ELECTRICAL PHENOMENA IN VISION 599 



as mentioned earlier, the fibers of the optic nerve are those of third-order 

 neurones, the retinal ganglion cells. Responses recorded in them will 

 have been modified by the presence of at least two synapses between 

 them and the primary sense cells of the retina. Horizontal and amacrine 

 cells may introduce facilitation or inhibition. Convergence is such that 

 several receptor cells may supply a single optic nerve fiber. 



Spatial Summation. Adrian and Matthews (1927a, b, 1928) measured 

 responses in the optic nerve of the eel, Conger vulgaris. Both on- and 

 off-effects were observed. Area and intensity were found to be recipro- 

 cally related, in the arousal of nerve impulses, for regions up to several 



EXPLORING 



SPOT ,• 



^/— , '^ SPOT 



EXPLORING 



I mm I mm 



(a) (6) 



Fig. 13-10. Receptive fields for single vertebrate optic nerve fibers. Contours at 

 various levels of intensity of the exploring spot. Each contour is labeled with the 

 logarithm of the intensity that is just capable of eliciting minimal responses in the 

 fiber (relative to maximum intensity of 2 X 10^ m-c). In {h) stimulation at high 

 intensity within the shaded region evokes a maintained discharge. Elsewhere, in 

 (a) and (6), discharge was not maintained, however intense the stimulating light. 

 {Hartline, 19406.) 



tenths of a millimeter in diameter on the retina. Spatial summation was 

 increased by the application of strychnine. Since this drug is known to 

 facilitate synaptic transmission, the authors concluded that the sum- 

 mation that they observed was the result of a lateral spread of excitation, 

 carried out for the most part by neural interaction. 



The most elegant demonstration of retinal interaction is to be found in 

 Hartline's work (1940a,b,c) on the optic nerve fibers of the frog. In 

 these experiments the eye was excised, and the retina was exposed by 

 removing the anterior half of the eyeball and draining away the vitreous 

 humor. Small bundles of nerve fibers w^ere then dissected free from the 

 anterior surface of the retina. These are third-order neurones making 

 their way from various parts of the retina toward the optic disc, where 

 they go to make up the optic nerve. By painstaking dissection it was 

 sometimes possible to obtain a bundle containing but a single active 

 nerve fiber. A cut end of the bundle was lifted onto a wick electrode. 



