Fig. 36-11. The compound eye of the horseshoe crab. (a)Normal view. (b)Cross section. Figures 36-7, 11, 12, 13 reprinted with permission from Goldsmith, Sensory Ce Figure 36-11(a) shows the compound eye of the horseshoe crab; it is not very much of an eye, it has only about a thousand ommatidia. Figure 36-11(b)is a cross section through the system; one can see the ommatidia, with the nerve fibers nat run out of them and go into the brain. But note that even in a horseshoe crab there are little interconnections. They are much less elaborate than in the human eye, and it gives us a chance to study a simpler example Let us now look at the experiments which have been done by putting fine electrodes into the optic nerve of the horseshoe crab, and shining light on only one of the ommatidia, which is easy to do with lenses. If we turn a light on at some instant to, and measure the electrical pulses that come out, we find that there is a slight delay and then a rapid series of discharges which gradually slow down to a uniform rate, as shown in Fig. 36-12(a). When the light goes out, the discharge stops. Now it is very interesting that if, while our amplifier is connected to this same nerve fiber, we shine light on a diferent ommatidium nothing happens response the nerve fibers of the eye of the horse hoe crab Now we do another experiment: we shine the light on the nd get the same response, but if we now turn light on another one nearby as well the pulses are interrupted briefly and then run at a much lower rate(Fig. 36-12b) The rate of one is inhibited by the impulses which are coming out of the other In other words, each nerve fiber carries the information from one ommatidium but the amount that it carries is inhibited by the signals from the others. So, for xample, if the whole eye is more or less uniformly illuminated, the information coming from any one ommatidium will be relatively weak, because it is inhibited by so many. In fact the inhibition is additive--if we shine light on several nearby ommatidia the inhibition is very great. The inhibition is greater when the om matidia are closer, and if the ommatidia are far enough away from one another, inhibition is practically zero. So it is additive and depends on the distance; here is a first example of information from different parts of the eye being combined in he eye itself. We can see, perhaps, if we think about it awhile, that this is a device to enhance contrast at the edges of objects, because if a part of the scene is light and a part is black, then the ommatidia in the lighted area give impulses that are