PHOTORECEPTION 175 



reversal of polarity of potentials (also noted by Naka and Kuwabara) 

 at a critical depth in the eye (retina plus optic lobes) corresponding to 

 the superficial part of the lamina ganglionaris, concluded that cells 

 in this layer, probably the giant monopolar cells, are the source of the 

 major part of the potential observed when the eye is illuminated. This 

 conclusion is in disagreement with the results of Bernhard (1942) from 

 Dytiscus, and has been criticized by Antrum (1958) as representing 

 pecularities of the recording technique. Ruck (1 957) has argued that an 

 interpretation of these data in a manner consistent with the distri- 

 bution of current in a volume conductor would in fact localize the site 

 of the origin of the ERG to the ommatidia. 



Still another electrical phenomenon observed in the optic tract of 

 insects is rhythmic spontaneous activity first found in Dytiscus 

 (Adrian, 1937) and subsequently in all eyes in which it was sought 

 (Roeder, 1939, 1940; CrescitelH and Jahn, 1942; Jahn and Wulff, 1942; 

 Bernhard, 1942; Massera, 1952; Autrum, 1951, 1952; Burkhardt, 

 1954; Burtt and Catton, 1958). It is clearly of ganghonic origin, since 

 extirpation of the optic ganglion abolishes it (Roeder, 1940; Burk- 

 hardt, 1954). 'Fast' eyes produce faster rhythms than 'slow' eyes 

 (Autrum, 1951, 1952; Burtt and Catton, 1958). In Dytiscus there are 

 differences between the day and night eye, the former having a higher 

 flicker fusion frequency than the latter; furthermore, the dark-adapted 

 night eye is more than 1 ,000 times more sensitive than the dark-adapted 

 eye, and the light-adapted day eye more sensitive than the light- 

 adapted night eye (Jahn and Wulff, 1941). The differences are un- 

 related to pigment migration in the eye. In Dytiscus there are actually 

 two rhythms: a dark rhythm and a bright (fast) rhythm. The latter 

 occurs only under maximum illumination, and was visualized by 

 Adrian (1937) as representing firing of all neurons. By contrast, 

 rhythms in butterflies and grasshoppers are found under any inter- 

 mediate conditions (CrescitelH and Jahn, 1942). They occur at high 

 temperatures as an after-discharge to a single flash of light and also 

 as a result of repetitive stimulation. The dark rhythm in Dytiscus 

 was explained by Adrian (1937) as the resting potential of all neurons 

 in the system being favoured by injury. In Mantis religiosa and 

 Romalea microptera there is also a light and a dark rhythm (Roeder, 

 1939, 1940). The light rhythm is somewhat similar to that found in 

 Dytiscus and unHke that described by CrescitelH and Jahn (1942). 

 Burkhardt (1954) has localized the bright rhythm as originating in the 

 medulla externa. This rhythmic excitation is related to the stimulus in 

 its amplitude but not in its frequency. The frequency Hes between 100 



