Vision 2: Central Processing of Vision

Vision 2: Central Processing of Vision

1. Example Case Study: David

   • 67 yrs old, noticed a translucent white sustain obscuring the upper part of his vision while driving, felt his motion perception was off, couldn't recognise a friend working next to him or his wife and family until they spoke
     • Visual fields indicate which part of the brain is affected due to patterns produced in the test
     • Diagnosis is a lesion affecting the fusiform gyrus (area IT) which also used a visual field defect

2. Ganglion Cells

   • Output neurons of the retina
     • M (parasol) ganglion cells (5%)
       • Magnocellular = large
       • Large receptive fields
       • Motion detection, flicker and analysis of gross features
     • P (midget) ganglion cells
       • Parvocellular = small
       • More numerous (discrimination of what we can see; fine detail)
       • Visual acuity and colour vision
   • Output targets of ganglion cells
     • Many brain regions
     • Majority target the lateral geniculate nucleus (thalamus)

3. Optic Chaism

   • Where the fibres from right an left optic nerves combine (cross)
   • Lies at the base of the brain, anterior to the pituitary
   • Allows information from corresponding points on the two retina to be processed by approx. the same cortical site in each hemisphere

4. The visual pathway

   • Nasal fibres cross at the Optic chasm where as the temporal fibres do not
   • The RIGHT visual hemifield is 'viewed' by the LEFT hemisphere
     • RVH = LEFT nasal + RIGHT temporal
   • The LEFT visual hemifield is 'viewed' by the RIGHT hemisphere
     • LVH = RIGHT nasal + LEFT temporal

5. Lateral Geniculate Nucleus (LGN)

   • Major target in the diencephalon of the thalamus
     • Neutrons send their axons to the cerebral cortex via the internal capsule (optic radiation)
   • Receive a functional stem of information
   • Constitutes 6 layers (numbered 1-6)
     • Magnocellular layers = layers 1, 2
       • Receive input from M (parasol) ganglion cells
       • Cells are bigger
     • Parvocellular layers = layers 3-6
       • Receive input from P (midget) ganglion cells

6. The Primary Visual cortex (V1)

   • Each half of the visual field is represented on the CONTRALATERAL visual cortex
     • The most back part of the cortex is important for central vision
   • LGN neurons project to the PVC
     • Area 17
     • Occipital lobe around the calcimine fissure
     • Input is into Layer 4C (region of cortex that receives input from the thalamus)
       • Layer 4 neutrons send they axons to other cortical layers; it is at this stage that the information from the two eyes converges onto individual neurons
     • Input from the LGN is segregated into small regions of the PVC called an ocular dominance column
       • Although the LGN receives inputs from both eyes
   • Retinotopic organization: neighbouring cells within the retina project to neighbouring cells in the LGN and VC

7. Example Case Study: Sarah

   • 43 yrs old, having difficulty crossing the road (cars appear out of 'nowhere', during coffee is very difficult (appears frozen until cup overflows), visual acuity and colour vision are normal, and visual fields is normal

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