An important principle of animal behavior, as pointed out by many ethologists [McFarland1971, Toates1980, Manning1979], is the feedback from the result of the animal's earlier actions. This feature is well reflected in our models of the mental state variables and corresponds to the ``satisfiability'' of the desires. As will be shown in the next section, the mental state variables are directly responsible for activating the matching behaviors. If a behavior is successful, its outcome should be able to satisfy, albeit perhaps gradually, the desire that triggered the behavior in the first place. It is essential to model satisfiability, otherwise a behavior may never terminate once activated.
The formulation of the mental state variables exhibits satisfiability as follows: The value of F depends only on the strength of the external stimulus. This means that if the appropriate stimulus does not exist or is weak, F will remain low (or satiated). This can be achieved by two means--first, the reactive fleeing action of the fish and, second, the dynamic nature of the virtual world. The values of H and L are mainly controlled by the corresponding internal urge component whose value decreases as the result of successful feeding and mating. The external stimulus components, as we have shown, represent dynamic opportunities and hence are kept at low values under normal conditions.
Note that in the formulation of H, the internal urge component
restrains the stimulus component such that the fish will not keep
taking opportunities to eat food (even when opportunities keep
presenting themselves) to the exclusion of all other behaviors. For
example, once the amount of food taken 
exceeds twice the value
of the fish's appetite 
, then H becomes zero. This situation
continues until 
indicating the complete
digestion of the food, at which moment, is reset to zero.
| Xiaoyuan Tu | January 1996 |