1. Behaviors

Before designing our own creature, we need to investigate some of the organizational principles underlying natural control systems. Let us start by breaking an organism's overall behavior into a collection of separate reflexes. This allows us to study and develop each sub-behavior in isolation from the others. To achieve some particular goal, we then put these reflexes back together and carefully coordinate their activities. Such a control system can be modelled as a set of "if-then" rules where each rule corresponds to a primitive reflex. A simple way to coordinate the animal's actions is to impose a fixed priority ordering on the rules. This structure is then used to resolve potential conflicts between behaviors. Experimental robotics research has shown that such systems, if cleverly designed, are powerful enough to accomplish relatively sophisticated tasks.

The functioning of such a rule-based system is best illustrated by an example. Consider the coastal snail. This creature spends its life at the edge of the ocean, eating algae off of rocks. Thus, the best place for a snail is in a crack right above the waterline, where there is a rich concentration of food. In addition, the creature is in no danger of being dried out by the sun or being gulped up by some passing bird. Unfortunately, the snails are occasionally washed off their rocks by large waves. To avoid starvation they must have some way of seeking out the optimal region again.

Ethological studies have revealed that the snail has two primitive drives: to climb upward and to avoid light. We will refer to these reflexes as UP and DARK. However, neither of these "instincts" are complete functions: there are some situations for which they do not suggest an action for the snail to take. For instance, if there is no appreciable intensity difference between directions, the DARK behavior is quiescent and the snail crawls straight upward. Similarly, when the snail is on a more or less flat surface, UP is inactive and the snail's direction of travel is determined solely by the illumination gradient. Overall, however, DARK is the stronger behavior. If a very bright light source is present, the snail will crawl away from it even if this means going downward. As shown in the figure, we can draw the two reflexes as boxes and indicate the priority between their outputs using a circle with an "S" in it. In the case of conflicting motion commands, the behavioral module which injects its signal into the side of such "suppressor" node always wins and gets to control the snail's body.

Surprisingly enough, if one turns the snail upside down, instead of avoiding light, it will now head toward bright areas. We can imagine that this is due to a third behavior, BRIGHT, which provides the animal with an urge to seek out light. Since BRIGHT ends up controlling the motion of the animal, it must override the output of DARK. Yet this new behavior only becomes active, "potentiated", when the animal is inverted. Otherwise the creature acts solely on the basis of the lower level behaviors. However, there is a further twist to the story. It has been observed that this light seeking behavior occurs only underwater. If the animal is in air it will invariably seek out dark areas, even if it is upside down! We can model this by adding a final behavior, called CRACK, to the creature's repertoire. When the creature is out of the water, this behavior takes precedent over all the other light sensitive behaviors.

This collection of four behaviors allows the snail to find the best foraging area, even if it has to negotiate major obstacles along the way. Imagine, as shown in the figure, that the snail starts off on the ocean floor a short distance off shore. Since the rocks are slightly darker than the surrounding sand, it crawls along the bottom towards them. When it reaches an outcropping it starts climbing the face. If it comes across a notch in the rock it is first drawn inward by the darkness. Upon reaching the end, it then starts climbing the rear wall and eventually reaches the ceiling. Here, it becomes inverted and thus moves outward toward light again. Having successfully overcome this impediment, the snail continues climbing toward the surface. When it reaches the edge of the water, it ascends still further until it comes across a crack. As before, the dark seeking behavior will take over and directs the snail into any crack encountered. However, since it is now above water, the snail does not turn around when it reaches the back, but instead stays deep in the crack.

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