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appear (or are "projected onto") the occipital lobe of the cerebral 

 cortex opposite to the half of the visual field containing the object. 

 Further, the area of maximum acuity around the fovea occupies a major 

 portion of the surface of the cortex. 



The stimuli are not simply transmitted through the synapses. At 

 various points in the midbrain, auxiliary fibers lead off to autonomic 

 systems, such as the feedback loops controlling the iris, and to tear, 

 blinking, and sudden withdrawal centers. Moreover, a great deal of 

 data processing may occur at these synapses. For example, potentials 

 at the retina follow a light blinking 1 ,000 times per second, those in the 

 midbrain barely follow 100 times per second, whereas the cortical 

 potentials can at most follow 10 per second. The potentials on the 

 surface of the occipital lobe occur first locally and then spread over the 

 entire cortex. Under the action of anesthesia, the local potentials do 

 not spread as far. 



No one yet knows the exact role of these potentials or their relation- 

 ship to conscious sensations. The complexity of the synapses and 

 responses of the visual pathway cannot but fill us with awe and wonder. 

 Unraveling the clues to the role of the various parts is a challenging 

 problem. 



6. Summary of Vision 



Vision can be studied from many different points of view. In Chapter 2, 

 the physical properties of light waves and optical systems necessary for 

 vision were discussed. Likewise, the gross anatomy and histology of 

 the vertebrate eye were described. These topics all are within the realm 

 of definitive, quantitative knowledge unlikely to change in the near 

 future. In Chapter 3, novel uses of vision in homing and navigation of 

 birds and bees were discussed. These uses depend critically on the 

 actions of the central nervous system. 



In this chapter, the neural aspects of vision were organized around 

 a model, illustrated by Figure 2. Many phenomena of vision can be 

 described in terms of this model, such as color vision, photopic and 

 scotopic vision, all experiments supporting a tricolor theory, all experi- 

 ments supporting an antagonist theory, kinetic data, coding in the optic 

 nerve and retinal potentials. The model uses most of the known 

 histological structures (as well as one unknown one, the "daylight rod" 

 8) . The model can be modified to bring it into accord with the experi- 

 ments by Land and his associates. This model also can explain all 

 varieties of visual defects. Nonetheless, one must expect that as more 

 data are gathered and new types of experiments are designed, the model 

 must eventually yield to a more sophisticated one. 



