Anatomy of Vision

1. Stimulus--radiant energy (periodic)
   • Range from Gamma rays to Radio Waves
   • Visual spectrum is between 400 & 700 nanometers
2. Refracted by cornea and lens
3. Rods and cones in retina-transducers
   • Resting potential -10 to -30 mv (without stimulation)
   • With stimulation, gradual hyperpolarity to -70 mv.
   • Produces graded response due to ability of NA+ crossing membrane in dark
   • Receptive field - specific area of stimulation

   Retinal Cells figure

   TOP

4. Horizontal cells - integrate information across broader region

5. Bipolar cells - function to integrate information
   • Primarily from rods and/or cones (excitatory)
   • Secondarily from horizontal cells (inhibitatory)
6. Amacrine cells - function as change detectors

7. Ganglion cells

8. Optic nerve

9. Optic chiasma (1/2 cross in man)

10. Optic tracts (4 types fibers)

11. Lateral geniculate (6 layers in primates)

12. Optic radiations (geniculocalcarine tract)

13. Area 17 (direct projections)

14. Areas 17 to 18 & 19

15. Areas 18-19 to posterior temporal lobe

16. Macular area - widespread in post 1/3 of cortex.

Visual Projections

Optic tracts (4 types fibers)

• Largest--Lateral Geniculate Nucleus of thalamus (LGN) to cortex
• Second in size--LGN to cortex and pulvinar
• Third in size--pretectal area
• Fourth in size--superior colliculi

• Layers 1,2,4--contralateral
• Layers 3,5,6--ipsilateral
• Affected by reticular activity

• Project to area 17 - direct connections only with 18 and 19.
• Cuneus gyrus - above Calcarine Fissure - inferior quadrant.
• Lingual gyrus - below Calcarine Fissure - superior quadrant.
• Columnar arrangement of cortical projection

Visual Reflex

• Topographical representation
• Reflex
• Accomodation focusing, tracking, etc.

• Light reflex--pupil constriction
• Edinger-Westphal nucleus

Visual Function

Strong optic tract stimulation

• First response to visual cortex
• 2nd response to lateral thalamic nucleus
• 3rd response to cortex via second tract

• Spontaneously active units
• Reticular formation influence
• Not all cells respond to visual stimuli

• Can respond to luminous flux
• Rats--nonvisual task impaired by bilateral visual cortex ablation
• Hubel and Wiesel--selective R - cortical units:
• Post cerebral artery damage - Homonymous Hemianopsia

Theories of Color Vision

1. Trichromatic Theory (Young-Helmholtz)
   • Thomas Young & von Helmholtz.
   • Three kinds of cones (Blue, Yellow, Red)
   • Based strictly on psychophysical observations.
   • Confirmed by recent research.
   
   
2. Opponent-Process Theory
   • Each bipolar cell receives information from 2 or 3 types of cones.
   • One type cone will stimulate maximum & another will inhibit.
   • Effect enhances discrimination ability.
   • Proposed by Ewald Hering.
   • Based upon negative afterimages.
   
   
3. Retinex Theory
   • If you look through red lenses you will still see color unless only one is present, then color will look red - color constancy.
   • Certain areas of the cortex follow the retinex theory - others don't.
   • More posterior 1/2 does, not anterior 1/2
   • Lesions - different effects in 2 areas.

• Several types - some people lack one type of cones.
• Red-green - most common form.
• On X chromosome - seen in males

Terms to Know

Links to Associated Areas

LINK Vision Sciences