VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 



49 



Fig. 14. Discharge of impulses in a single optic nerve fiber of Limulus, in response 

 to exposures of the eye to light of the same intensity for durations of 0.97 sec, 0.096 

 sec and 0.010 sec. (top to bottom). Signal marking period of exposure blackens out 

 white line above time marks. Time in }£ sec. The times from the onset of illumination 

 to the successive occurrences of corresponding impulses in the upper and middle 

 records were the same for the first 10 impulses. From the 11th impulse on, the occur- 

 rences were significantly earlier in the upper record than in the middle. Therefore, 

 0.096 sec was the "critical duration" for the 10th impulse, for which the time of oc- 

 currence = 0.32 sec. For the 1st impulse (time of occurrence = 0.14 sec) the critical 

 duration at this intensity was 0.04 sec. (Reproduced from Hartline et al. 1952.) 



which nothing observable happens but during which the response is already 

 determined and we suppose that this must be occupied by intermediate proc- 

 esses, the nature of which is yet unknown. 



Finally, I should like to mention the inhibitory effect investigated by Hart- 

 line (1949) and by Hartline and Ratlif (1955). Illumination of adjacent ele- 

 ments causes a decrease in frequency of a unit responding to illumination. This 

 is shown in Fig. 15. The ommatidium from which the recording was made 

 was illuminated by a small spot of light which was turned on several seconds 

 earlier than the portion of the record shown in the slide. The black band indi- 

 cates the time of illumination of a neighboring area of the eye by a separate 

 spot of light. You can see that there is a decrease in the frequency of response 

 during the illumination of the neighboring elements. When that illumination is 



