18 INVERTEBRATE PHYSIOLOGY 



vironmental change in the single unit. Superimposed on these is the possi- 

 bility of gaining reliability of small signals and hence sensitivity to actual 

 stimuli by summing the activity in independent parallel channels or afferent 

 fibers. This very probably occurs as an important normal mechanism in 

 those organs or animals that can afford to have many channels, and per- 

 mits the central nervous system to demand a high level of significance. 



The problem of extracting reliable information from a continuously 

 active background of fluctuating impulse intervals seems likely to be quite 

 a general physiological problem. Spontaneous activity has emerged in 

 recent years as a feature of many sense organs. Even in the best cases of 

 rhythmicity, the constancy of successive intervals is relative and small 

 signals look much like noise. The possibilities suggested may be difficult 

 to test physiologically, but would be extremely interesting if they approxi- 

 mate the specifications for actual integrative centers. It can be anticipated 

 that in different animals and in dift'erent modalities the relative roles of 

 different parameters of the integration will differ significantly. In arthro- 

 pods, following the line of argument developed a few years ago by Wiersma 

 (1952b), it may be true for some sense modalities that only a few parallel 

 channels are available and reliability or sensitivity or resolution may be 

 sacrificed. 



We have not attempted here to extend the consideration to the problem 

 of resolution ; but it is clear that there must be a somewhat similar integra- 

 tion in cases like the vertebrate eye, ear, temperature, and tactile senses, 

 where central discrimination far exceeding that of individual receptor units 

 is achieved by comparing the firing frequencies of many channels from 

 overlapping units of slightly different maxima. Little is known about this 

 process in invertebrates and we shall only pause to note that it appears 

 quite amenable to explanation in terms of the properties of units. 



Expectation of New Levels of Complexity 



So far we have attempted to explain or describe integrative phenomena 

 in terms of familiar properties of units. This effort could be carried a good 

 deal farther and might well account for a considerable part of the function- 

 ing of nervous tissue in higher animals. Certainly a treatment of the pres- 

 ent theme must begin with, and today consists largely of, a list of such fa- 

 miliar properties. 



The question whether the known properties of units suffice, when com- 

 bined in great and intricate enough permutations, is so risky that few 

 neurophysiologists will be caught guessing, though some have recently 

 answered not only "no," but have implicitly or explicitly denied that physio- 

 logical (matter-energy) mechanisms will be found adequate to account 



