2.08 Visual Pathways 2

Layers of the retina from anterior to posterior

1. Internal limiting membrane

2. Nerve fiber layer

3. Ganglion cell layer

4. Inner plexiform layer

5. Inner nuclear layer

6. Outer plexiform layer

7. Outer nuclear layer

8. External limiting membrane

9. Photoreceptor layer

10. Retinal pigment epithelium

What layer of the retina are glanglion cells found

Ganglion cell layer duhh

What layer of the retina do amacrine cells operate

Inner plexiform layer (where bipolar cells and retinal ganglion cells form synapses)

What layer of the retina are horizonal cells found

Inner nuclear layer

What layer of the retina are bipolar cells found

Inner nuclear layer

What layer of the retina are rods and cones found

Photoreceptor layer

How do photoreceptors, bipolar cells and ganglion cells work together

Rods and cones (photoreceptors) respond to light

The bipolar cells process the signal and send a signal to the ganglion cells

Ganglion cells form the output of the retina

These neurones axons form the optic nerve which leaves the retina via the optic disc

What is the visual receptive field of a neuron

The visual receptive field of a neuron is the region of the visual field in which a stimulus can modulate the firing of the neuron

Response of on bipolar cells in the light

On bipolar cells are active in the light

Cones release less/no glutamate (neurotransmitter)

On BP cell has an inhibitory receptor so if theres glutamate it inhibits the BP cell

But since theres no glutamate released, the BP cell is active - its not inhibited so it releases neurotransmitter to ganglion cells

Response of off bipolar cells in light

Off bipolar cells are inactive in light/active in dark

When theres light, cones release less/no glutamate (neurotransmitter)

Off BP cells have an excitatory glutamate receptor

Since no glutamate is released in the light the off bipolar cell receptor is not excited, the BP cell is inacitve and doesnt release any neurotransmitter

Response of on bipolar cells in the dark

On bipolar cell is inactive in the dark

Cones release glutamate

On BP cells have an inhibitory glutamate receptor so since cones release glutamate to BP cell it inhibits the BP cell

BP cell doesnt release glutamate to ganglion cells

Response of off bipolar cells in the dark

Off BP cells are active in the dark

Cones release glutamate in the dark

Off BP cells have exitatory glutamate receptor so since glutamate is released it excites the off BP cell/ makes it active so BP cell releases glutamate to ganglion cell

Which bipolar cells respond to light images against a dark background

On bipolar cells have inhibitory glutamate receptors so they respond to light images against a dark background

Which bipolar cells respond to dark images on a bright background

Off bipolar cells have excitatory glutamate receptors so respond to dark images on a bright backgrouns

On bipolar cells connect with _

ON bipolar cells connect with ON ganglion cells

(And off BP cells connect with OFF ganglion cells)

What do ON and OFF ganglion cells form

They form the output signals from the retina

What do horizontal cells do

They carry out horizontal processing (at the photoreceptor synapse) which helps refine the responses in the ON and OFF pathways

What do amacrine cells do

Increase the quality of signals

Rods only have _ bipolar cells

On

Which cells in the retina generate action potentials

Only ganglion cells

What are the types of ganglion cells

Magnocellular

Parvocellular

Bistratified/koniocellular

(More detail in eye intro 2)

Where does the thalamus transfer sensory information

To the cerebral corticies

What is the main component of the thalamus

The dorsal lateral geniculate nucleus (dLGN/LGN)

Which ganglion cells project to which layers of the LGN

Layers 1 and 2 = magnocellular cells

Layer 3 to 6 = parvocellular cells

Bistratified cells project to cells between the layers called konicellular

Each layer recieves retinal input from only one eye - This means the same visual space is represented multiple times in each LGN

Right visual field = left LGN

Left visual field = right LGN

What is the retintopic map

Nearby cells in each layer of the LGN represent a nearby location in the visual fiels

Where do 90% of retinal outputs terminate

In the dLGN

Where do 10% of retinal outputs terminate

To non visual functions eg circadian rhythm

What is the magnification factor

Fovea has a larger respresentation than peripheral retina (around half the mass of the LGN represents fovea bc it has the highest density of cones)

Parvocellular cells (LGN) :

