ELECTRICAL PHENOMENA IN VISION 615 



a difference of several millivolts between the (positive) cornea and the 

 (negative) fundus of the eyeball. 



3. In response to hght the retina develops an action-potential wave, 

 known as the "electroretinogram," whose components have been rather 

 extensively studied in many animals, including man. It seems likely 

 that the vertebrate action-potential wave arises primarily in the sensory 

 cells or bipolar cells of the retina, though no direct evidence for this is 

 yet available. 



4. The responses of single optic nerve fibers, in certain invertebrates, 

 provide evidence for the properties of visual sensory cells. Many impor- 

 tant visual processes, such as Ught and dark adaptation, appear to depend 

 primarily upon events within the sensory cell. 



5. Optic nerve fibers in vertebrates are those of third-order neurones 

 which display relatively complex responses. There are responses to both 

 the onset and cessation of light. The basis for specific wave-length dis- 

 crimination is found in many animals. Evidence of spatial and temporal 

 summation and inhibition is also found. 



6. Visual centers at the levels of the geniculate body and cortex exhibit 

 increasingly variable responses whose wave form is complex. It is the 

 neurones within these centers which are most easily influenced by drugs 

 and anoxia. 



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