vi The Physiology of Sense Organs 



as isolated single elements; usually they are found in large populations 

 — grouped or spatially separate — and they differ variously in terms of 

 their sensitivities to the different parameters of a stimulus. There are 

 thus a host of interesting facets of receptor cell organization and 

 interaction which are outside the scope of this book, as well as the 

 immense problems concerning the mechanisms used by the brain in 

 processing sensory information. How does the brain predict events 

 which are discontinuous in space and time; to take an example, how does 

 a predator track the course of a prey which has momentarily disappeared 

 from sight behind an intervening obstruction? Certainly such a com- 

 plex procedure involves not only elements of a memory of the preceding 

 sensory events, but also the ability to extend the sequence of these events 

 into the future, and to act on the assumption that such an extension will 

 reflect the actual state of affairs at a forthcoming moment in time. Ex- 

 citing work is now being done on the cortical arrangements involved 

 with visual perception in manunals, and these findings may well turn 

 out to be the very least that is necessary before we can turn to the type 

 of question presented above. But it all starts with the absorption of 

 stimulus energy by the peripheral nervous system, and the limits of 

 what we perceive are set by the accuracy of our sensory systems. Just 

 how the various sense organs cope with different energy forms presents 

 some unique and baffling problems. Moreover, the electrical events 

 leading to and including impulse initiation are the same for all neurons, 

 and in this respect the findings obtained from more conveniently acces- 

 sible sensory structures are entirely applicable to cells buried in the 

 central ganglia. 



Some excellent collected accounts of symposia on sensory mechan- 

 isms have appeared in recent years, and these are recommended to the 

 reader who finds the present book unsatisfactorily general, or specifically 

 unclear. They include: Sensory Communication, ed. W. Rosenblith, 

 1961. M.I.T. Press, Cambridge, Mass.; ' Biological Receptor Mech- 

 anisms,' 1962. Symp. Soc. Exp. Biol., 16; ' Sensory Receptors,' Cold 

 Spring Harbor Symp. quant. Biol., 1965. 30. 



DeF. M., Jr. 



Zoological Laboratory, 



Cambridge. 



January, 1967 



