594 RADIATION BIOLOGY 



made less systematically in this country by the writer and others. It is 

 notable that some disorders (e.g., retinitis pigmentosa) abolish the electro- 

 retinogram even when the degeneration has not progressed very far. 

 This finding suggests the possible use of the electroretinogram as a device 

 for early diagnosis of such disorders. Figure 13-7 presents a comparison 

 between a normal electroretinogram and one obtained from an eye 

 affected by retinitis pigmentosa. 



The effects of anoxia upon the action potential of several animals, 

 including man, have been reported by Noell and Chinn (1950; Noell, 

 1951) as a part of their research on the visual pathways (see later). In 

 general, the 6- wave of the electroretinogram is depressed relatively soon, 

 so that only the negative PHI component remains after prolonged oxygen 

 deprivation, as in Granit's experiments (1933). 



CONCLUSIONS WITH REGARD TO THE RETINAL ACTION POTENTIAL 

 In summary, the action potential has the following features: (1) It is 

 relatively easy to record in most animals and is the only electrical response 

 that is suitable for recording in the human eye. (2) In invertebrates it 

 appears to originate in the visual sensory cells. (3) In vertebrates it 

 contains a number of components, the most prominent of which, the 

 6-wave, is thought to originate at some point between the sensory cells 

 and the ganglion cells of the retina. (4) In man the 6-wave has scotopic 

 characteristics; earlier waves appear to reflect photopic or specific red- 

 receptor activity. (5) The usefulness of the electroretinogram as a tool 

 for exploring particular retinal areas is seriously hmited by the necessity 

 of using high intensities and large areas of stimulation. (6) The action 

 potential has certain applications to clinical problems. 



OPTIC NERVE IMPULSES 



It is evident from this discussion that the electroretinogram can 

 scarcely provide the sort of detailed information which is needed for 

 analyzing visual receptor processes. This can be done only by the far 

 more difficult procedure of placing electrodes directly on the separate 

 elements of the retina (sensory cells, retinal neurones, and optic nerve 

 fibers). Again, as in the case of the electroretinogram, the complexity 

 of the vertebrate retina has made it very difficult to obtain clearly differ- 

 entiated responses from sensory cells alone. More primitive eyes are 

 more easily used for this purpose. 



VISUAL RESPONSES OF PRIMITIVE EYES 



Hartline and Graham (1932) developed an admirable method for 

 studying basic sensory processes in the lateral eye of Limulus polyphemus, 

 the horseshoe crab. A small bundle of fibers is dissected free from the 



