600 



RADIATION BIOLOGY 



25 mm 



The indifferent electrode was a second wick in contact with the surface 

 of the retina. 



HartUne used a small exploring spot of light to locate the portion of 

 the retina whose stimulation would arouse impulses in the isolated nerve 



fiber. Typically the sensitive field so located 

 had the appearance shown in Fig. 13-10. It 

 is clear that a wide area is served by this fiber 

 and that a small region at the center of this 

 area is most effectively represented. 



Further experiments were performed in 

 which the degree of spatial summation was 

 measured for a region within the receptor field. 

 Figure 13-11 illustrates the region so employed, 

 and Fig. 13-12 shows a sample of the resulting 

 responses of the nerve fiber. It is clear that a 

 higher frequency of nerve impulses results 

 from the stimulation of the large area than 

 from stimulation of the most sensitive small 

 patch within it. 



The experiments just described show clearly 

 the following: (1) Spatial summation extends 

 over a relatively large area of the eye of a frog. (2) Subliminal excitation of 

 small regions may result in a discharge when several such regions are 

 illuminated together; hence more than one impulse must originate in the 



Fig. 13-11. Stimulus area 

 on the retina of a frog. 

 Left: frequencies of main- 

 tained optic-nerve discharge 

 (single fiber) for each of nine 

 small squares tested indi- 

 vidually. Right: frequency 

 of discharge in same fiber to 

 stimulation of the entire area 

 covered by these squares. 

 Intensity 300 m-c in all 

 cases. {HartUne, 1940c.) 



Fig. 13-12. Records showing spatial summation in the arousal of impulses in a single 

 optic nerve fiber of the frog. Top: maintained discharge in response to illumination 

 of most effective region (small square labeled 2.3 in Fig. 13-11). Bottom: response to 

 illumination of the entire area (large square labeled 4.3 in Fig. 13-11). {HartUne, 

 1940c.) 



converging pathways in order to arouse the final common path, the optic 

 nerve fiber. (3) With very high intensities of stimulation there is a 

 smaller response from a large region of stimulation than from a small one. 



