Start | basic neuroanat. | reading | zeki extracts | table of links | List of Refs | Journals
[page 15 of handout]
NEUROANATOMICAL SUMMARY (VISUAL SYSTEM)

  • Sensory input for vision, hearing and touch is initially mapped onto distinct areas of the cerebral cortex, called primary sensory areas.

  • These are topographically organized – that is, the peripheral sensory areas (skin, retina) are mapped to cortical representations in a coherent way.

  • Visual input goes to the occipital lobe, auditory input to the temporal lobe, and touch to the parietal lobe.

  • There are also secondary sensory areas for each modality, and it is supposed that these have highly specialized functions.
Most is known about secondary sensory areas in the case of vision. (e.g. Zeki 1992)

V1=“Striate cortex”= primary visual cortex.
Other visual areas may be referred to as “extrastriate cortex”, “prestriate cortex” or secondary visual cortex.

“The first two visual areas, V1 and V2, are segregators, containing separate groupings of orientation, wavelength, and direction selective cells.” (Zeki and Shipp, 1988 p316)

Outputs from V1 and V2 assemble separate signals in three pivotal areas

V3= dynamic form;
V4= colour and static form;
V5= movement (this area is sometimes called MT)

These pivotal areas marshal and redistribute the output of V1 and V2 to higher areas in the parietal and temporal lobes.

Broadly —

  • the temporal lobe deals with “what is it” questions
  • the parietal lobe deals with “where is it” questions [see Ungerlieder, 1998]

 

 

SEGREGATION AND SPECIALIZATION

Specialized pathways begin in the retina, and continue through the subcortical relay in the Lateral Geniculate Nucleus (LGN), which has Magnocellular (M) and Parvocellular (P) layers.

Magnocellular System

  • Is faster, with large receptive fields and greater sensitivity to contrasts: Sensitive to movement, not colour
  • This system is relatively more concerned with the form of moving objects and with generating structure from motion.
Parvocellular System
  • The (P)arvocellular system goes to one colour system and one for detailed form.. But both go through V2 to V4, which is specialized for colour. Not known what happens to form in V4 Speed of “Magno” make it best for movement, but both P and M have orientation.
  • Cells in V4 have larger receptive fields and more complex properties than their counter parts in V1.
  • The strategy of generating larger receptive fields - that is, gathering information from a wider part of the field of view - is probably essential to the computational process.
-- more --
 


Start | basic neuroanat. | reading | zeki extracts | table of links | List of Refs | Journals