Vision 2 Flashcards
Retinofugal pathway
- Optic nerve leaving retina
- Portions of optic nerves corresponding to nasal retina decussate at optic chiasm
- Optic tract leaves the optic chiasm
- Optic tract terminates at the lateral geniculate nucleus (LGN) of thalamus
- Projection of LGN to cortex is optic radiation
- Visual cortex (V1) is in the occipital lobe
Diagram of retinofugal pathway
Where is the visual cortex (V1)?
Occipital lobe
Portions of optic nerves corresponding to
nasal retina decussate at the ___
Optic chiasm
The ___ leaves the optic chiasm
Optic tract
Where does the optic tract terminate?
At the LGN of the thalamus
Diagram of visual field: retina
Visual field: retina
What happens if the left optic nerve is cut (left temporal retina and left nasal retina)?
- Left peripheral visual field is lost
- This is because only the left nasal retina carries information on the left peripheral visual field (left temporal retina sees right central visual field but this is also seen by right nasal retina)
What happens if there is damage to the left optic tract (left temporal retina and right nasal retina)?
- Right hemifield is lost
- This is because the optic tract carries information on solely the contralateral hemifield
What happens if the optic chiasm is transected?
- Right and left peripheral visual fields lost
- This is because both peripheral visual fields are only seen by the ipsilateral nasal retina
What damage has this person suffered?
Damage to the left optic tract, causing loss of the right hemifield
What damage has this person suffered?
Damage to the left optic chiasm
What damage has this person suffered?
Damage to the optic chiasm (transected), causing loss of right and left peripheral visual fields
Oh no! I’ve been really clumsy and have somehow managed to lesion my roommate’s right optic nerve. What visual field deficits would my roommate observe?
- Loss of the right peripheral visual field.
- Remember, only the right nasal retina sees the right periphery
What is the receptive field of a neuron?
The area on a sensory surface where a stimulus will modify the firing or electrical activity of that neuron
‘Best’ or ‘optimal’ stimulus for a cell is one that causes the maximal response
● Photoreceptor: Spot of light on retina
● Bipolar cell, ganglion cell, thalamus: Have center/surround antagonistic RFs i.e. ON-center/OFF-surround or OFF-center/ON-surround
● V1: Bar of light with a particular orientation
● Dorsal secondary visual area: Spots of light moving in particular direction → motion processing
● Ventral secondary visual area: Biologically significant objects → form/color processing
PR –> BP + Horizontal table (separate into flashcards)
On-off center surround table (separate into flashcards)
Bipolar cell response
1
Bipolar cell response diagram
Describe features of ON bipolar cells
- Metabotropic glutamate receptor: glutamate causes Na+ channels to close –> hyperpolarization (exception to glutamate being depolarizing)
- Sign inverting: PR hyperpolarizes to light –> less glutamate –> contrarily makes the ON bipolar cell depolarize
- ON: the light activates/depolarizes this cell
Describe features of OFF bipolar cells
- Ionotropic glutamate receptor: glutamate causes Na+ channels to open –> depolarizing
- Sign preserving: PR hyperpolarizes to light –> less glutamate –> makes the OFF bipolar cell also hyperpolarize
- OFF: the light deactivates/hyperpolarizes this cell
Are ON bipolar cells sign preserving or inverting?
Sign inverting
Are OFF bipolar cells sign preserving or inverting?
Sign preserving
Horizontal cells
- At the level between photoreceptors and bipolar cells
- Receive input from surround PRs (PRs in surround area of receptive field) and send output to center PR (PR in center area of receptive field)
- Input of horizontal cells: glutamate (excitatory)
- Output of horizontal cells: GABA (inhibitory)
- These characteristics make center-surround antagonistic RFs possible
Input and output of horizontal cells
- Input: Glutamate (ex. released by surround PRs) → excitatory effect on horizontal cells
- Output: GABA (ex. Released onto
center PR) → inhibitory effect - In other words, a surround PR can affect a center PR through a horizontal cell.
A single PR is not “only surround” or “only center”. The effect of a PR’s response to light depends on the bipolar cell that receives that input
Light on surround PR –> ___
Center PR depolarized
- Light hyperpolarizes surround PR
- As a result, surround PR releases less glutamate (glutamate is excitatory)
onto horizontal cell - As a result, horizontal cell releases
less GABA (GABA is inhibitory) onto
center PR - The end result is that less GABA is
released onto the center PR, causing it to be depolarized.
