282 



H. K. HARTLINE, F. RATLIFF AND W. H. MILLER 



of the eye actually play an important role in human vision has been shown in 

 experiments in which an optical device is used to cancel all the effects of eye 

 movements. When a retinal image is formed that is stationary with respect 

 to the retinal mosaic, all contours and discontinuities gradually fade out, and 

 within a few seconds the visual field appears uniform, although the image is 

 physically unchanged on the retina (Riggs, Ratliff, Cornsweet and Cornsweet, 

 1953; for a review of work by Ditchburn and his associates see Ditchburn, 

 1955). 



Fig. 28. Oscillogram of a "synthetic" on-off discharge in a single fiber of Limulus 

 optic nerve. The typical response to steady illumination of a single receptor is a 

 sustained discharge. In the record shown, the receptor was illuminated simul- 

 taneously with other nearby receptors which exerted inhibition on it. By properly 

 balancing the excitatory and inhibitory influences against one another transient 

 burst of impulses at the onset and cessation of illumination were obtained. A 

 section 0-8 sec long (no impulses) was cut from the center of the record (cf. 

 Ratliff and Mueller, 1957). 



Responses to movement are but one instance of a retinal action that 

 serves as a step in the integration of sensory information ; the wide diversity 

 of response characteristics of the ganglion cells of the vertebrate retina un- 

 doubtedly subserve other integrative processes that have their beginnings in 

 the sense organ itself. Receptive fields of retinal ganglion cells have an 

 elaborate functional organization (Kuffler, 1953; Barlow, 1953). The full 

 significance of this organization is one of the important problems of visual 

 physiology; in some animals, antagonistic actions within the receptive fields 

 of single ganglion cells are "color-coded" (Wagner et al., 1960). The neural 

 mechanisms involved in these various phenomena are not fully understood, 

 but it is clear that they are based on an interplay of excitatory and inhibitory 

 influences, and on the interactions of retinal receptors and neurons (Granit, 

 1955). In the simpler eye of Limulus, these mechanisms are not elaborated, 

 but a simple integrative process is nevertheless present in the form of the 

 inhibitory interaction. 



Ahhough "on-off" and "off" responses are never observed in the optic 

 nerve fibers of Limulus under ordinary conditions of stimulation, such re- 

 sponse patterns can be "synthesized" by careful balancing of the excitation 

 furnished directly by illumination on an ommatidium and the inhibition from 

 its neighbors (RatUff and Mueller, 1957). An example is shown in Fig. 28, 



