130 Neural Aspects of Vision \1 : 3 



individual retinal rods and cones of vertebrates would follow this same 

 pattern. 



Another type of experiment carried out by Hartline and his co-workers 

 involved the vertebrate eye. These experiments were more difficult to 

 perform and also much more difficult to interpret in a quantitative 

 fashion. Nonetheless, the results molded the thinking of everyone who 

 has worked in the field of vision since then. In these experiments, the 

 vertebrate eye was removed with the optic nerve intact. The nerve 

 was dissected until just one fiber remained. Through many experi- 

 ments, a variety of types of fibers were found. All showed a spontan- 

 eous, rhythmic background firing. Some increased this rate on stimu- 

 lation; more of them were almost completely "silent" during intense 

 stimulation showing a strong "on" and a strong "off" burst of spike 

 potentials. In other words, these experiments produced just the results 

 expected from the model in Figure 2. (Or maybe one should reverse 

 this, since the experiments came first.) 



Another method of obtaining electrophysiological data is to remove 

 the cornea, lens, and vitreous humor of an intact eye. Electrodes are 

 passed over the surface of the retina until the response is that of a single 

 nerve fiber. Granit, in Sweden, has used this method in detail. In 

 snakes, rats, frogs, and guinea pigs, he found that most fibers gave the 

 normal photopic threshold curve. He calls these dominators. Other 

 fibers giving different, characteristic spectra Granit calls modulators. 

 In most animals, Granit found three, sometimes four modulators, whose 

 spectra differed from the photopic threshold curve. 



Granit's data show very clearly the need for an inhibition mechanism 

 during continuous illumination. The animal data are hard to interpret 

 in terms of human vision owing to controversies over whether the animals 

 really see colors as separate sensations. Moreover, Granit's criterion for 

 observing antagonistic effects was very weak. These experiments with 

 exposed retinas do provide evidence for a mechanism such as that 

 provided by the i cells in Talbot's model. 



In summary, then, the direct neural measurements indicate that 

 vertebrate nerve fibers of the optic nerve show more response when a 

 light intensity changes than during continuous illumination; in many 

 cases, the rate of spike formation is depressed or abolished during strong 

 illumination. This is in direct contrast to the response of individual re- 

 ceptors whose rate is apparently increased on direct stimulation. Complex 

 neural interaction (that is, computation) is an important part of retinal 

 function. In this respect, the retina acts like a part of the brain. The 

 retina is a subdivision of the brain in terms of its embryological formation. 

 It further resembles the brain in giving rise to electrical potentials, 

 which are similar in some ways to the electroencephalographic potentials. 



