The Primary Visual Pathway & Cortex Flashcards
What is the relay between the retina and the cortex?
The dorsal lateral geniculate nucleus (dLGN)
-therefore, it needs to be faithful
What are the Receptive fields of dLGN and retinal ganglion cells?
dLGN receptive fields strongly resemble those of retinal ganglion cells
How are dLGN receptive fields similar to those of RGCs?
- RGC axons make big, powerful ionotropic synapses
- Predominantly on proximal dendrites
- Single RGCs make many connections with single LGN cells, which will be activated synchronously each time the RGC fires
What are the Inputs to the LGN?
The retinal axons however only make up about 7% of the terminal boutons within the LGN. All the rest of the input comes from other places.
- Brainstem inputs (mainly Ach) ~30%
- Local inhibitory cells ~30%
- Primary visual cortex ~30%
What are the local inhibitory inputs to the LGN?
They are from LGN inhibitory interneurones which inhibit the LGN cells
What are Inhibitory inputs to LGN?
Inhibitory inputs strengthen the surrounds of LGN cells compared to their RGC inputs
How did experimenters find that inhibitory inputs to LGN strengthen surround of LGN cells?
When looking at reversal potential, experimenters found indication of GABAa-type inhibition (not just withdrawal of excitation from RGCs), proving:
if there is an on-centre RGC driving an on-centre cell in the LGN, both will have a weak surround
however, if you have an off-centre cell RGC in the surround, and you apply a bright stimulus in the surround to excite it, the off-centre RGC, giving input into an LGN inhibitory interneuron, will excite the LGN inhibitory interneuon, and therefore inhibit the LGN cell
there is enhancement of the receptor field (push-pull arrangement), the LGN inhibitory interneurons strengthen the centre-surround antagonism and therefore the contrast sensitivity system
To understand cortical inputs to the LGN, we need to…
know about cortical cell response properties
What are the parallel pathways from retina to primary visual cortex?
retina→M and P cells which feed into respective layers in dLGN→ then feeding to respective layers in visual cortex
What are V1 cells?
cortical cells of the primary visual area (V1) which is the first stage of cortical processing of visual information
What are V1 cells?
cortical cells of the primary visual area (V1) which is the first stage of cortical processing of visual information
what are the Properties of V1 cells?
· Unresponsive to flashing spots · Preferring elongate stimuli · Orientation tuned · Direction tuned · Velocity tuned · Length tuned
V1 cells are classified as…
classified as simple cells or complex cells, depending on receptive fields
What are Simple V1 cells?
distinct/separate ‘on’ and ‘off’ zones, like LGN and retina (distinct excitatory and inhibitory regions)
what are Complex V1 cells?
give ‘on’ and ‘off’ responses to stimuli throughout their receptive fields (no distinct ‘on’ and ‘off’ zones)
respond equally well to an optimal orientation regardless of where it is within the cell’s receptive field
How are simple and complex cells similar?
They are both orientation selective neurones in V1, meaning they respond optimally to a stimulus with a specific orientation
They respond best to a stimulus with a particular orientation, and then as the orientation gets larger or smaller, the response of the cell decreases
How are simple and complex cells similar?
They are both orientation selective neurones in V1, meaning they respond optimally to a stimulus with a specific orientation
They respond best to a stimulus with a particular orientation, and then as the orientation gets larger or smaller, the response of the cell decreases
What are Hypercomplex V1 Cells?
Cells in the visual cortex that respond to lines (bars, edges) in specific orientations and specific lengths
-short-length tuning, meaning they prefer a short elongate bar than a long one
stimuli that are shorter or longer than the ideal length will produce a reduced response.
What is the Non-primate hypothesis in the 1960s?
Hierarchy scheme of processing:
- Simple cells driven by LGN cells
- Complex cells driven by simple cells
- Hypercomplex cells driven by complex cells
Layer 4 simple cells…
inherit their zones from the LGN
What is a classical cortical response property?
