HORMONES AND BEHAVIOR Psychology

1
Vision (cont.)

• Neural Basis of Visual Perception (6.2)
• light transduction
• color vision
• brain regions involved in vision
• visual fields
• blind sight

2
Light Transduction

• receptors contain photopigments
• photopigments chemicals that release energy
  when struck by light
• photopigments 11-cis-retinal (a lipid) bound
  to opsins (protein)

• light converts photopigments (11-cis-retinal to
  all-trans-retinal)
• this inhibits the inhibitory photoreceptors and
  results in depolarization of
• bipolar and ganglion cells

3
The Trichromatic Theory of Color Vision

• three kinds of receptors for
• human color vision (color cones)
• determined by the particular opsin
• within a photoreceptor
• each cone is maximally sensitive
• to a different set of wavelengths
• (short, medium long wavelengths)
• color is perceived through the
• relative rates of response of each cone
• a response by any one cone is ambiguous (CNS
  must process relative activity across all cones)
• retina contains equal numbers of red and
  green cones much smaller number of blue
  cones (8 )

4

• Disorders of Color Vision
• color blindness (lack of functional cones)
• very rare (1 in 100,000)
• usually due to an inherited lack of cones
• see objects through stimulation of rods
• perceive only shades of gray have poor
  visual acuity

• protanopia/deuteranopia (red-green color
  deficiency)
• difficult to distinguish red from green
• see most objects in shades of yellow/brown
• normal visual acuity (suggest presence of 3
  cones in the appropriate numbers)
• problem red cone is filled with green
  opsin or green cone is filled with red opsin
• inherited transmission (linked to the X
  chromosome 8 men vs. 1 women)

• tritanopia (blue color blindness)
• dont see shades of blue
• retina lacks blue cone
• blue sky looks green
• good acuity
• very rare (equally common in men and women)

5
Brain Regions Involved in Vision
Retino-Geniculo-Cortical Pathway Retinal
ganglion cells - optic nerve - optic chiasm -
optic tract - LGN optic radiations - visual
cortex (striate cortex V1) - visual association
areas
6

• Brain Regions Involved in Vision
• Retina
• axons of ganglion cells form the optic nerves
  which cross at the optic chiasm
• Lateral Geniculate Nucleus (LGN)
• receives inputs from the retina
• projects to primary visual cortex
• contains six layers of neurons
• each layer receives input from only 1 eye
• magnocellular layer (2 inner layers)
• large cell bodies color-blind brief response
• form movement depth
• parvocellular layer (outer 4 layers)
• small cell bodies color fine details slow
  response
• koniocellular sublayers
• lie ventral to magno/parvo layers
• transmits info. from short wavelength (blue)
  cones

7

• Primary Visual Cortex
• primary visual cortex V1 striate cortex
• surrounds the calcarine fissure
• arranged in 6 primary layers (parallel to
  cortex)
• receives inputs from parvo/magno LGN (layer 4C)
• receives input from konio LGN (layers 2,3)
• projects to visual association areas (secondary
  visual cortex, temporal cortex,
• posterior parietal cortex

8
Light activates receptors in the retina
nasal hemiretinas
right temporal hemiretina
left temporal hemiretina
9
Visual Fields

• visual perception depends on integrity of the
  connections between the retina and the striate
  cortex
• damage in one or more of these areas results in
  particular visual deficits

10
Blindsight

• damage in striate cortex
• Blindsight (Weiskrantz, 70-80s)
• ablation of left striate cortex causes
  blindness in right visual field
• BUT, if an object is placed in the right visual
  field, a person with this visual deficit is
  pretty good at reaching out and grasping the
  object (can also detect movement/orientation)
• HOWEVER, subject will report that they cannot
  see the object