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.