Orientation tuning
- LGN cells expected to respond to all orientations of light, which they do
- with cortical cells, cells respond better at one orientation, less for others and sometimes no response
What are Orientation columns?
Columns of visual cortical neurones stretching from layer II to layer VI that responds best to the same stimulus orientation
What does not fit into the 1960s model?
That cortical cells have no responses to certain orientations
- therefore, for the cell to have no responses to certain orientations, there must be inhibition to get this tight orientation tuning
- inhibition has been shown to create/enhance a wide range of visual selectivities
Describe Re-evaluation the hierarchical model of receptive field organisation.
There is not really a hierarchy. Both simple and complex cells can be “length tuned” and all 3 types (simple, complex and hypercomplex) may be LGN-recipient cells.
What is all the other cortical circuitry doing?
More complex stimulus regimes reveal more complex characteristics. Cortical cells respond to discontinuities in the stimulus.
What are Corticofugal connections?
Cortical inputs to the LGN create “cortex-like” properties.
The hypercomplex characteristic is found in the dLGN because the cortex is telling it to (but not in the retina).
One function of the corticofugal feedback is to generate length tuning in the dLGN.
What happens if you inhibit layer 6 of the visual cortex at a retinotopic location and you get rid of the cortical input to the LGN cell?
LGN cell starts to respond like a retinal cell
LGN cells show a strong preference for a stimulus that has…
discontinuity (just like in the cortex)
-this discontinuity could be spatial frequency/temporal frequency/orientation etc.
What happens if you get rid of the cortical feedback to the LGN?
The inhibition disappears
- therefore, the LGN is considered a part of the cortical circuit, and not just a faithful relay
- it is actually taking a very active part in carving out these stimulus response properties
Where do LGN afferents terminate?
In blocks known as ocular dominance (OD) columns
What is ocular dominance?
Ocular dominance is the property of the receptive fields of striate cortex neurones by which they respond more vigorously when a stimulus is presented in one eye than when it is presented in the other.
What areOcular Dominance Columns?
Striped shape regions of neurones in V1, all of which respond preferentially to input/stimuli from one eye or the other
The columns span multiple cortical layers and the stripes lie perpendicular to the orientation columns.
The columns are innervated by inputs from the LGN into cortical layer 4
What do ocular dominance columns play a role in?
Binocular vision
Each neurone in layer 4 of V1 receives afferents from a layer of the LGN representing the left or right eye. These neurones are monocular, responding to light only in one of the eyes.
But, axons leaving layer 4 diverge and innervate more superficial cortical layers, mixing the inputs from the two eyes. Therefore, most neurones in layers superficial to layer 4 are binocular, responding to light in either eye.
What is the Binocular visual field?
the portion of the visual field viewed by both eyes
Animal having 2 eyes is able to perceive a single 3D image of its surroundings:
- object closer than a fixation point → focused on slightly mismatched locations
- object further than fixation point → reverse
What is steropsis?
the impression of depth that is perceived when a scene is viewed with someone with normal binocular vision
binocular viewing creates 2 different images of the scene in 2 eyes (because of the different positions of the eyes in the head) → binocular disparity
How is orientation selectivity arranged?
Like ocular dominance, also in columnar fashion.
What is a hypercolumn?
2 sets of columns (full set of orientation and ocular dominance columns), each covering every possible orientation (0-180 degrees) with 1 set preferring input from the left eye and 1 set preferring input from the right eye
What is interspersed in hypercolumns?
Interspersed in the hypercolumns are blobs ‐ columns that are not orientation sensitive and appear to process colour information
Describe the Blobs in hypercolumns.
Blobs in hypercolumns are found in the cortex and are regions of heightened metabolic activity. They are strung along the centres of the ocular dominance columns, forming the pin wheels in the orientation columnar system. They receive koniocellular LGN input - cells tend to code for colour.
