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4. Neural Sharpening and Analyses 



Inhibition in the retina can be demonstrated in other ways. One of the 

 more striking is the process called neural sharpening. Similar effects in 

 hearing were discussed in the last chapter. Sharpening within the 

 retina was demonstrated directly in the experiments of Hartline and 

 co-workers with limulus eyes. The nerve fibers from the ommatidia go 

 through a complex plexus, not clearly understood anatomically, in 

 which the various fibers apparently synapse with one another. If two 

 receptors are stimulated instead of one, as described in Section 3, their 



Inhibition 

 Steady Inhibited Partially 



Dark On Rate by B Relieved 



I I I IIHI 11 1 Fiber 



Light 



Inhibited by 

 Dark On Inhibited by A A + C 



Light 

 Dark On 



B 



Fiber 



C 

 Fiber 



Time 



Light 



Figure 4. Diagrammatic representation of three ommatidia. 

 A and C are so far separated that there is no mutual interaction. 

 However, both A and C interact with B. After H. K. Hart- 

 line, H. G. Wagner, and F. RatlifT, " Inhibition in the Eye 

 of Limulm" J. Gen. Physiol. 39: 651 (1956) ; H. K. Hartline and 

 F. RatlifT, "Inhibitory Interaction of Receptor Units in the 

 Eye of Limulus" J. Gen. Physiol. 40: 357 (1957). 



responses can be shown to be interrelated. These relationships exist at 

 the ommatidia themselves but are abolished if the nerve fibers are dis- 

 sected free (that is, removed from the plexus) from the ommatidia to the 

 points of observation (and cut thereafter) . Thus, the interrelationships 

 depend on the neural plexus. As a result, the stimulation of one 

 ommatidium raises the threshold and decreases the steady-state firing 

 rate of the second ommatidium used. These effects are reciprocal and 

 are important only for very close neighbors. 



The response of an individual receptor, then, depends on the state of 

 stimulation of its neighbors (or more correctly, on the firing rate of its 

 neighbors). For example, one may choose three receptors, A, B, and C, 

 such that A and B inhibit each other and B and C inhibit each other 

 but A and C are too far apart to have an appreciable mutual effect. 

 The results of this experiment are illustrated in Figure 4. If one 



