16 INVERTEBRATE PHYSIOLOGY 



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Fig. 6. Frequency-time plots of the activity of a single receptor unit in the infrared 

 sense organ of the rattlesnake, Cro talus, to three different intensities of physiological 

 stimulation. Two simultaneous plots of the same activity : the finer spots in a more 

 nearly smooth curve are from an integrator which averages the frequency with a 

 time constant of 2.5 seconds. The coarser spots with a wide scatter are from a pulse- 

 interval-plotter which places the spot higher on the ordinate the shorter the interval 

 since the last impulse. The ordinate scales are not given here as they are not im- 

 portant for the present purpose. Time scale at lower right is one second. The case is 

 chosen because the wide scatter of spots indicates a very arrhythmic activity except 

 at high frequency under strong stimulation ; this is typical of this sense organ. In A 

 the stimulus is weak though well above threshold defined as a significant change in 

 the integrated frequency ; but the unintegrated frequency record signals the environ- 

 mental event very poorly and unreliably. While the integrated frequency greatly im- 

 proves the detection, it is still dependent for its usefulness upon the absence of slow 

 fluctuations in the spontaneous background; this short-term record does not show 

 such fluctuations. B and C show the disadvantage of integrated frequency as a signal 

 at high stimulus intensities — its slow response; the unintegrated frequency reliably 

 reports much sooner. Note partial adaptation and postexcitatory silent period before 

 spontaneous background returns. (From Bullock and Diecke. ) 



Instead of the actual frequency we may consider the ratio of, or the 

 difference between, frequency recently transpiring and a background fre- 

 quency. 



(3) The ratio, frequency-integrated-over-some-short-time- just-past 

 ( Frci ) to frequency -integrated -over -some -longer -period -just -past, 

 ( Frc2 ) • Sensitivity would increase as the time constants RCi and RC2 

 increase. The greater RCo is, the better (within a limit depending on the 

 relative role of this channel in change detection and steady-state detection) . 

 RCo cannot be less than several minutes if it is to avoid loss of information 

 by an adaptation of the central nervous system more rapid than that of 

 some receptors. RCi cannot be longer than a few seconds if loss of informa- 

 tion is to be avoided, because sensory adaptation to weak stimuli already 

 occurs in that time in many receptors. It should be, in fact, a fraction of a 

 second in order to keep down reaction time and loss of information on 

 brief stimuli and flicker. 



Steady-state input from nonadapting receptors, like position and some 

 temperature receptors, could not be processed by such a formula, for long 



