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 |