The Visual System Flashcards
The Andromeda Galaxy
Can see w naked eye
Can’t tell if it is an ellipse or circle tilted on it’s side
This is problem w visual system
Sensation is an abstraction, not a replication of the real world
The eye is like a camera
Focused light is projected on retina
Optic nerve creates optic disc which is blind spot
Horizontal cells and Amacrine cells
Connect photoreceptors together
Connect ganglion cells
What is closest to the retina?
Retina-
Photoreceptors
Horizontal cells
Bipolar cells
Amacrine cells
Retinal Ganglion cells
-Front of eye
How many layers in retina?
10 layers
Fovea
High-acuity center of visual field
Cells are pushed out of the way
Photoreceptors have direct access to light in the fovea
It is intended
17 distinct
Do all neurons fire action potentials? Give an example of what doesnt
NO
Rods and cones do not
Are cones or rods concentrated in the fovea?
Cones, no rods
Nasal side
Rods concentration, no cones
Optic disc
Temporal side
Rods, no cones
Light sensitivity
Rods are more sensitive to light but have low acuity
Cones are less sensitive to light but have high acuity
Examples: seeing a faint star, will be brighter if you look slightly to the side of it because rods will be perceiving it
In dark
cGMP flowing around cells, high concentration
It binds to ion channel that is permeable to sodium, sodium flows in
In the dark the photoreceptor is depolarized
When light hits photreceptor?
cGMP phosphodiesterase activated and reduces concentration of cGMP so Na channels close
Photoreceptors are _____ in the dark
depolarized
Photoreceptors are _____ in the light
hyperpolarized
How is rhodopsin activated?
Light causes conformational change
Transducin (G-protein)
Activated by Rhodopsin
cGMP Phosphodiesterase
Breaks down cGMP which results in sodium channels closing
Do any of the 5 cell types in the retina fire action potentials
Retinal Ganglion
Intensity of light is transformed…
Into the frequency of APs in retinal ganglion cells
Horizontal cells
Inhibit NT release from cone in the dark
If light shines on central cones
AP increases
If on horizontal cells
It is inhibited
In dark
Calcium flows in
Cone releases Glutamate
In light
Calcium stops
Cone does not release NT
How does glutamate affect the bipolar cell
Glutamate actually hyperpolarizes the bipolar cell which is weird bc we usually think of glutamate as excitatory
Does bipolar cell fire AP?
No
Bipolar cell synapses on RGC and…
Excitatory glutamate at the synapse
Is RGC firing AP in the dark?
No
Why is glutamate inhibitory b/w cone and bipolar cell?
Bc receptors are metabotropic
In the dark the cone…
Releases a lot of glutamate
In bright light
Amount of glutamate is dramatically reduced
Intermediate light
Cone will only be somewhat hyperpolarized so it will release sort of less glutamate
Amount of glutamate being released is a continuous function of…
How much light the cone absorbs
The change in membrane potential is going to be a continuous function of…
How much light the cone absorbs
How much glutamate the cone releases
The amount of glutamate that it releases is going to be a continuous function of…
How much light the cone absorbs
If light is dim…
Cone hyperpolarizes a little bit
If cone hyperpolarizes a little bit…
Bipolar cell depolarizes a bit
If bipolar cell depolarizes a little bit
The bipolar cell will release some glutamate onto the ganglion cell and ganglion cell will increase it’s firing
Explain the idea of inhibition by horizontal cells
Imagine 3 photoreceptors.
One of them is connected to a bipolar cell which is connected to a ganglion cell
When the photoreceptors surrounding it are hit by light this causes the horizontal cell to INHIBIT NT release from the center photoreceptor connected to this RGC
What determines if RGC will be on center or off-center
Bipolar cell
Cones in fovea…
Each cone is connected to a glutamate (inhibitory) - ON bipolar cell
AND
A glutamate (excitatory) - OFF bipolar cell
What happens to receptive fields as you move away from the fovea?
They become larger
Multiple PR’s contribute to the center and to the surround
Lotta convergence
This why they are more sensitive to light but less precise
There are more then a dozen distinct types of RGC’s. How many?
17
Each type of retinal ganglion cell tiles the retina
Jus like somatosensory system (corpuscles)
3 best characterized channels from the retina. 3 kinds of RGC’s that we know the most about
Midget ganglion cells
Parasol ganglion cells
Bistratified ganglion cells
% of RGC’s that are midget ganglion cells
70%
% of RGC’s that are parasol ganglion cells
10%
% of RGC’s that are bistratified ganglion cells
8%
Small cone and wavelength
Blue cone - 445nm
Medium cone and wavelength
Green - 535nm
Large cone and wavelength
Red - 575nm
Midget cells
Can see how much green vs red
Very concentrated in the fovea
They are the first step(neurons) of the p-pathway
Center of their receptive fields is made of one cone, very high resolution
Midget cells are high precision but not good at detecting rapid changes
How many types of midget ganglion cells?
4
Where do midget cells synapse?
Parvocellular layer in the LGN (p-pathway)
For green center and red surround what cones are where
Green cone in center, red cones surrounding it
How are midget cells also edge detectors?
White light makes everything activate
Parasol cells
m-pathway
Have larger receptive fields
Only recognize illumination
Really good at detecting rapid changes
Both center and surround or mixed up w red and green cones
These can’t tell diff b/w red and greed they are just activated anyways
Bistratified cells
k-pathway
Convey relative amount of blue vs yellow
Project to koniocellular layers of LGN
Yellow is…
Mix of green and red light
Parasol
Luminance
L+M
Midget
L vs M
Red/green
Bistratified
S vs (L+M)
Blue/Yellow
Which are more complex? The receptive fields of RGCs or PR’s?
