Central Visual Pathway notes - CH 29

Retina -> Pretectal area

• pupillary reflex

Retina -> Superior Colliculus

• SC gets input from retinal ganglion and indirect input from VC.
  • Has visual, auditory and somatic maps, spatially organinzed, and spatially similar.
  • Has a motor map which controls coarse saccades.
    • SC Gets three types of visual stimulus: motion, attentiveness, broad outlines.
  • fine saccades controlled by frontal eye fields in frontal cortex.
• SC also projects to tectospinal tract for reflexive head and neck movements
• SC also projects to tectopontine tract for reflexive head and neck movements;

Retina -> Lateral Geniculate Nucleus (LGN)

• Majority of retinal ganglia project to LGN
• Optic nerve from nasal hemiretinas cross at optic chiasm. Nerves from temporal hemiretinas do not.
• 6 layers of cells - first 2 are magnocellular, last 4 are parvocellular. Their main input is from M cells in retina. Each layer receives input from only one eye.
• Fibers from contralateral nasal hemiretina go to layers 1, 4, and 6
• Fibers from ipsilateral nasal hemiretina go to layers 2, 3, and 5
• Receptive fields are small, concentric, about 1 degree in diameter.
• on-center or off-center pathways are independent, and divided into M and P channels.

LGN -> Primary Visual Cortex

• first place where receptive fields topology changes is in V1
• PVC (V1) receives information from contralateral half.
  • 6 layers - layer 4 mostly from LGN. M & P channels terminate in different layers/sublayers.
  • Layer 4 projects up to 4B, 2, 3
  • Cells in 2 and 3 project down to pyramidal cells in layer 5.
  • From layer 5, go to pyramid cells in layer 6
  • Cells do not respond to spots of light - they do respond to lines and bars.
• Simple Cells of PVC
  • Resemble cells of LGN, but w/ rectangular fields.
  • Best response when bar aligned (in theta) with center of receptive field Every axis of rotation is represented for every retinal position. This is a LARGE amount of fields.
  • Hubel&Wiesel - rectilinear receptive fields built up from collection of circular cells in layer 4C of PVC.
• Complex Cells
  • Larger receptive fields
  • Have a critical axis of orientation, but position does not have to be as well defined, because there are no clearly defined on or off zones.
  • input thought to be from layer 4C, but mostly from families of simple cells w/ similar orientation.
• Many Many LGN cells go to simple cells, many simple cells go to complex cells. Abstraction is increasing w/ depth.
• both simple and complex cells get input from magnocellular and parvocellular pathways.
  • Magnocellular - more concerned w/ movement and coarse outlines
  • Parvocellular - more concerned w/ color, texture, patterns
• Leads to "primal sketch" - Marr - an inital 2-D approximation of stimulus shape and contour
• PVC is organized in columns
  • each column contains simple and complex cells, receptive to a single orientation.
  • columns change orientation in ~10 degree increments.
  • ocular dominance columns - alternating system of columns devoted to left/right eyes. Helps in binocular vision.
  • hypercolumn - set of columns of arbitrary orientation referring to the same spot in both eyes.
• Columns are horizontally linked
  • columns w/ same orientation linked.
  • possibly integrating information - possibly for motion?
• 3 major functions
  • deconstruct visual world into short line segments
  • segregates color information from form and movement
  • combines input of both eyes, beginning depth perception.
• Output of V1
  • mostly pyramidal cells
  • layers 2 and 3 go to V2, 3, 4; Also to reciprocal cortical areas on other side of brain via corpus callosum.
  • layer 4b goes to V5 (medial temporal lobe)
  • layer 5 to superior colliculus, pons, and pulvinar.
  • layer 6 goes back to LGN and claustrum