaks down the bound GTP of the active form into GDP, so transducin is no longer active and can no longer activate cGMP phosphodiesterase, so cell remains depolarized and does not fire
How does activated rhodopsin end up terminating the light response?
activated rhodopsin (metarhodopsin II) becomes a target for phosphorylation by opsin kinase -once phosphorylated, it binds to arrestin, leading to its inactivation, preventing it from interacting with transducin
What is light adaptation?
negative feedback via Ca that provides variable sensitivity and amelorates saturation; a way to termiante the light response
What two events mediate light adaptation?
- slow recovery of membrane potential (from -70mV to -40mV)
- desensitization of the photoreceptor
What is the importance of cGMP gated ion channels allowing both Na and Ca in?
- the cGMP gated ion channels allow both Na and Ca to enter the neuron
- Ca inactivates guanylyl cyclase, which produces cGMP, so when Ca is present, it decreases cGMP levels
- Ca also inhibits activation of the photopigment
Light adaptation
light -> low concentration of Ca -> increases cGMP production, recovers the membrane potential by closing Na channels; increases spped of photopigment inactivation, leading to receptor desensitization -> more intense light stimulus required to close the ame number of cGMP gated cation channels
What roles does light adaptation play in perception?
- discards information about ambient light while retaining info about object reflectances
- to match the small dynamic range of firing in a retinal ganglion cell to the large range of light intensity in the environment (gain control)
If the dark current is metabolically demanding, why does our visual system bother?
because it creates high gain (aka contrast sensitivity)
High gain
- amplification via biochemical enzymatic cascade
- no threshold, so there is modulation of an ongoing signal because of the gradient in signal
variable gain
- adaptation to changes in light intensity; change in gain in response to ambient light
- contributes to large operating range of retinal ganglion cells
Retinal ganglion cells are ___ cells of the retina, generate ___, and are part of the ___ pathway
output, action potentials, vertical pathway
ganglion cells have ____ surround receptive fields and come as ___ and ___
center-antagonistic; ON-center, OFF-Center
Why are retinal ganglion cells contrast detectors?
- have poor responses to diffuse illumination
- best response to differences in light and surround (respond best to sharp contrast)
Why use center-surround?
- enhances stimulus contrast and so transmits differences
- contributes to response invariance, where luminance is interpreted in context of the background
In dim light, the visual system is low or high pass?
low pass, so is more sensitive to lower frequenices
Contrast sensitivity
1/contrast threshold
contrast threshold
the lowest contrast needed to percieve spatial frequency
What does higher contrast sensitivity indicate?
that a lower contrast is need to percieve spatial frequency
spatial frequency
cycles/degree
Very low spatial frequency is like what kind of light? Are RGCs activated by this?
diffuse light; yes, because a bar takes up the entire on or off part, mimicking light
On-center cells give strong responses to small ____ in light
increases
Off-center cells give strong responses to small _____ in light
decreases
Magno and parvo cells are the beginning of what type of processing?
parallel processing
What percent of retinal ganglion cells are parvo cells?
80%
What percent of retinal ganglion cells are magno cells?
10%
Characteristics of magno cells:
- can be ON or OFF
- have small receptive fields
- signal fast changes in the stimulus
- involved in perception of gross features and motion of a stimulus
- vision requiring low spatial and high temporal resolution
Characteristics of parvo cells:
- can be ON or OFF
- have small receptive fields
- are wavelength specific, recieve inputs from cones
- involed in color/form perception and fine details of a stimulus
- vision requiring high spatial and low temporal resolution
K cells recieve inputs specifically from ____
blue cones
Horizontal cells and bipolar cells respond to light with ____ changes in membrane potential
graded
Retinal ganglion cells and amacrine cells respond to light with ____
action potentials
In OFF bipolar cells, glutamate:
opens an Na channel (AMPAR), excites the cell
In ON bipolar cells, glutamate:
opens a K channel (hyperpolarizes), which activates a GPCR, this activates cGMP phosphodiesterase, closing cGMP-gated channels carrying inwared Na current- hyperpolarizing and inhibiting the cell
light and OFF bipolar cells
light decreases excitation of bipolar cells leading to hyperpolarization of OFF-center bipolar cells
light and ON bipolar cells
light decreases inhibition of bipolar cells leading to depolarization of ON-center bipolar cells
generation of antagonistic surround for ON-center cell
- when light hits the surround of an ON-center cell’s receptive field, the horizontal cell is activated by the OFF cell releasing glutamate
- the activated horizontal cell inhibits ON center cells so they dont excite ON center bipolar cells
What type of connection do rod bipolar cells make with amacrine cells?
excitatory
What does an amacrine cell do to an ON cone bipolar cell? OFF bipolar cell?
ON: makes an excitatory, sign preserving connection due to gap junction
OFF: makes a sign inverting connection due to it releasing glycine
trichromatic theory
one can match all of the visible colors by mixing any 3 primary colors, as long as mixing two of them does not produce the third
opponent process theory
trichromatic signals from the cones feed into subsequent neural stages and exhibit two major opponent classes of processing.
