Central Visual System

Question 1

Process of Visual Pathway

Answer 1

• Retina (Extracts information from visual field) -> CNS (analyses + interprets -> picture)
• Retina -> Lateral Geniculate Nucleus -> Visual Cortex (Area 17, V1 or Striate Cortex) -> O/T/P lobes for specialised analysis

Question 2

Retinal Input: describe

• Rentinalfugal projection :

Answer 2

Rentinalfugal projection : ‘fleeing of retina’ (from eye -> brain stem)

• Rentinalfugal projection : ‘fleeing of retina’ (from eye -> brain stem)
• Optic nerve -> Optic chiasm -> optic tract (N->C->T) (Nice Chamomille Tea)
  
  • Optic chiasm = decussation/ crossing of optic nerves (@ bottom of brain infront of dangling pituitary gland)
  • Optic tract = proceeds after crossing of nerve which forms chiasm, runs under pia along lateral sides of diencephalon

Question 3

• Visual Hemifields (VH)

Answer 3

• Hemifields= portion of perception viewed by each eye, separated by midline there is a right and left hemifield.
  
  • Binocular vision = middle of hemifields viewed by both eyes.
  • Left visual hemifield -> viewed @ Nasal retina (left eye) + Temporal Retina (Right eye)
• Nasal retina’s ganglion cell axons -> cross @ optic chiasm
  
  • Significance of chiasm = crossing allows: left VH is viewed by right brain

Question 4

• Optic Tract Targets

Answer 4

• OT’s axons split off and go on their own little journeys -> some go to meet and form synaptic connections with cells @hypothalamus, midbrain (10% will innervate this area) but mostly go to dorsal thalamus to meet cells at LATERAL GENICULATE NUCLEUS
• Ganglion cells from retina can have axons that go on non thalamus journeys (innervates places other than LGN):
  
  • Hypothalamus: sleep n wake cycles, tells you whether light or dark
  • Pretectum (part of mid brain): help control pupil’s size and some eye movements
  • Superior colliculus (part od midbrain tectum, 10%): helps move fovea to objects in view (has neurons connecting to motor neurons of brain stem), you cant see everything, so you need to move to orient to new stimuli and Sc helps this to happen, so you can focus on object to detect whether threat or not
• Optic radiation: p/w of LGN’s axons goin to cortex

Question 5

Lesions to Retinofugal projection

Answer 5

• Lesions to Retinofugal projection ( Optic NGT -> LGN -> cortex (optic radiation) = partial or full blindness to visual field
  
  • Damage to nerve = damage on SAME SIDE
  • Damage to optic tract = blindness to OPPOSITE side of visual field
  • Damage to chiasm = BOTH SIDE affect fibers which cross which located @ NASAL retinas (responsible for PERIPHERAL vision of both eyes) therefore -> blindness of visual field received
  • by nasal retina

Question 6

Optic tracts end goal target is?

Answer 6

• Optic tracts end goal target -> left and right LGN

Question 7

LGN layout looks like what?

Answer 7

• LGN separated in layers, sits like pancake on top of each other, responsible for different aspects of visal perception

Question 8

Role & Input of Lateral Geniculate Nucleus

Answer 8

• Information processed in PARALLEL = because of layers
• Receives input from not just retina!
  
  • Thalamus parts and
  • Brain stem- alertness and attention (startled by flash of light)
  • 80% excitatory synapse @ primary visual cotex (talking back and forth between Primary visual cortex and LGN - top down (PVC -> LGN), bottom up (LGN -> PVC)
  • Monocular vision= input not yet together

Question 9

magnitude of LGN’s response depend on…

Answer 9

• Brain stem neurons can affect magnitude of LGN’s response

Question 10

Layers of Lateral Geniculate Nucleus

Answer 10

• 2 ventral= large cells (magnocellular LGN layers)
  
