PSYC 460 -- Dr. King

Overview of The Visual System (as an example of a neural network)

I. Retina - 3 layers of cells
     A. photoreceptor layer - rods and cones, make synapses in...
     B. bipolar cell layer - the sensory neurons, make synapses in...
     C. ganglion cell layer - axons form the optic nerve
     D. there are also two kinds of lateral elements
          1. horizontal cells - interconnect adjacent photoreceptors
          2. amacrine cells - at the level of bipolar-to-ganglion synapses
II. Optic Nerve
     A. axons of ganglion cells leave the retina in bundles called the optic nerves
          1. some axons leave the optic nerve and go to the hypothalamus -
             biological rhythms (day-night cycle)
          2. some axons leave the optic nerve and go to the midbrain visual areas -
             the primitive visual areas of the brain (this is all there is in frogs!)
               a. axons of these cells project to the thalamus (different areas from
                  optic tracts)
               b. and then to the secondary visual cortex
               c. this system does not give rise to conscious visual awareness
               d. other axons project to the muscles that control eye movement
                  (partially voluntary)
               e. projections to the muscles of the iris (involuntary)
     B. partial decussation at the optic chiasm - the axons become segregated
        in such a way that axons serving the right vision field are traveling
        to the left hemisphere and vice versa
     C. after the optic chiasm these axons are called the optic tracts
          1. there appears to be two distinct sets of axons
               a. one is responsible for color vision
               b. one is responsible for shape, brightness, depth, movement
     D. the optic tract axons make their synapses in the thalamus (in a six-layered
        structure called the lateral geniculate nucleus)
III. Optic Radiations
     A. axons of cells in the thalamus terminate in the primary visual cortex
     B. the visual field is mapped onto V1 (primary visual cortex)
          1. the right visual field is mapped into the left hemisphere, and
             vice versa
          2. the central visual field is represented bilaterally
     C. destruction of V1 results in loss of visual awareness (but not total
        loss of visual capabilities)
IV. Primary Visual Cortex
     A. at the very back of the brain
     B. appears to be critical for visual awareness
     C. projects (sends axons to) the surrounding secondary visual areas
V. Dorsal Stream vs. Ventral Stream
     A. dorsal stream axons go to the top of the brain - "where" system
     B. ventral stream axons go the side/bottom of the brain - "what" system
     C. there are also special areas for processing color
     D. and possibly faces
VI. Projections to Heteromodal Sensory Association Areas - the "big picture"