Darkness on surround PR –> ___
Center PR hyperpolarized
Dark on the surround PRs
- Surround PR releases glutamate (excitatory) onto horizontal cell
- As a result, the horizontal cell releases more GABA (inhibitory) onto the center PR
- The end result is that more GABA is released onto the center PR, causing it to be hyperpolarized
Light on the surround PRs
- Light hyperpolarizes surround PR
- As a result, surround PR releases less glutamate (excitatory) onto horizontal cell
- As a result, horizontal cell releases less GABA (inhibitory) onto center PR
- End result is that less GABA is releases onto the center PR, causing it to be depolarized
Two PRs are connected by a horizontal cell. If I shine a light on PR 1, what cellular effects should I expect from PR 2?
The horizontal cell will release less GABA onto PR 2 than in the no light condition → PR 2 is relatively depolarized.
What happens if we add LIGHT on the PRs in both the center and surround if they are ON-center and OFF-surround RFs
Light in center –> center PR hyperpolarizes (all PRs hyperpolarize to light)
If we add light on the PRs in both the center and surround:
- This results in the depolarization of the center PR due to horizontal cells
- However, because opposing things are happening to the center PR (light in center is hyperpolarizing it vs. light in surround is depolarizing it through the horizontal cell effects), the center will be weakly hyperpolarized
What happens if we add DARK on the PRs in both the center and surround if they are ON-center and OFF-surround RFs
- This results in the hyperpolarization of the center PR due to horizontal cells
- Because the stimulus in the center and stimulus in the surround are doing the same thing (both are hyperpolarizing the center PR), this will result in the strongest possible response
- Because you get an especially strong hyperpolarization of the center PR, this would strongly activate an ON bipolar cell. We call this an ON-center/OFF-surround RF
What combination of center and surround do ON bipolar cells prefer?
ON bipolar cells prefer ON-center/OFF-surround RFs
What happens if we add DARK on the PRs in both the center and surround if they are OFF-center/ON-surround RFs?
Dark in center –> center PR depolarizes (all PRs depolarize in the dark)
If we add dark on the PRs in both the center and surround:
- This results in the hyperpolarization of the center PR due to horizontal cells
- However, because opposing things are happening to the center PR (no light in center is depolarizing it vs. no light in surround is hyperpolarizing it through horizontal cell effects), the response will be weak depolarisation
What happens if we add LIGHT on the PRs in both the center and surround if they are OFF-center/ON-surround RFs?
Now let’s add light on a surround PR:
- This results in the depolarization of the center PR due to horizontal cells
- The stimulus in the center and stimulus in the surround are doing the same thing (both are depolarizing the center PR), resulting in strong depolarization
- In this case, because you get an especially strong depolarization of the center PR, this would strongly activate an OFF bipolar cell. We call a receptive field like this an OFF-center/ON-surround RF
What combination of center and surround do OFF bipolar cells prefer?
OFF bipolar cells prefer OFF-center/ON-surround RFs
What is the purpose of lateral inhibition?
To enhance visual contrast (e.g. the edge between a white surface and black surface)
Lateral inhibition
- Recall: Light in surround depolarizes center PR vs. dark in surround
hyperpolarizes center PR - Preferred stimulus in center and the opposite stimulus in surround
produces best response
○ The bipolar cell gets its input from the center PR
○ ON-center/OFF-surround stimulus in the RF of an ON-bipolar cell results in maximum
hyperpolarization of the center PR and maximum depolarization of the ON-bipolar
cell.
○ OFF-center/ON-surround stimulus in the RF of an OFF-bipolar cell results in
maximum depolarization of the center PR and maximum depolarization of the
OFF-bipolar cell.
Retinal Ganglion Cells (RGCs)
- 92 million rods and 5 million cones → 1.5 million ganglion cell axons
- Affected by glutamate release of bipolar cells
- ON-center bipolar cells release glu onto ON-center ganglion cells
- OFF-center bipolar cells release glu onto OFF-center ganglion cells
- Only cells that fire action potentials! Number of AP based on level of
depolarization from bipolar cells - Like bipolar cells, RGCs have center/surround antagonistic RFs → RGC fires the most APs when the surround and
enter are getting preferred opposing light
input
What are the three types of retinal ganglion cells?