  • Where Retina’s magnocellular/ M type ganglion cells project into
• 4 dorsal= small cells (parvocellular LGN layers)
  
  • Where Retina’s P type cells project into
• Konio cellular LGN layers= heaps of tiny neurons sitting in front of each layer ( AKA K1- K6)
  
  • Receives info from retina’s NON M and non P ganglion cells -> projects to visual cortex
• Receptive fields
  
  • Cells respond to stimulus within a field around them, how big. Cells in retina are compared and found that their identical to LGN.
  • Magnocellular = large cells = large receptive field
• Cells from retina interact with corresponding cells @ LGN
• Ipsilateral = no crossing over
• Non retinal inputs LGN
  
  • Main info is coming back from SC
  • ~ How you perceive what you see is dependant on if you have seen it before.
  • Brain influences what activities are occuring, not causing activity. ~ telling you to stop looking at things, emotionally detected

Question 11

Where does LGN go to?

Answer 11

• GN goes to striate cortex/ Area 17/ Brodmann’s area/ PVC ~ 30 mm thick

Question 12

The striate cortex AKA?

Answer 12

Area 17/ Brodmann’s area/ PVC ~ 30 mm thick

• @ medial surface of brain, surrounded by fissure (calcarine)
  *

Question 13

Retinopy is….?

Answer 13

• Retinopy = Retina -> LGN -> V1/ SC
  
  • Maps of visual field -> target (V1)
  • Fovea (central vision) over represented on map bc more ganglion cells.here compared to periphery
  • Light = activates lots of cells in target region due to overlapping of fields received
  • Perception= brain interprets pattern distribution (not map)
  • If you look to number of cells next to sensual visual field (you have lots of cells that will respond to that stimulus) ~ retina cells = same.

Question 14

• describe the Lamination of Striate Cortex

Answer 14

• Neocortex + Sc= 6 layers
• Layer 4=4 ABC layers
  
  • Layer 4C= a/b layers

Question 15

• Inputs to Striate cortex

Answer 15

• LGN Mags -> 4C-a
• LGN pavs -> 4C-b
• LGN kinocellular axons (can bypass layer 4 which is really important for motion and shape wherease 1,2,3 = wavelength sensitive/ colour) -> 1,2,3 layer

Question 16

• Intracortical connection patterns from layer 4C

Answer 16

• Radial = maintains retinopic organisation coming from layer 4

Horizontal C= layer 3

Question 17

Ocular dominance columns

Answer 17

= thick bands of cells radiating @ SC in alternation. Layer 4 is striped

Question 18

• where does binocular occcur?

Answer 18

• 2 eyes = mixed information
  
  • Binocular neurons @ SC in layer 3 (NOT layer 4)
  • LAYER 3 = 3 mono-> binocular.
    
    • Regions in layer 3 (L and R ocular dominance) where you see OVERLAP of information

Question 19

• SC has different projections to where?

Answer 19

Question 20

Cytochrome oxidase blobs

Answer 20

• = mitochondrial enzyme for metabolism viewed as pillars/ ocular dominance colum @ SC (layer IV , centered)
  
  • Gets info @ koniocellular layers @ LGN & 4C (mags and pavs layers of SC)
  • These blobs vary in layer, they form pillars together
• Nldt input @ koniocellular from LGN into these blobs. Activity = in pillars (ocular dominance colums) = colours= not mixed. Can see motion, shapes, so colour bothe sides (no mixing yet)

Question 21

• Physiology of SC and relation to monocular/binocular vision

Answer 21

Question 22

Visual Selectivity

Answer 22

• Orientation selectivity
  
  • Different orientation of stimulus= different responses
  • V1 Neurons can have optimal orientation to certain angle of stimuli
  • Centre surround (receptive field not perfect round but approximate centre point)
    
