74 



VISION 



"ON" UNIT 



"OFF" UNIT 



light alone 



light and 

 depolarizing 

 current 



polarization of 

 horizontal 

 cell 



light and 

 hyper- 

 polarizing 



20 n A 

 5 sec 



Figure 20 Responses of ganglion cells in the retina of Mustelus canis. Scheme shows the 

 interaction between light and injection of current into a horizontal cell connected 

 through the retina to a ganglion cell. Response of "on" units to light is enhanced by 

 subliminal depolarization of an interconnected horizontal cell. Hyperpolarization results 

 in depression of the ganglion response to light. The inverse is observed with "off" units. 

 (Taken from Naka and Witkovsky 1972 by permission of the authors and the Journal 

 of Physiology (London) © 1972 Cambridge University Press Limited.) 



the surround will inhibit spontaneous firing when turned on but will evoke 

 firing of the cell when extinguished. A second type of ganglion cell, the 

 "off-center" type, also occurs. Onset of illumination in the center of this cell 

 causes inhibition, while offset causes excitation. In the surround the 

 opposite effect is seen. A third type of cell is termed the "on-off " type. All 

 three types have been found in elasmobranchs. Stell et al. (1975) reported 

 that most of the units in the dogfish Mustelus were of the on- center variety, 

 while Dowling and Ripps (1970) reported finding an equal number of on- 

 center and off-center units in the retina of Raja. 



In addition to the units just described, Stell et al. (1971) found ganglion 

 cells in Mustelus whose receptive center responded at offset as well as at 

 onset of illumination. When the stimulus was moved to the periphery, the 

 cell responded both at on and at off. These cells were termed "double 

 opponent cells." However, the conditions under which they were obtained 

 were not specified. Double opponent receptive fields were described for the 