- Magno
- Parvo
- nonM-nonP
Magno cells
- 5% of RGCs
- Large RFs
- Color insensitive
Ventral stream
Parvo LGN –> 4Cβ –> 2/3 –> Occipital/Temporal
Parvo cells
- 90% of RGCs
- Small RFs
- Color sensitive
nonM-nonP cells
- 5% of RGCs
- Small RFs
- Color sensitive
Color opponency
Color opponency includes red-green and blue-yellow opponency
I’m a neuroscientist experimenting with a new drug in my lab. I’ve found that injection of this drug causes constant release of glutamate from cones in the fovea. What effects should I see in an OFF-center retinal ganglion cell?
- The RGC should fire more APs.
- Remember that PRs are depolarized and
release glu in the dark and that OFF-bipolar cells are ionotropic (depolarize to glu). - If the drug is causing an increase in glu being released by the PR onto the OFF-bipolar cell, this will lead to greater depolarization of the OFF-bipolar cell and thus greater depolarization and more APs in the RGC.
Retina Summary
Summary of OFF-center bipolar cells
- Ionotropic receptors that preserve sign of input
- E.g. hyperpolarization in presynaptic cell –> hyperpolarization in post-synaptic cell
Summary of ON-center bipolar cells
- Metabotropic receptors that invert sign of input
- E.g. hyperpolarization in presynaptic cell –> depolarization in post-synaptic cell
Do off-center bipolar cells have ionotropic or metabotropic receptors?
Ionotropic
Do on-center bipolar cells have ionotropic or metabotropic receptors?
Metabotropic
How many LGNs are there?
2, because the thalamus is a bilateral structure
Diagram of LGN
Where does input into the LGN come from?
Only 20% of input is from retina. The other 80% of input (majority) is from V1.
Structure of LGN
- Layers numbered 1 to 6 from ventral to dorsal
Ipsilateral vs. contralateral
○ 1, 4, and 6 get input from contralateral eye
○ 2, 3, and 5 get input from ipsilateral eye (2+3=5, SAME as 5, SAME side)
RGC types in the LGN
- Layers 1 and 2 get magno RGC input (darkest, sink to the bottom)
- Layers 3, 4, 5, and 6 get parvo RGC input
- Koniocellular layers (K1 - K6, ventral to and share same eye input with corresponding LGN layer) get nonM-nonP input
At this level, info from the 3 RGC types is still largely separated.
LGN functions
- Thalamic relay between optic tract and V1
(through optic radiation)
2 firing modes:
- Spiking mode: LGN firing follows
ganglion cell input
- Intrinsic burst mode: LGN cut off from
sensory input
- Dreams in burst mode
- Also plays a role in visual attention:
Only perceive a fraction of input to eye - LGN may gate which activity gets
through to V1 (filter out unattended
visual input)
Dorsal stream
Magno LGN –> 4Ca –> 4B –> Occipital/Parietal
Primary Visual Cortex (V1, Area 17, Striate Cortex) - table
Retinotopy
- Retinotopic projections are examples of topography / topographic maps: Spatial relationships preserved from one region to the next
- In other words, if A projects to B, neighboring areas of A project to neighboring areas of B
- In the specific case of retinotopy, projections preserve relative position in retina
- Different areas can be represented by different amount of cortex: Amount of V1 dedicated to fovea is greatly magnified (reflects higher visual acuity in fovea)
All the following are valid distinctions between magno and parvo visual streams EXCEPT
a) Magno cell bodies in the LGN tend to be bigger than parvo cell bodies
b) Magno cell axons from the LGN make their first synapse in a more superficial layer of
V1 than axons from parvo LGN cells
c) Magno cells are less likely to be color selective
d) Magno processing projects more to ventral cortical areas beyond area V1
d) Magno processing projects more to ventral cortical areas beyond area V1
Which type of retinal ganglion cell is least likely to be color-selective?
a) konio
b) magno
c) nonM-nonP
d) parvo
b) magno
Which lesion would cause complete blindness in your left eye?
a) left optic nerve
b) left optic tract
c) right optic radiation
d) optic chiasm
a) left optic nerve
