Lecture 4 - the retina, LGN and visual pathways

Question 1

Problems for the eye and brain

Answer 1

Resolution:
-If whole retina was high-resolution the blind spot would take up most of the retina = so only small part is high-resolution

Energy:
-If all cells in retina were active all the time the amount of energy required would be huge

Question 2

Rece[tive fields

Answer 2

• No retinal cell sees all the image
• Each one only gets light from small part of space = receptive fields = responds to small proportion of sensory environment
• Photoreceptors respond when light falls on receptive field –> causes AP in retinal ganglion cells, whose axons from the optic nerve
• Single ganglion cell only sees small proportion of visual field

Question 3

Retinal ganglion cells - the output of the eye

Answer 3

• Only info from retina ganglion cells is relayed to the brain
• The axons transmit info which forms the optic nerve –> relays info to visual pathway
• Brain only has access to retinal ganglion cells and doesn’t have direct access to photoreceptors

Question 4

Ganglion cells

Answer 4

• Classified in terms of the way the receptive field is configured
• On centre retinal ganglion cells = type of cell where centre portion is recieving info from small population of rods and cones = turn on retinal ganglion cell when activated
• Surrounding proportion of receptive field of this cell will turn off retinal ganglion cell = constantly receiving info from 2 populations of rods and cones (one off and one on)

Question 5

Retinal output signalling changes over space = edges

Answer 5

• Sensitive to edges and borders
• Info being relayed out of retinal ganglion cells is in simple black and white picture consisting of edges
• Higher visual pathways add colour

Question 6

Consequences of only edges

Answer 6

• Ganglion cells only respond to edges
• Image containing only edges passes through retinal unchanged
• Normal image gets converted to edges
• End result = output of retina will look much the same for 2 images
• Rest of brain fills in missing areas so they look more similar than they really are

Question 7

Signalling changes over time and space

Answer 7

• Inhibition over time causes neurones to reduce their activity = save energy
• Lack of crisp edges means spatial location not signalled well
• Reduced neural signal due to inhibition and lack of crisp edges allows filling in of grey regions by cortex
• Troxler fading = if stimulus stays on for long time will appear to fade away
• When stimulus removes time lag on inhibition it produces a negative after image

Question 8

Inhibition over time results in

Answer 8

• Troxler fading
• After images
• People saying opposite of what they have looked at
• Disappearance of stimuli

Question 9

What is the LGN

Answer 9

• Relay centre in brain where info is coming out eye via optic nerve –> enters LGN before output from LGN relayed to visual cortical regions
• Complex structure
• Info from eyes being sorted into certain portions of the lGN
• Each layer contains complete retinotopic representation of half visual field
• Bi-lateral and contralateral structure
• But each LGN receives info from both eyes

Question 10

3 types of cells in LGN

Answer 10

Magnocellular:

• Large receptive field
• Important for detecting motion and depth
• Respond fast to stimulation
• Receive info from retinal ganglion cells that reside in periphery of retina = useful because want to be aware of things approaching quickly
• Respond to coarse details

Parvocellular:

• Smaller receptive fields
• Sensitive to red and green
• Receive info from retinal ganglion cells that sit in centre of retina
• Responds to fine details

Koniocellular cells:
-Recede info about blue and yellow wavelengths

• LGN has alternative portions of Parvo and Magno cellular cells
• Layers 1 and 2 = magno
• Layers 3-6 = Parvo
• Koniocellular = sits between layers

Question 11

How retinal ganglion cells travel

Answer 11

• Output from these cells starts to go towards high visual processing areas
• Before reach cortex reach sub-cortical structures called LGN

Question 12

The visual pathways

Answer 12

-After LGN organisation of info starts to accumulate in higher visual processing areas

Ventral processing stream:

• WHAT pathway
• Along temporal part of brain
• Tells you what an object is
• V1, V2, V4, V8, occipital face area, fusiform face area, parahippocampal place area

Dorsal processing stream:

• WHERE pathway
• States where something is in space
• Gets relayed to parts of the parietal cortex
• V1, V2, V3, V5, V6, V7