7:3/ Neural Aspects of Vision 129 



discrimination have been developed around a single cellular model 

 based on histological findings and the actions of neurons. This model 

 has helped to organize and combine the experimental data obtained 

 from a variety of approaches using many techniques. It is useful in 

 that it orders past knowledge about the visual mechanisms in a form that 

 is easy to remember; it will be used in succeeding sections to describe 

 evidence from electrical measurements of spike potentials, as well as to 

 interpret neural sharpening and analyses. 



3. Direct Neural Measurements 



Measurements of neural spike potentials were made by Hartline and his 

 co-workers who recorded impulses from the optic nerves of limulus and 

 vertebrate eyes. The eye of the king crab, limulus, is particularly simple 

 because it consists of many individual rodlike receptors called ommatidia. 

 Each of these receptors is connected to an individual nerve fiber. When 

 the nerve is dissected until just one fiber remains intact, a slow natural 



Dark On Off 



Time 



Light 



Figure 3. Diagrammatic representation of response of a single 

 limulus ommatidium. The vertical lines represent spike 

 potentials. Solid horizontal line represents light on. Note 

 dark rate, on-burst, steady rate in light, off-burst, and return 

 to dark rate. After H. K. Hartline, H. G. Wagner, and F. 

 Ratliff, "Inhibition in the Eye of Limulus,'" J. Gen. Physiol. 39: 

 651 (1956); H. K. Hartline and F. Ratliff, "Inhibitory Inter- 

 action of Receptor Units in the Eye of Limulus,'''' J. Gen. Physiol. 

 40:357 (1957). 



firing rate is observed in the dark. This is illustrated in Figure 3. If a 

 threshold stimulus is applied to this single ommatidium, an extra spike 

 is observed. If light stimuli considerably above threshold are used, the 

 response is somewhat more complicated as is also shown in Figure 3. 

 Initially, there is a very rapid (transient) burst of spikes as the light is 

 turned on. This is followed by a slower steady-state "firing" rate 

 far faster than the dark rate. The steady-state rate is a function of the 

 intensity of the light stimulus. When the stimulus is removed (that is, 

 the light is turned off), there is another transient burst of spikes, followed 

 by a gradual return to the dark rate. There is no reason to doubt that 