As a result of the convergence of multiple PR’s individual RGC’s the receptive fields of RGC’s are more complex then those of photoreceptors
Visual pathway
Optic nerve–> optic chiasm –> optic tracts –> LGN
Midget and parasol ganglion cells project to how many layers in LGN?
6 distinct layers
1 is most medial
Each layer has
Full retinotopic map
Lateral projections…
Ipsilateral
Layers 2,3,5
Nasal projections
Contralateral
Layers 1,4,6
Do LGN neurons relay inputs from RGCs to V1 faithfully without significant change in the receptive fields?
YES
Projections that go on lower pathway from LGN to V1 represent…
Top half of visual space
Projections that go on higher pathway from LGN to V1 represent…
Bottom half of visual space
How does LGN output project to V1?
Optic radiation
Lot more projections going to LGN from cortex or vice-versa?
Lot more from V1 to LGN
Superior colliculus
Involved in saccadic eye movements
Pretectum
Pupillary reflexes
Superchiasmatic nucleus of the hypothalamus
Modulates circadian rhythms
Distribution of projections from RGC’s?
90%
- LGN
10% split b/w:
- superior colliculus
- pretectum
- superchiasmatic nucleus
What separates occipital lobe from rest of brain?
Parieto-Occipital sulcus
V1 neurons respond best to bars of light?
Bars of light at a particular orientation
Torsten Wiesl and David Hubel
Learned how brain neurons encode visual stimuli
V1 Location
Brodmann’s area 17
Around the Calcarine sulcus
Left primary visual cortex represents…
Right side of visual space
Ventral part of primary cortex reps..
top half of visual space
Top half of V1 reps…
Bottom half of visual space just like optic radiation
Inputs from thalamus terminate in…
Layer 4
LGN neurons project mainly to layer…
4c
K-pathway
From thalamus to superficial layer (2,3)
Directly to BLOB
BLOB projects to the VENTRAL - WHAT pathway
P-pathway
From LGN to layer 4Cbeta
Projects to Blob and inter-blob then to VENTRAL - WHAT pathway
M-pathway
From LGN to 4Calpha to 4B
To blob and inter-blob to the Ventral-WHAT pathway
4B projects to Dorsal, Where pathway
Where does 4B from M-pathway project?
Dorsal, Where pathway
The elongated receptive fields of simple cells are built from convergent input from…
many layer 4 cells with roughly circular receptive fields
Blob
Involved in color vision
Cortical columns
Orientation-selective columns
Arranged in a complex pattern of swirls
A cycle around a swirl contains neurons responding to the full range of orientations for a particular location in space
Inter-blob
Shape of objections
Mixing of P-pathway and M-pathway
Hypercolumn
Column containing all possible orientations of a region of visual space
Simple cells
On center, off-surround
Complex cells
Don’t have inhibition, just need an edge anywhere in their receptive field
Some neurons have a motion preference
Subset of neurons, bars of light have to move in a particular direction
Some V1 neurons are sensitive to bar length
Blobs found in…
Layer 2 and 3
Blobs are interspersed within the orientation columns
They respond to color
Interblob are orientation
Ocular dominance columns
Superimposed on the orientation columns and blobs are columns corresponding to alternating input from the ipsilateral and contralateral eyes.
These are called ocular dominance columns.
Dorsal stream
Where, goes along parietal lobe
Location and movement
Ventral stream
Encodes form and color
What stream
Area MT
In the dorsal stream is involved in detection of motion
If dots move at random, 50% correlation and 100% correlated
What is threshold
What is threshold of detection for people to know that dots are moving in particular direction
Usually 10% threshold
If lesion in MT neurons
Deficit in detecting global motion, understanding which direction things are moving
Aperture problem
V1 receptive fields are small, may give misleading info on direction of movements
Can be deceived about direction of motion.
Dorsal stream multimodal sensory pathways from…
Parietal cortex to premotor cortex
How is aperture problem fixed?
Neurons in area MT integrate input from neurons with smaller receptive fields so they can detect the overall direction of movement of an object
People who have lesions to these particular color areas can not imagine color even if they had color vision before
Idea about memory, how are erasing this piece of memory?
Fusiform face area
Faces
Prosopagnosia
Loss of ability to recognize familiar faces
feedback connections may be involved in visual attention and..
top down anticipator mechanisms that enable us to differentiate an object from the background
Predictive processing
Prediction error signals are fed forward to update predictive model at the next level in the hierarchy
Expectation vs input = error
Error goes up to correct error
Blindsight
Blind man can avoid obstacles
V1 on right side of brain so he can’t see things on left side
Superior colliculus is connected to parietal and frontal lobes. even though V1 gets no info, there is this extra stream that allows blindsight to happen
Involved in detecting motion and controlling eye movements
Another unconscious pathway goes from thalamus (LGN and pulvinar) to the amygdala. what is it involved in?
Rapid, unconscious emotional responses to visual stimuli. Like seeing a bear pass you at night.
Projections to prefrontal cortex are involved in…
Visual working memory
The binding problem
How is visual stuff unified? How are the diff components of vision (form, color, location, motion) spread out over disparate regions of cortex, bound together to form unified, coherent percept
