The Primary Visual Pathway & Cortex

Question 1

What is the relay between the retina and the cortex?

Answer 1

The dorsal lateral geniculate nucleus (dLGN)

-therefore, it needs to be faithful

Question 2

What are the Receptive fields of dLGN and retinal ganglion cells?

Answer 2

dLGN receptive fields strongly resemble those of retinal ganglion cells

Question 3

How are dLGN receptive fields similar to those of RGCs?

Answer 3

• 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

Question 4

What are the Inputs to the LGN?

Answer 4

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%

Question 5

What are the local inhibitory inputs to the LGN?

Answer 5

They are from LGN inhibitory interneurones which inhibit the LGN cells

Question 6

What are Inhibitory inputs to LGN?

Answer 6

Inhibitory inputs strengthen the surrounds of LGN cells compared to their RGC inputs

Question 7

How did experimenters find that inhibitory inputs to LGN strengthen surround of LGN cells?

Answer 7

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

Question 8

To understand cortical inputs to the LGN, we need to…

Answer 8

know about cortical cell response properties

Question 9

What are the parallel pathways from retina to primary visual cortex?

Answer 9

retina→M and P cells which feed into respective layers in dLGN→ then feeding to respective layers in visual cortex

Question 10

What are V1 cells?

Answer 10

cortical cells of the primary visual area (V1) which is the first stage of cortical processing of visual information

Question 11

What are V1 cells?

Answer 11

cortical cells of the primary visual area (V1) which is the first stage of cortical processing of visual information

Question 12

what are the Properties of V1 cells?

Answer 12

· Unresponsive to flashing spots
· Preferring elongate stimuli
· Orientation tuned
· Direction tuned
· Velocity tuned
· Length tuned

Question 13

V1 cells are classified as…

Answer 13

classified as simple cells or complex cells, depending on receptive fields

Question 14

What are Simple V1 cells?

Answer 14

distinct/separate ‘on’ and ‘off’ zones, like LGN and retina (distinct excitatory and inhibitory regions)

Question 15

what are Complex V1 cells?

Answer 15

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

Question 16

How are simple and complex cells similar?

Answer 16

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

Question 17

How are simple and complex cells similar?

Answer 17

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

Question 18

What are Hypercomplex V1 Cells?

Answer 18

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.

Question 19

What is the Non-primate hypothesis in the 1960s?

Answer 19

Hierarchy scheme of processing:

• Simple cells driven by LGN cells
• Complex cells driven by simple cells
• Hypercomplex cells driven by complex cells

Question 20

Layer 4 simple cells…

Answer 20

inherit their zones from the LGN

Question 21

What is a classical cortical response property?

Answer 21

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

Question 22

What are Orientation columns?

Answer 22

Columns of visual cortical neurones stretching from layer II to layer VI that responds best to the same stimulus orientation

Question 23

What does not fit into the 1960s model?

Answer 23

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

Question 24

Describe Re-evaluation the hierarchical model of receptive field organisation.

Answer 24

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.

Question 25

What is all the other cortical circuitry doing?

Answer 25

More complex stimulus regimes reveal more complex characteristics. Cortical cells respond to discontinuities in the stimulus.

Question 26

What are Corticofugal connections?

Answer 26

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.

Question 27

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?

Answer 27

LGN cell starts to respond like a retinal cell

Question 28

LGN cells show a strong preference for a stimulus that has…

Answer 28

discontinuity (just like in the cortex)

-this discontinuity could be spatial frequency/temporal frequency/orientation etc.

Question 29

What happens if you get rid of the cortical feedback to the LGN?

Answer 29

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

Question 30

Where do LGN afferents terminate?

Answer 30

In blocks known as ocular dominance (OD) columns

Question 31

What is ocular dominance?

Answer 31

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.

Question 32

What areOcular Dominance Columns?

Answer 32

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

Question 33

What do ocular dominance columns play a role in?

Answer 33

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.

Question 34

What is the Binocular visual field?

Answer 34

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

Question 35

What is steropsis?

Answer 35

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

Question 36

How is orientation selectivity arranged?

Answer 36

Like ocular dominance, also in columnar fashion.

Question 37

What is a hypercolumn?

Answer 37

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

Question 38

What is interspersed in hypercolumns?

Answer 38

Interspersed in the hypercolumns are blobs ‐ columns that are not orientation sensitive and appear to process colour information

Question 39

Describe the Blobs in hypercolumns.

Answer 39

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.