Lecture 3 - Visual cortex

Question 1

What is the visual pathway?

Answer 1

• Retinal ganglion cells produce action potentials that project visual information through the optic nerve
• Projections from the contralateral hemifield switch sides at the optic chiasm
• The optic tract projects to the lateral geniculate nucleus in the thalamus
• From there, optic radiations project to the visual cortex

Question 2

What is the lateral geniculate nucleus (LGN)?

Answer 2

• It’s part of the thalamus, a hugely important relay station within the brain (receives info from eyes and sends to visual cortex)
• 80% of axons from RCGs project to the LGN, the rets mostly go to the superior colliculus (eye movement) and hypothalamus (circadian rhythm)
• The LGN has 6 layers
  a) Cells in layer 1&2 are larger than other layers and get unput from M cells (magnocellular layers)
  b) Cells in layers 3-6 are smaller and receive inputs from P cells (parvocellular layers)
  c) Cells with inputs from K cells are sandwiched between the M and P layers

Question 3

The LGN connections and different fibres

Answer 3

• Different layers will also get inputs from different eyes
• The optic tract (after optic chiasm) has nerve fibres from both eyes
• Ipsilateral fibres (have not crossed) input to the LGN in layers 2,3,5
• Contralateral fibres (have crossed) input to layers 1,4,6
• Each hemisphere in the brain has an LGN, so the eye provides contralateral input to one LGN and ipsilateral input to the other

Question 4

What is LGN retinotopy?

Answer 4

• Each layer of the LGN is retinotopically organised
• Retinotopically = neighbouring areas of the retina (in your eye) are represented in the neighbouring regions of each layer of the LGN (i.e. two light points close together in real world, the LGN neurons that respond will also be physically close to each other)
• The spatial relations of the retina are therefore preserved (retinotopic map)
• Each of the 6 LGN layers has a retinotopic map with the matching regions stacked on top of each other. So, the same retinal region is represented in the same location within each layer
• LGN cells are sensitive to specific region of visual space

Question 5

LGN cells receptive fields and subdivision

Answer 5

• Cells in the LGN have receptive fields too
• These RFs are circular in shape, with centre-surround configuration and two opposing types (ON/OFF)
• Both are found in magnocellular and parvocellular layers
• The inhibitory influence of the surround is stronger in RGC RFs, this amplifies differences between neighbouring regions of the RGC RF
• This subdivision of input to the LGN (M vs P) suggests a sub-division in visual function
  1. Colour – p cells are colour sensitive. M cells respond to all colours/ not colour sensitive
  2. Acuity – LGN RF sizes vary in each layer, the smallest devoted to the fovea. The largest area in M sirs, so best spatial resolution in P layers
  3. Temporal sensitivity – M layer cells respond well to rapid change in light intensity, P cells are slower the respond. M cells more sensitive to motion

Question 6

What is the structure of the visual cortex?

Answer 6

• The LGN projects to the primary visual cortex (V1) via optic radiations (V1 aka striate cortex)
• V1 has about 100m cells per hemisphere and is organised in 6 layers
• LGN input comes into V1 at layer 4 (magnocellular in upper layer 4, parvocellular with lower layer 4)
• They then connect to upper and lower layers
• K cells go straight to layers 1-3

Question 7

What are ocular dominance columns? and its importance

Answer 7

• Cells in layer 4 are driven by the input from one eye only
• If a block of cells receive input from right eye the cells above and below will also receive input from right eye
• BUT adjacent cells either side will receive input from opposite eye (creating zebra stripes of right eye, left eye, right eye, left eye, etc)
• This creates a pattern of ocular dominance columns that penetrate perpendicular to the surface. They are organised in patterns visible to staining
• Helps brain compare images from each eye and create depth perception

Question 8

What is visual cortex retinotopy?

Answer 8

• Like LGN, adjacent regions of retina are mapped onto adjacent regions of the cortex – the retinotopic map is maintained
• However, the distribution of cells associated with each retinal region is distorted
• This disproportionate weighting of cortical power is referred to as cortical magnification
• This mirrors how the vast majority of RCG’s are devoted to the fovea

Question 9

What are the functional properties of cortical cells that are similar and different to RGC/LGN

Answer 9

similarities: they maintain reinotopic map, arent particularly sensitive to illumination level, respons best to abrupt changes in luminance (lies, bars)

difference: selectivity to orientation, sensitive to size in a diff way, can be binocular, more sensitive to colour, sensitive to direction of motion

Question 10

What is orientation selectivity in cortical cells and how does this work?

Answer 10

• Most cortical cells have marked preference for particular orientations
• Cortical RFs are organised and shaped different so that they obtain a maximum response to a line of specific orientation
• This selectivity of cortical neurons can be demonstrated using cortical adaption
  1. Adaptation stage: obliquely oriented gratings are presented at a rapidly changing phase
  2. Static stage: just a pair of grating pointing straight up/down
• Task: focus on the cross in between the gratings and they will appear to orient in the opposite direction to the adaptors
  How does this work?
• The adaptors are stimulating the cells tuned to that orientation and following adaptation, the sensitivity of the active cells is reduced, changing the response of the population to new stimuli
• The perception of the static stimuli is distorted
• Staining can tell us about the orientation preference of an array of cortical cells

Question 11

Size and location for 3 types of cortical cells

Answer 11

• Cortical cells come in different types, some sensitive to location and some to size
  1. Simple cell: optimum response to an appropriately oriented stimulus and a certain position within the RF (phase sensitive)
  2. Complex cell: optimum response to an appropriate oriented stimulus placed anywhere within the RF (phase insensitive)
  3. Hypercomplex cell: optimum response depends not only on orientation but also on contour length. Maximum response occurs when the bar length matches the width of the receptive field (“end stopping” or “length-width inhibition”)

Question 12

Binocularity in cortical cells

Answer 12

• Cells in V1 layer 4 are monocular
• Other layers are binocular (they can be driven by either eye)
• If a visual stimulus is delivered to each eye in turn, the cell will respond
• If the same stimulus is the delivered to both eyes, the response is more vigorous
• Binocular cells have two RFs (LE & RE): they are matched in type and respond to similar preferred orientations, locations and directions of motion

Question 13

Colour selectivity in cortical cells

Answer 13

• Colour sensitive cells are concentrated in the cortical blobs
• Each blob is centred on an ocular dominance column
• Within a blob, cells will either have red/green opponency or blue/yellow opponency (these are not mixed in a single blob)
• Blobs receive their input form lower layer 4 (parvocellular LGN layers)
• These cells show no preference for a particular orientation, unlike, most other V1 cells

Question 14

Direction selectivity in cortical cells

Answer 14

• A large proportion of cortical cells display preferences for stimuli moving in a particular direction
• Motion-sensitive cells usually respond only to one direction (direction selectivity)
• Simple cells respond to slow motion, complex cells respond to faster motion

Question 15

Columns and hyper columns: advantages

Answer 15

• The visual cortex is composed of columns of cells. Each column consists of cells with the same orientation preference & ocular dominance preference
• A set of 18-20 columns transverse a complete range of orientations and ocular dominance (hyper column)
• This structural arrangement is known as the ice cube model
• Each hyper column contains the neural machinery required to simultaneously analyse multiple attributes of an image (i.e. size, colour, direction of motion) falling on a localised region of the retina
• Advantage: minimises distance between neurons that respond to similar stimulus properties to maximise processing speed. Clustering of neurons in groups allows brain to minimise number of neurons required to analyse different characteristics of a visual stimulus