• cell body size

• Receive input from where

• Receptive fields organisation

• Subtypes

• Chromatic or achromatic

• Receptive field size

• Good or bad Spacial acuity

• Preferred temporal frequency

Magnocellular cells

• cell body size

• Receive input from where

• Receptive fields organisation

• Subtypes

• Chromatic or achromatic

• Receptive field size

• Good or bad visual acuity

• Preferred temporal frequency

Koniocellular cells (LGN)

• cell size

• Input from where

• Function

Where do axons leaving the LGN go

They make up the optic radiations that project (mainly) to the primary visual cortex (striate cortex)

What is the neocortex

The outer layer of the brain

How many layers in the neocortex

6 horizontal layers (but can be subdivided)

what stain is used to see cell bodies

Nissl staining

Cells in layer 1 of the visual cortex

The outer surface is celll sparse (arent many cells)

The axons go to the next cortical layer

Cells in layer 2 of cortex

The external granule layer

Mainly small spherical cells

Cells in layer 3 of cortex

The external pyramidal layer

Variety of cell tyes, mainly pyramidal cells, cells deeper in the layer are larger than the superficial layers

Cells in layer 4 of cortex

(Internal) granule cell layer

Mainly granule cells

Cells in layer 5 of cortex

The internal pyramindal cell layer

Ppyramidal cells - Cells are typically larger than layer 3

Cells in layer 6 of cortex

Polymorphic or multiform layer

Heterogenous mix. ‘Blends’ into white matter

Which layers of the cortex are supragranular

1, 2 and 3

Which layers of the cortex are infragranular

5 and 6

Which layers of the cortex are granular

4

Function of layer 2 and 3 of the cortex

Project info to extrastriate cortex

Recieves input from konicellular cells

Contains colour tuned cells within the CO blobs and orientation tuned cells outside the blobs (both project to V2 separately)

Function of layer 4A of the cortex

Recieves input from magnocellular and parvocellular cells

Function of layer 4B of cortex

Projects info to extrastriate cortex (processes motion)

Associated with motion perception - Contains direction selective cells

Function of layer 4Ca of cortex

Recieves input from magnocellular cells

Info process in 4B

Function of layer 4Cb of cortex

Receives input from parvocellular cells

Info processed in 2/3

Function of layer 5B of cortex

Projects/output to superior colliculus

Function of layer 6 of cortex

Projects/Feedbacks to LGN - Helps modulate responses eg helps you pay more attention to something or enhance response to stimulus

Receives secondary inputs from magnocellular and parcocellular cells

Projects/outputs info to extrastriate cortex

What is retinotopy

the way visual information from the retina is mapped onto areas of the brain, especially the visual cortex. Neighboring parts of the retina connect to neighboring neurons in the brain, preserving the spatial layout

What is ocular dominance

Left eye info is kept separate from right eye info in the LGN

Axons from the different layers of the LGN (each layer only responds to one eye) go to separate ‘zones’ in the cortex, forming ocular dominance columns. Theres equal space devoted to each eye, each about 500 micrometres in width forming a zebra like pattern

Left and right eye info project next to eachother allowing you to compare visual representation from LE+RE for the same location in space - important in depth perception

What is orientation selectivity

Cells in the cortex are most excited when stimulus is at its orientation. Other cells nearby will also prefer a similar orientation

The orientation is the same as you penetrate the cortex vertically (apart from layer 4) but gradually shifts as you walk along horizontally

This property is not found in the LGN or retina

Ice cube model

Orientation columns run perpendicular to ocular dominance columns and orientation columns vary smoothly from location to location

What is a hypercolumn

One block of tissue in the visual cortex will represent both eyes and represent all orientations for one location in the visual space

How do neurons become orientation selective

Concentric receptive fields (CRFs) from several neurons can be combined in a line. When a bar of light with the right orientation hits them, they all activate together. This creates a new neuron with an elongated receptive field that responds best to that bar angle — giving it orientation selectivity. ON and OFF zones in the combined field help it respond to light-dark patterns

The new cell is called a simple cell

What is a simple cell

Cells which have distinct regions that like bright/dark bars

Sensitive to spacial phase

Simple cell properties

• receptive fields

• What are they selective for

• Small RFs → like __ SFs

• Large RFs → like __ SFs

• RFs sizes increase with __

• Sensitive to __

What is a complex cell

Cells that sum up multiple simple cells to produce a receptive fields that has overlapping on and off responses

