The significance of simulating explicitly the influence of external stimuli on H and L is that it gives rise to opportunism. Opportunism is an important aspect of animal behavior and has been especially addressed by researchers in designing action selection mechanisms for animats [Maes1990, Maes1991a, Sahota1994]. We model opportunism to increase the realism of the behavior of the artificial fishes.
It is reasonable to assume that the mental state of hunger and libido are governed mainly by the corresponding internal urge, while external stimuli will have an impact only if they are strong enough. This indicates that and should resemble a step function. The detailed calculations are as follows. For efficiency, we assume that only the stimulus of the closest food and mate contributes to and , respectively. In addition, the strength of such a stimulus depends solely on the time-varying distance d(t) between the stimulant and the fish--the smaller the d(t), the stronger the stimulus. Let s(t) = 1/d(t) denote the strength of the stimuli, then and are given by
where x denotes either h or l and , are threshold values. When x=`h', we use and ; When x=`l', we use and . A graph of and is shown in Fig. . The parameters are and in our implementation.
Figure: The form of the stimulus functions and .
|Xiaoyuan Tu||January 1996|