- spectrally opponent processes (red vs green; yellow vs blue)
- spectrally non-opponent processes (black and white)
What theory of color vision operates at the receptor level? Neural level?
receptor level: trichromatic theory
neural level: opponent processes theory
How does the retina know what color light is?
- through the use of color-opponent parvo retinal ganglion cells: Red ON/Green OFF, Green ON/Red OFF, and Blue ON/Yellow OFF (also exist with ON and OFF switched)
- so in Red ON/Green OFF, the cell fires when red light hits the center
What brain region performs color constancy?
V4
In regard to color, what do neurons in LGN and V1 respond to? V4?
LGN and V1 actual wavelength of light
V4 percieved color
Describe achromatic magno retinal ganglion cells
can be ON or OFF center; and recieve equal amounts of input from L and M on center and outside so can’t distinguish color
Describe the visual pathway
eyes to optic nerve/tract to optic chiasm to LGN to V1
What region of vision takes up the most space in V1?
the fovea
How is the visual field organized in LGN and V1?
V1 and LGN represent the contralateral hemifield in a retinotopic map, such that nearby points in the visual field are close together in LGN and V1
How are the magnocellular and parvocellular channels organized in the LGN?
- one layer recieves input from one eye’s retinal gagnlion cells
- parvo layers are: 6-contralateral eye, 5-ipsilateral eye, 4-contralateral eye
- magno layers are: 2-ipsilateral eye, 1-contralateral eye
- the koniocellular/intralaminar pathway recieve input from S cones
What retinal features are maintained in the LGN?
- segregation of afferents from the two eyes
- segregation of M, P, and K pathways
- retinotopy
- receptive fields are center-antagonistc surround
- receptive fields are ON-center or OFF-center
What are the non-retinal inputs to the LGN?
- from retinal formation of the brainstem for arousal and sleep
- feedback from cortex to gate flow of info from retina to cortex and so may be involved in visual attention
Is magno or parvo responsible for high resolution pattern vision?
parvo
Is mago or parvo responsible for crude form and motion vision?
magno
Luminance contrast
difference in luminance between dark and light bars divided by total luminance
spatial frequency
gratings cycles per degree (so lots of skinny bars is high spatial frequency)
temporal frequency
gratings cycles per second
color contrast
bars of different colors but the same luminance
Difference in sensitivity of M and P cells to color contrast
M not sensitive to color; P sensitive to color
Difference in sensitivity of M and P cells to luminance contrast
M cells more sensitive to contrast than P cells
Difference in sensitivity of M and P cells to spatial frequency
M cells less sensitive to spatial frequency; P cells more sensitive to spatial frequency and so have higher spatial resolution
Difference in sensitivity of M and P cells to temporal frequency
M cells have higher temporal resolution than do P cells
What percentage of the cortex are parvo cells? Magno cells?
Parvo cells 80% of cortex
Magno 8% of cortex
contrast sensitivity
inverse of lowest simulus contrast that can be detected; so with high contrast sensitivity can tell the difference between very similar stimuli
Parvo cells (P channel) project to which layers of the LGN?
layer 4Cbeta, 4A, and layer 6
Magno cells (M channel) project to which layers of the LGN
layer 4Calpha, layer 6
How are inputs from each eye organized in layer 4C?
into occular dominance layers, so inputs from one eye are one layer and inputs from the other eye are another layer
What are the V1 output layers?
layers 2/3, 4A, 4B, 5, and 6
Where do V1 layers 2/3, 4A and 4B project to?
extrastriate cortex
Where does V1 layer 5 project to?
superior colliculus, pulvinar nucleus in thalamus, pons
Where does V1 layer project to?
sends feeback projections to the LGN, also pojects to calustrum
What are the interlaminar connections that integrate activity within V1?
- from input layes upward to layers 2/3
- from layers 2/3 downward to 5 then to 6
- feedback from 6 to 4c and 5 and then onto 2/3
Pyramidal cells are in all layes of V1 except:
layer 4C
What type of cells are pyramidal cells? input or output? Neurotransmitter?
Output cells, glutamatergic
What cells are local neurons of V1 layer 4? What cells do they contact? What is the exception to this pattern?
spiny stellate cells; contact other spiny stellate cells; those in 4B are glutamatergic output cells
What are the inhibitory local neurons of V1?
the aspiny stellate cells and non-pyramdial cells
What ways can you classify neurons?
morphology, physiological properties like spike rate, molecular properties, tuning, gain response
What are the three major classes of inhibitory neurons in V1? What is their function?
parvalbumin, somatostatin, VaP; in general moderate tuning and gain responses
Function of parvalbumin cells
contact some excitatory cells and shut down the response of post synaptic cells
Function of somatostatin cells
modulate responses of post synaptic cells
Where do the intralaminar/konio afferents form the LGN project in V1?
layers 1, 2/3 blobs, and 4A
