6i8 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



vertebrate retina and of the compound eye of insects 

 is ample evidence that, in the more highly developed 

 eyes, sensory information from the transducer ele- 

 ments is acted upon almost immediately by highly 

 organized ganglionic structures. The physiological 

 studies that bear out this expectation are reviewed in 

 Granit's chapter, where the conception of the retina 

 as a nervous center is thoroughly developed. In the 

 vertebrate retina, excitatory and inhibitory influences 

 spread and converge, and interplay in a complex 

 manner to generate patterns of optic nerve activity 

 that are much more than mere copies of the patterns 

 of light and shade on the receptor mosaic. Even in 

 more primitive eyes, simple interactions of receptor 

 units take place (8) that serve to accentuate certain 

 significant features of the stimulus pattern, at the 

 expense of exact fidelity of reproduction. Integrative 

 nervous processes begin \ery early indeed in the 

 visual pathway. More than this, Granit's chapter 

 reveals that the visual receptor organ, like other 

 sense organs, is under a certain amount of centrifugal 

 control from the higher nervous centers. This new 

 development in neurophysiology is already having 

 far-reaching effects on ovn- understanding of sensory 

 physiology. 



With the study of the physiology of the higher visual 

 centers of the brain, taken up in Hartley's chapter, 

 visual physiology merges with other branches of 

 neurophysiology. In this area, contributions come 

 from workers not primarily concerned with vision, 

 for brain physiology involves the integration of all 

 forms of neural activity that govern the organism's 

 behavior. Quite properly, many references to the 

 physiology of the visual centers will be found scat- 

 tered in other chapters throughout this work. 



In the analysis of central nervous system mecha- 

 nisms, extensive u.se has been made of experimental 

 animals in which parts of the brain have been ab- 

 lated, fiber pathways interrupted or specific areas 

 stimulated artificially. The resulting modifications of 

 behavior then reveal important physiological rela- 



tionships. Applied to the visual system, such studies 

 require more than a casual familiarity with specific 

 principles of retinal physiology and with overall 

 visual performance. This is brought out in Hartley's 

 chapter. Especially in the field of animal behavior it 

 should be emphasized that great advances have been 

 made recently by experimental psychologists. Ani- 

 mal behavior can now be controlled more effecti\ely 

 and studied with greater precision than was possible 

 only a few years ago. The present day neurophysiolo- 

 gist must master these powerful new techniques or 

 work in close collaboration with colleagues who have 

 mastered them. 



The aim of the studies that have been outlined 

 above and taken up in detail in the chapters that 

 follow is to understand vision. This broad aim can 

 be expressed quite explicitly, so far as many ol the 

 behavioral manifestations of vision are concerned. 

 The reactions of intact animals to stimulation by 

 light and the reports of human observers in response 

 to visual presentations have been studied by truly 

 scientific methods for many years. Experimental 

 psychology provides a vast amount of very detailed 

 and very precise information about just how animals 

 do react, what human subjects do report, in carefully 

 controlled visual experiments. Students of visual 

 phenomena have not neglected the analysis of their 

 observations in attempts, often very successful, to 

 provide an understanding of underlying mechanisms. 

 Indeed, many such mechanisms are now being veri- 

 fied by direct neurophysiological experimentation. 

 Psychological studies of vision are vitally important 

 to the visual neurophysiologist, for it is this field of 

 science that sets many of his ultimate problems. No 

 matter how far we may progress in the analysis of 

 the neurophysiological mechanisms of the visual 

 pathway, our task of acquiring .scientific understand- 

 ing will not be complete without the complementary 

 act of synthesizing our detailed knowledge into a 

 coherent whole. 



REFERENCES 



1. DeRobertis, E. J. Biophyi. & Biochem, Cylol. 2, .Supple- 

 ment: 209, 1956. 



2. Detwiler, S. R. Vertebrate Photoreceptors . Experimental 

 Biology Monographs. New York: MacMillan, 1943. 



3. Fessard, a. Recherches sur I'Activite Rytlmiique des NerJ 

 holes. Paris: Hermann, 1936, p. 130. 



4. Goldsmith, T. H. and D. Philpott. J. Biophys. & Bio- 

 chem. Cylol. 3: 429, 1957. 



5. Granit, R. Receptors and Sensory Perception. New Haven; 

 Yale Univ. Press, 1955. 



6. Hartline, H. K. Harvey Lectures Ser. 37: 39, 1942. 



7. Hartline, H. K., H. G. Wagner and E. F. MacNichol, 

 Jr. Cold Spring Harbor Symp. Qiiant. Biol. 17: 125, 1952. 



8. Hartline, H. K., H. G. Wagner and F. Ratliff. J. 

 Gen. Physiol. 39 : 65 1 , 1 956. 



9. Hecht, S. Physiol. Rev. 17: 239, 1937. 



10. Miller, W. H. J. Biophys. & Biochem. Cytol. 3: 421, 1957. 



11. PoLVAK, S. L. The Retina. Chicago: Univ. Chicago Press, 



194" • 



