VISUAL SYSTEM: STATE OF THE ART 



Log I _ .^^^^^^ 



«- £f *— U ' ' ' ' ' 111! 



71 



■8 

 ■7 



^stimulus/time 



20mV 



Figure 18 Relation between stimulus intensity 

 and horizontal cell activity in the dark-adapted 

 retina of Raja. Maximum stimulus intensity was 

 1.22 mW/cm 2 ; duration was 0.2 s. Both ampli- 

 tude and duration of S-potential are directly pro- 

 portional to log stimulus intensity. (Taken from 

 Dowling and Ripps 1971&. Reproduced by kind 

 permission of the authors and the Journal of 

 General Physiology © 1971 Rockefeller Uni- 

 versity Press.) 



As the area of a photic stimulus is increased, the response amplitude of 

 the S-potential also increases (Norton et al. 1968). This area effect, known 

 from many vertebrate eyes, has also been confirmed by Dowling and Ripps 

 (19715) for the skate. 



The retinal area over which light will evoke a response from a given cell is 

 termed the "receptive field" of that cell. Dowling and Ripps (1971b) have 

 measured receptive fields of skate horizontal cells using bright stimuli. The 

 average diameter was 3 mm. Upon reduction of stimulus intensity, receptive 

 fields increased to 4 mm. Receptive fields up to 10 mm were reported from 

 the horizontal cells of Mustelus by Kaneko (1973). 



The large size of these receptive fields can probably be attributed to 

 electrical connections between horizontal cells of the same layer. Yamada 

 and Ishikawa (1965) provided anatomical evidence for the existence of gap 

 junctions between adjacent horizontal cells; Kaneko (1973) provided the 

 electrophysiological evidence for electrical coupling by showing that current 

 passed into a horizontal cell can be recorded in an adjacent horizontal cell. 



