PHOTORECEPTION 171 



(about three hours). In all of the earlier work the preparations were in- 

 tentionally allowed to stand for approximately three hours in the dark 

 to stabilize. On the other hand, it is clear from all records that the 

 ERGs of Diptera and Hymenoptera are more complicated than those 

 of Orthoptera. Autrum (1950) suggested that those of Lepidoptcra, 

 with their small a waves, may represent an intermediate form. 



According to Autrum, there are fundamental physiological differ- 

 ences, other than wave form, between 'slow' and 'fast' eyes. The 'slow' 

 ERG increases in magnitude with increase in stimulus intensity, it 

 changes markedly with light and dark adaptation, it responds poorly 

 to flickering light. Above frequencies of forty-fifty flashes per second 

 it fails to follow the stimulus. For stimuli of durations up to about 0-08 

 seconds, the magnitude of response is related to energy (intensity x 

 time). The 'fast' ERG increases in magnitude with an increase of 

 stimulus intensity, it is unaffected by light and dark adaptation, its 

 absolute sensitivity is less than that of 'slow' eyes {Tachycines is 

 approximately 140 times more sensitive than Callfphora), it follows 

 flicker up to about 300 flashes per second, the rate depending upon 

 stimulus intensity and temperature. The on-effect for all stimuli longer 

 than 5 msec, is dependent only upon intensity; the off-effect in the 

 range 5-200 msec, is dependent upon the total energy (i.e., intensity x 

 time). Ruck (1958 a, 1958 b), on the other hand, maintained that 'slow' 

 and 'fast' eyes differ only in their flicker fusion frequency and that the 

 wave form of the ERG, absolute sensitivity, and rate of dark adapt- 

 ation are independent visual functions not necessarily related to each 

 other, to the flicker fusion frequency, or to the form of the ERG. In 

 contesting these conclusions, Autrum and Hoffmann (1960) demon- 

 strated that subjecting the eye of Calliphora to oxygen lack (0-1 per 

 cent O2 plus 99-9 per cent N2) caused the ERG to revert to a mono- 

 phasic negative type until reoxygenated. The flicker fusion frequency 

 of this negative ERG drops to 60 per second ; it is very sensitive to light 

 adaptation. It was also pointed out that damage to the eye and long 

 use of preparations leads to the conversion of the diphasic potential 

 into a slow monophasic potential. The observations of Hassenstein 

 (1957) and Naka and Kuwabara (1959) confirm this. It is noteworthy 

 that in practically all of the work reported prior to 1948 preparations 

 were held for a period of one to four hours in the dark in order to 

 stabilize. 



The shape of even the simple 'slow' type ERG has suggested to all 

 workers that it is the summation of several components. Even the 

 ERG of Limulus, the simplest of them all, is now believed to be dual 



