602 



RADIATION BIOLOGY 



temperature, state of adaptation, asphyxia, carbon dioxide, ion imbal- 

 ance, or type of stimulation. The eyes of certain fish, amphibia, and 

 reptiles all gave essentially similar results. 



Granit and his coworkers have used a different technique from that of 

 Hartline for recording the responses of retinal fibers. They have removed 

 the cornea and lens but have done no retinal dissection. Instead they 

 have used a microelectrode (fine platinum wire insulated except at the 

 tip) as their active lead. An indifferent electrode is applied to the back 

 of the eye (see Fig. 13-1). Granit (1947, p. 304) has expressed some 



Fig. 13-13. Responses of three types of retinal fiber in the frog. (A) Fiber responding 

 with an initial burst and maintained discharge. (B) Fiber responding to onset and 

 cessation of light. (C) Fiber responding only to cessation of light. In each record the 

 signal marking the duration of the stimulus fills the white line above the time marker. 

 Time is in units of one-fifth of a second. {Hartline, 1938a.) 



concern about the possibility of confusing the response of two well- 

 synchronized adjacent fibers with that of a single fiber. He points out 

 that, when the technique is faulty, so that single fibers are not obtained, 

 the only "final common path" in the experiment is the microelectrode 

 itself. He beheves that this objection holds generally, however, for all 

 isolation techniques except those in which a single end organ is stimulated. 

 Some of the principal findings of Granit's laboratory (Granit, 1947, 

 1950a, b) are the following: 



1. Discharge types: Mammahan eyes show the three types of discharge 

 found by Harthne in the frog. The maintained-discharge type, the 

 on-elements, predominate in the guinea pig, whose retinal receptors are 

 mostly rods. In the cat retina they have the spectral sensitivity charac- 

 teristics to be expected of responses to the bleaching of visual purple in 

 rod receptors. The off-elements are more numerous in cone retinas. 

 Granit associates these off-responses with the d-wave (off-response) of 

 the electroretinogram. The on-off' elements are characteristic of cones, 

 though both they and the off-elements may represent some rods as well. 



2. The off/on ratio: This is the ratio between stimulus thresholds for 



