ELECTRICAL PHENOMENA IN VISION 



595 



optic nerve and allowed to rest on two recording electrodes, as shown in 

 Fig. 13-1. Stimulation of the eye by light results in a train of nerve 

 spike potentials that are recorded by the use of a suitable amplifier and 

 oscillographic camera. Further dissection may be used to isolate a bun- 

 dle containing but a single active nerve fiber. A sample of the resulting 

 response to light is shown in Fig. 13-8. 



The anatomical evidence is that a fiber of the Limulus optic nerve 

 stems directly from an individual sensory cell. Impulses recorded as 



Fig. 1.3-8. Responses of a single optic nerve fiber of Limulus. In each record the 

 vertical spikes represent impulses recorded by electrodes placed as in Fig. 13-lc. The 

 time line has notches every fifth of a second. Duration of the stimulus is denoted by 

 the horizontal black line. 



described are accordingly those of a true sensory nerve fiber whose 

 impulses have not traversed any synapse or ganglion cell. Recent 

 experiments (Hartline, 1949) have shown some degree of interaction 

 among units of this sort and have raised the possibility that synaptic con- 

 nections may exist, though undetected as yet by histological examination. 

 Characteristics of the Single-fiber Response. Impulses recorded in a 

 single fiber show the following characteristics: (1) The first impulse in 

 response to illumination appears after a latent interval whose duration is 

 progressively shorter for higher intensities of stimulation. (2) The fre- 

 quency of the discharge is high during the first burst of impulses, particu- 

 larly for high stimulus intensities (see Fig. 13-8). (3) There is a "silent 

 period" during which the impulses come less frequently or cease alto- 

 gether. (4) A steady discharge begins after this silent period and con- 

 tinues as long as the light remains on. (5) There is not usually an 



