Visual Perception

1
Visual Perception

• Neural Pathways
• Retina
• Photorecpetors
• Rods
• Cones
• Middle layer
• Bipolar cells
• Horizontal cells
• Amacrine cells
• Ganglion cells
• Fovea

2
Visual Perception

• Neural Pathways
• Retina
• Optic Nerve
• Visual half fields
• Optic chiasm
• Retino-geniculo-cortical pathway
• Lateral geniculate nucleus of thalamus (90)
• Primary visual cortex (V1, Area 17) via Optic
  radiations
• Retino-collicular pathway
• Superior colliculus (10)
• Pulvinar nucleus of thalamus

3
Visual Perception

• Organization of LGN
• 6 layers
• 1, 4, 6 Contralateral nasal hemiretina
• 2, 3, 5 Ipsilateral temporal hemiretina

4
Visual Perception

• Organization of LGN
• 6 layers
• 1, 4, 6 Contralateral nasal hemiretina
• 2, 3, 5 Ipsilateral temporal hemiretina
• Retinotopic organization across layers
• On-center/Off-surround
• Receptive fields consistent across layers
• Simple cells edge perception yields orientation
  information

5
Visual Perception

• Organization of LGN
• 6 layers
• 1, 4, 6 Contralateral nasal hemiretina
• 2, 3, 5 Ipsilateral temporal hemiretina
• Retinotopic organization across layers
• 1 2 are Magnocellular (M system) 20 in
  macaque
• 3 - 6 are Parvocellular (P system) 80 in
  macaque

6
Visual Perception

• Cortical Pathways
• Geniculo-cortical
• Primary visual cortex V1, Brodmann Area 17,
  striate

7
Visual Perception

• Cortical Pathways
• Geniculo-cortical
• Primary visual cortex V1, Brodmann Area 17,
  striate
• M system terminates in Layer 4
• P system terminates in Layer 4, then on to
  Layers 2 3
• Blobs areas of high metabolic activity
• Interblobs areas of low metabolic activity

8
Visual Perception

• Cortical Pathways
• Geniculo-cortical
• Primary visual cortex V1, Brodmann Area 17,
  striate
• Secondary visual cortex V2, Brodmann Area 18,
  prestriate
• Thick stripes M system
• Thin stripes P blob sytsem
• Interstripes P interblob sytsem

9
Visual Perception

• Extrastriate Cortical Organization best
  understood in macaque
• Massively parallel multiple areas have
  convergent and divergent pathways
• Visual perception is analytical
• Extraction of basic features
• Integration of features into percept
• Different areas specialized for different
  features, e.g.
• Dorsal, "where," pathway
• Ventral, "what," pathway
• Retinotopic distribution in each area

10
Visual Perception

• Visual perception is analytical
• Geniculo-cortical pathways
• project to different brain areas
• sensitive to different combinations of features

11
Visual Perception

• Visual perception is analytical
• Extraction of basic features
• Treisman visual search task
• Feature search
• Pop out
• Flat slope across distractor density
• Visual primatives color, shape, orientation,
  movement direction and speed, depth

12
Visual Perception

• Visual perception is analytical
• Extraction of basic features
• Treisman visual search task
• Feature search
• Conjunctive search
• Serial search
• Search time increases across distractor density

13
Visual Perception

• Visual perception is analytical
• Extraction of basic features
• Treisman visual search task
• Illusions and feature independence
• Evidence for parallel processing of independent
  features
• Ponzo illusion and apparent motion illusions
  disappear with isoluminant colors

14
Visual Perception

• Visual perception is analytical
• Deficits in feature perception
• Achromatopsia - inability to see color
• Lesions in V4 region
• Deficit in P blob pathway

15
Visual Perception

• Visual perception is analytical
• Deficits in feature perception
• Achromatopsia
• Akinetopsia - deficit in motion perception
• MT area lesions (perhaps only with bilateral
  lesions)
• M system pathway deficit
• Can be induced using TMS

16
Visual Perception

• Dissociations of cortical (retino-gleniculo-crotic
  al) and subcortical (retino-collicular) pathways
• Scotoma due to lesion in primary visual cortex
• perimetry

17
Visual Perception

• Dissociations of cortical and subcortical
  pathways
• Schneider's experiments with hamsters
• Double dissociation of cortical and subcortical
  pathways
• Striate lesions produced deficit in
  discrimination task
• Superior collicular lesions produced deficit in
  localization task

18
Visual Perception

• Dissociations of cortical subcortical pathways
• Weiskrantz "blindsight" experiments with patient
  DB
• Present visual stimulus within DB's scotoma and
  have him move his eyes to stimulus
• Systematic relation of eye movement to stimulus
  location within about 20 degrees

19
Visual Perception

• Associations of cortical and subcortical pathways
• Rafal's experiments
• Patients with severe scotomas make eye movements
  to visual stimuli presented in intact visual
  field
• On test trials, simultaneously present visual
  stimulus in scotoma