ELECTRICAL PHENOMENA IN VISION 601 



(4) Stimulation of the area outside the receptive field of a nerve fiber has 

 no effect on the responses of the fiber. (5) All these effects have been 

 observed under conditions that preclude scattered light, chemical spread, 

 or electrotonic spread as mechanisms for the observed summation. 



Hartline's receptive-field experiments have furnished detailed infor- 

 mation about the process that is presumably responsible for the extra- 

 ordinarily great sensitivity of human peripheral vision. Graham, Granit, 

 and others have shown that off-center vision is highly sensitive to low 

 levels of illumination (see Graham, 1934). Convergence of neural path- 

 ways has achieved this result at the expense of visual acuity. Central 

 vision, on the other hand, shows higher acuity but fails completely at 

 low levels of illumination. It is known that there is relatively little con- 

 vergence of pathways within the central fovea of the human eye. 



Specificity in Optic Nerve Fibers. The law of specific nerve energies 

 states that every impulse is similar to every other impulse within a nerve 

 fiber, so that qualitative differences cannot be directly signaled along a 

 single pathway. Sensory end organs are specifically tuned, however, to 

 mediate sensory qualities such as warmth, cold, or pain. Likewise there 

 are nerve fibers that, like some found in the vagus nerve, have inhibitory 

 rather than excitatory effects. 



The vertebrate retina is easily the most complex nervous region out- 

 side the central nervous system. As such, it may contain inhibitory as 

 well as excitatory fibers, and its individual receptor cells may respond 

 selectively to separate characteristics of the visual stimulus. The tri- 

 chromatic theory demands that at least three types of color receptor be 

 present. The duplicity theory asserts the presence of two types of recep- 

 tor, the rods and the cones. The existence of aftereffects and contrast 

 phenomena suggest, at least, that some sort of inhibitory mechanism 

 may also be important. With these speculations in mind, let us review 

 some of the information that is obtainable from optic nerve fibers of the 

 vertebrate retina. 



Hartline (1938a), using his dissection method on the opened eye of the 

 frog, found that only about 20 per cent of the fibers responded, as do 

 those of Limuhis, with an initial burst followed by a maintained discharge. 

 About 50 per cent showed an initial burst when the light appeared and a 

 final burst after the light went off, wdth no discharge during steady illumi- 

 nation. The remaining 30 per cent of the fibers showed no response at 

 all to illumination but gave a vigorous and prolonged discharge after the 

 light was turned off. Figure 13-13 shows samples of the three types of 

 response. 



On-responses were most vigorous Avhen elicited after a long period of 

 darkness, whereas off-responses were at their best following prolonged 

 illumination of the eye. Off-responses were suppressed by reillumination. 

 The specific character of the response did not change with variations in 



