The Primary Visual Pathway and Cortex Flashcards
(24 cards)
What does the lateral geniculate nucleus connect?
The retina and the cortex
What doe dLGN receptive fields resemble?
Retinal ganglion cells
How do dLGN receptive fields resemble retinal ganglion receptive fields?
- Retinal ganglion cell axons make big, powerful ionotropic synapses
- They are predominantly on proximal dendrites
- Single RGCs make multiple connections with single LGN cells which will be activated synchronously each time the RGC fires
What percentage of RGCs make the terminal boutons in the LGN?
7%
What do the brainstem inputs do?
- Controls state of a system including the sleep-wake cycle
- These inputs send the thalamus to sleep
- Also switch up the gain of responses in the LGN
What are the 2 primary types of ganglion cell receptive fields?
- ON centre/OFF surround:
- flashing bright spot in the centre subregion increases the cell’s response. Flashing bright annulus in the surround subregion inhibits the cell’s response. There is little to no response to a large spot of light that covers both the centre and the surround = lateral inhibition
- OFF centre/ON surround:
- gets inhibition form a small spot of light in the centre and excitation from an annulus in the surround
What type of cells can be found in the primary visual cortex?
Simple and complex cells
What are simple cells?
- Cells which have separate zones that respond to light or dark
- The do not respond to flashing spots but will respond to an elongated bar
- If you flash on and off in some places, you get an on response but in other places, you get an off response
What are complex cells?
- On and off response wherever you flash bar
How are V1 cells classically described?
- Unresponsive to flashing spots
- Prefer elongated stimuli
- Orientation tuned - orientation of elongation matters
- Direction tune - might respond in one direction but not another
- Velocity tuned - might prefer slower or faster stimuli
- Length tuned - means the cell will only respond to a very short stimulus
What are the benefits of binocular vision?
- Allows 3D vision
- Increased depth perception
- Able to use only one eye in case other one is damaged or blinded
What is the hypercolumn
A therotecial cube of cortex containing a full det of orientaiton and ocular dominance columns and function as a unit that enables us to see one specific portion of the visual field
inputs to the LGN
- Retinal GCs: ~7%
- Brainstem inputs (mainly Ach): ~30%
* Modulates the thalamus’ activity e.g. sleep vs awake, concentrating vs - Local inhibitory cells: ~30%
- Primary visual cortex: ~30%
* Where the LGN is projecting to
what are Receptive field subregions:
: The area within the receptive field is subdivided into two regions, center and surround.
what is the action of the inhibitory cell inputs
Recorded intracellular activity from LGN cells
They stimulated the LGN cells by either a spot on the centre of the receptive field or by not stimulating the centre
but the surrounding area (annulus)
Blue: ↓ illumination
Red: ↑ illumination
Just like in the retina…
Red spot (↑L) –> depolarised the cell
Blue spot (↓L)–> hyperpolarised the cell
On centre cell, bright surround is inhibitory
On centre cell, dark surround is excitatory\
On-centre cell in the retina driving an on-centre cell in the LGN
Light in the centre will excite the on-centre cell–> excite the cell in the LGN
Light in the surround will excite an off-centre cell—> gives input to an inhibitory interneurone–> inhibits the LGN
cell
what does the reversal potential indicate
GABAa-type inhibition adn not just withdrawal of excitation
- Inhibitory inputs strengthen the surrounds of LGN cells c.f. their RGC inputs
* If there’s no contrast in the image then the LGN cells do not respond
what are the V1 cells
V1 (primary visual cortex) cells are classically described as:
▪ Unresponsive to flashing spots
▪ Prefer elongate stimuli
▪ Orientation tuned
▪ Direction tuned
▪ Velocity tuned
▪ Length tune
what is orientation tuning
Orientation tuning is a classical cortical response property
* LGN cells, with a concentric receptive field pattern, can respond at all orientations of a bar of light
* Cortical cells respond best to certain orientations
what is the corticofugal
Corticofugal= a nerve fibre that originates in and runs from the cerebral cortex
* One function of corticofugal feedback is to generate length tuning in the dLGN
LGN input to the primary visual cortex and ocular dominance (OD) columns
One of the eyes is injected with a radioactive tracer
The tracer is picked up by the projection cells and travels to the cortex
* The tracer shows that in the cortex, the LGN afferents terminate in blocks- ~500 microns wide, that aren’t
labelled, where inputs from the other eye comes in
vertical arrangement of cells that all responded to cells in the same orientation in the same retinal location
Inputs from the 2 eyes go to the LGN
Then to the cortex
They terminate alternately in the cortex
Cells sitting directly above these blocks are monocular
In the junction between the 2 blocks are cells which are binocular (get input from both eyes)
binocular visual fields
Both eyes have the same fixation point
Blue area= visual field
Object sitting at same point as fixation point
* i.e. if you look at the position in the retina that the object is focused on, the distance is the same
* The object is being focused onto matched locations in the 2 eyes
Object sitting closer than the fixation point
* The image will be focused on slightly mismatched locations
Object sitting further away than the fixation point
* The image will be focused on slightly mismatched locations
what are the Perceptual benefits of binocular fields
- Allows 3D vision
- Increased depth perception
- able to use only one eye in case the other one is damaged or blinded
what is a hyperccolumn
Hypercolumn: a theoretical cube of cortex, 1mm x 1mm in surface area, containing a full set of orientation & ocular
dominance columns with one set preferring input from the left eye and one set preferring input from the right e
ocular dominance columns
Ocular dominance columns alternate systematically between left eye and right eye dominance.
Orientation columns change systematically across orientations (Hubel & Wiesel, 1962). When ocular dominance
columns and orientation columns are combined, they form something that Hubel and Wiesel called a hypercolumn.
A hypercolumn is a 1 mm block of V1 containing both the ocular dominance and orientation columns for a particular
region in visual space.
