inhibited by all the other light in the neighborhood, so it is relatively weak. On the other hand, an ommatidium at the boundary which is given a"white"impulse is also inhibited by others in the neighborhood, but there are not as many of them, since some are black; the net signal is therefore stronger. The result would be a curve, something like that of Fig. 36-13. The crab will see an enhancement of → Response of Ommatidium the contour The fact that there is an enhancement of contours has long been known fact it is a remarkable thing that has been commented on by psychologists many [lumination times. In order to draw an object, we have only to draw its outline. How used ye are to looking at pictures that have only the outline! What is the outline? The outline is only the edge difference between light and dark or one color and another It is not something definite. It is not, believe it or not, that every object has a line around it! There is no such line. It is only in our own psychological makeup that Fig. 3613. The net response of there is a line; we are beginning to understand the reasons why the"line"is enough horseshoe crab ommatidia near a sharp of a clue to get the whole thing. Presumably our own eye works in some similar change in illumination manner-much more complicated, but similar Finally, we shall briefly describe the more elaborate work, the beautiful advanced work that has been done on the frog. Doing a corresponding experiment on a frog, by putting very fine, beautifully built needlelike probes into the optic nerve of a frog, one can obtain the signals that are going along one particular axon and, just as in the case of the horseshoe crab, we find that the information does not depend on just one spot in the eye, but is a sum of information over several spots. The most recent picture of the operation of the frogs eye is the following One can find four different kinds of optic nerve fibers, in the sense that there are four different kinds of responses. These experiments were not done by shining on-and-off impulses of light, because that is not what a frog sees. A frog just sits there and his eyes never move, unless the lily pad is flopping back and forth, and in that case his eyes wobble just right so that the image stays put. He does not turn his eyes. If anything moves in his field of vision, like a little bug(he has to be able to see something small moving in the fixed background), it turns out that there are four different kinds of fibers which discharge, whose properties are sum- marized in Table 36-1. Sustained edge detection, nonerasable, means that if we bring an object with an edge into the field of view of the frog, then there are a lot of impulses in this particular fiber while the object is moving, but they die down to a sustained impulse that continues as long as the edge is there, even if it is standing still. If we turn out the light, the impulses stop. If we turn it on again while the edge is still in view, they start again. They are not erasable. Another kind of fiber is very similar, except that if the edge is straight it does not work. It must be convex edge with dark behind it! How complicated must be the system of inter connections in the retina of the eye of the frog in order for it to understand that a convex surface has moved in! Furthermore, although this fiber does sustain some what, it does not sustain as long as the other, and if we turn out the light and turn it on again it does not build up again. It depends on the moving in of the convex Types of response in optic nerve fibers of a frog Speed field 1. Sustained edge detection(nonerasable) 2. Convex edge detection 0.5 m/sec 3. Changing contrast detection 4. Dimming detection Upto是m/sec Upto15° 5. Darkness detection Very large