They can also be produced by summing lots of concentric (LGN-like) RFs together

Complex cell properties

• receptive field size

• Insensitive to what

• Response time

What are hypercomplex cells

Cells that are length tuned - they respond maximally to a defined length and are inhibited if the stimulus is longer than this

Thought to be involved in processing corners

Dont care where bar is or if its black/white

Care about length of bar therefore only respons when the central region is activated at the right orientation

Phase insensitive mixed on and off subregions

Why do we need oriented recepetive fields

Edges define when objects start and end

What is direction selectivity

Cells tuned to the direction the stimulus is moving at

What layer of the cortex is direction selectivity found And where does it project to

Direction selectivity is found in layer 4B

4B projects out of V1 to V2 and also directly to MT (these regions are involved in motion processing)

What is disparity selectivity And wheres it found (depth selectivity)

Selectivity for the difference between the two eyes views

Found mainly in the upper layers

What are interblob cells selective to

Orientation

What are blob cells selective to

Wavelength

They respond the same to all orientations (not orientation selective)

Konicellular cells (blue/yellow) project directly to these blobs

Revised ice cube model

How is information processed in V1

Info is inputted to layer 4

Vertical and horizontal connections between neurons

V: Its then projected upwards and downwards to layers 2/3 and 5 and other sublaminar

Filled pyramindal cells in layer 2/3 - axon collarerals branch off to neighbouring cells

Lateral connections in visual cortex

Lateral connections are like to like. Cells in close layers are connected. Cells connect to cells that are similar eg same orientation

These connections may be involved in grouping responses together

Or they mat play a role in sharpening responses to the orientaiton of a stimulus or to the length of a stimulus or all of the above

What is the dorsal stream

The where pathway

Its involved with processing the objects spatial location relatitve to the viewer (motion and location)

Goes to the prefrontal cortex

What is the ventral stream

The what pathway

Involved with object and visual identification and recognition

Recognising objects, form, colour

Goes to the inferotemporal cortex

Full Dorsal pathway

V1 → V2 → V3 → MT → MST → prefrontal cortex

Full ventral pathway

V1 → V2 → V4 → inferotemporal cortex

What do connections between the ventral and dorsal pathways lead to

Object identification and location

Damage to the ventral pathway can lead to what

Prosopagnosia - cant recgonise people

Damage to the dorsal pathway can lead to what

Optical ataxia - dont know where things are in space

Where do parallel streams from V1 originate from

From layers 2/3 and layer 4B (project info out of V1)

Wher is V2

Surrounds V1

Thick stripes in V2 are selective for

Movement direction

Disparity

Thick stripes in V2 receive input from which layer of V1

4B

Thin stripes in V2 are selective to

Colour

Contrast

Thin stripes in V2 receive input from where in V1

Blobs (layer 2/3)

Interstripes in V2 are selective for

Orientation

Contrast

Interstripes in V2 recieve input from where in V1

Interblobs - layer 2/3

Full V2 thick stripes stream

4B → thick stripes → V3 (depth perception)

Full V2 thin stripes stream

Blobs → thin stripes → V4 (colour and form)

Full V2 interstripes stream

Interblobs → interstripes → V4 (colour and form)

Where does V3 receive input from

Magnocellular cells

Receives inputs from layer 4B in V1 and thick stripes in V2

What is V3 selective for

Has a large number of cells for orientation, direction and disparity

Where does V3 make connections to

V1, V2, V4 and MT

What is V4 associated with

Processing colour vision and form vision

First area to show strong modulation by attention

Where does V4 receive input from

V2 (thin stripes and pale stripes) and direct connections from central V1)

Where does V4 send outputs to

Strong outputs to Posterior inferotemporal cortex (PIT)

Also weaker outputs to MT/V5

What is V4 selective for

Orientation, colour and object features of intermediate complexity

Inferotemporal cortex Properties

Very large receptive fields

All include fovea

May include both hemifields ie input from opposite cortex

Cells tuned to specific shapes

Inferotemporal cortex (IT) selective for

Dont care about the position or size of the object within the recpetive fields

Cells respond to faces

What are face cells

Cells that are responsive to faces and hands

Cells may be more or less responsive according to familiarity

What is MT And where does it get input from

Part of V5 is involved in motion processing and receives mainly magnocellular inputs from V1 directly and indirectly from V2, V3 and V4