SINGLE SYNAPSE: TWO NEURONS 



45 



being maximal at r 



and symmetric about r = \ . In terms of 



visual stimulation /'* is the frequency above which the typical response 

 to intermittent stimulation ceases, and thus /* may be identified with 

 the critical flicker-frequency. Equation (16) then states that the criti- 

 cal flicker-frequency increases with the logarithm of the intensity for 

 large /*. This is essentially the Ferry-Porter law. Furthermore, with- 

 in a limited range and for a fixed stimulus-intensity, this frequency 

 is the same for a given value of the light-dark ratio, LDR = r/(l— r) , 

 as for its reciprocal. 



For very small frequencies, we find from equation (15) that in- 

 dependently of r and /*, when S > h', there results a response to flick- 

 ering (intermittent) illumination, whereas when S < h' there results 

 no response, h' being a constant which is the effective threshold. Thus 

 a plot of /*(log S/h x ) begins at (log h'/h lf 0), rises vertically at 

 first, then flattens off while approaching a final slope which depends 

 upon r . 



The relationship between f* and r is generally determined for a 

 constant apparent brightness given by S' = Sr . Using the approxi- 

 mate expression (16) with S' = constant, we find that f*(r) rises 

 rapidly from zero to a maximum for r < ^(LDR < 1) and then falls 

 to zero for r = 1 (Figure 4). The position and the height of the 





.4 



Figure 4 



.8 



1.0 



maximum depends upon S". However, when r is near zero or unity, 

 the approximations break down. Furthermore, equation (16) holds 

 only for large enough /*. Hence the exact relation f*{r) may be of 

 considerable complexity. For the various experimental relationships, 

 one may consult Bartley (1941). Most of the results quoted agree 

 with the above prediction that for constant S', f*(r) is decreasing in 