    • Stimulus-> cell stimulus measured (stop/ elicit)
• Direction Selectivity
  
  • Neurons fire in relation to moving bar of light (optimally oriented light -> perpendicular movement to orientation in one direction)
• Simple vs Complex receptive fields of V1= binocular, sensitive to orientation stimulus (~ colour and direction of motion ~ diff neurons sensitivity)
• Bob receptive fields for colour
  
  • Monocular
  • Can’t detect direction
  • ~ orientation + colour opponency
  • Not sure about inter blob function
    
    • They have variety of colour opponency and have surround org (cells most responsive to ~ Blue/ wavelngth are surronded by cells with same interest
    • Blog/ kinocell responsive to colour NOT MOTION, DIRECTION OR SHAPE
• Simple cell receptive field. Lots of cell with surround receptive field combine -> provide for one neuron

Question 23

• Parallel Pathways of visual system

Answer 23

3 pathways

• Mags: Mtypes @ retina -> LGN mags -> V1 (layer 4C-a -> 4B)
• Pavs- P type @ Retina ->LGN Pavs -> IVC B -> INTER BLOB= SHAPE
• Blobs: Non M and P @ Retina -> LGN koniocellular layer = COLOUR
  
  • Colour not important= can survive without.

Question 24

~Cortical modules

Answer 24

• Striate cortex is made up of 1000 modules. Blocks of V1 neurons which analyse form and colour. If you remove this, it will make a blind spot.
• 2x2mm of thick chunck of SC looks like:
• Ocular dominance columns, combined. Blobs in each layer, cells surrounding blobs = parvo
• Selective orientation in column
• If you shine light on right eye, most responsive cells are lined up etc

Question 25

Ventral + Dorsal Streams (input -> perception)

Answer 25

Process of Visual Pathway

• Retina (Extracts information from visual field) -> CNS (analyses + interprets -> picture)

Beyond the Striate Cortex (first area where information is received from LGN) there is another ~2 dozen extra striate regions of cortex.

• Dorsal Stream: @ cells direction
  
  • Info @ V1 -> V2 and V3 -> Dorsal or ventral stream
  • Dorsal stream = Area MT (V5/ middle temporal)= percieves motion
    
    • Is innervated by cells from the striate cortex (layer 4B) which are transient, large receptive fields, direction sensitive
  • Beyond MT= Parietal lobe (e.g. medial superior temporal) can also have cells detecting LINEAR motion, RADIAL motion and circular motion.
    
    • MST still not fully known believe it has to do with ~ navigation, directing movement of eyes and perception of movements
  • Damage to MT or MST= no motion percieved= like seeing the world in photos!
• Ventral = Visual, most parvo cells start project to temp regions- memory thus important place for colour and shape
  
  • Continues @ V1, V2 and V3, running front towards temporal lobes= responsible for VISUAL looks
  • Area V4 = most studied of visual pathway.
    
    • blob and interblob areas @ striate cortex -> relay infor @ V2 -> info goes to V4
    • Damage to v4= no colour or shape, grey banana (achromatopsia )
    • Called V4 recieves info @ SC
  • Area IT=
  • Further than V4 of ventral stream, below temporal lobe
    
    • Stimulated by many colours and abstract shapes
    • Damage= prosopagnosia- forget faces

Question 26

Visual perception

Answer 26

• Meaning to object
• Heirarchy of complex receptive fields
  
  • Retinal cells = centre surround to light etc
  • LGN = relay centre btw retina and SC
  • In SC we exhibit orientation shape and direction selectivity + BINOCULARITY
  • Extrastriate (dorsal and ventral ) = more complex perception like face
  • Retinal ganglion = small centre surround –> layer 3 SC recptive fields overlap, cells with diff RF input together -> extra striate = input from 10000000’s cell = complexity of receptive fields.

Parallel process = when 3 pathways and monocular pathways until layer 3 SC, when you meld together in dorsal and ventral streams when no colour have been put to image (ventral stream before it hits these regions is COLOUR BLIND) like we process image (no detail first) and then other details like colour is added :