Unlike a fish's habits, which are static, its mental state varies
over time. The mental state of an animal can be viewed as consisting
of several distinct desires. For example, the desire to drink or the
desire to eat. In order to model an artificial fish's mental state, it
is important to make certain that the modeled desires resemble the
three fundamental properties of natural desires: (a) they should be
time varying; (b) they should depend on either internal urge or
external stimuli or both; (c) they should be satisfiable (see
Section 
for more details).
Currently, the artificial fish has three mental state variables, hunger H, libido L, and fear F. The range of each variable is [0,1], with higher values indicating a stronger desire to eat, mate and avoid predators, respectively. H and L are determined both by internal urge and by external sensory stimuli, while F is solely induced by the latter, i.e., the perception of predators. The variables are calculated as follows (overlined terms simulate the corresponding internal urge):
where
is the amount of food consumed as measured by the number of
food particles or prey fishes eaten,
with constant 
is the digestion rate,
is the time since the last meal,
is a constant that indicates the appetite of the fish (bigger
fishes have a larger ),
captures the influence of external sensory stimuli,
i.e. the perception of nearby food (explained in more detail shortly),
with constant 
is the libido function,
is the time since the last mating,
calculates the influence of the perception of
potential mates;
is a constant,
and 
are, respectively, the fear of and distance to sighted
predator i.
and 
. Certain choices
can result in ravenous fishes (e.g, 
) or sexual mania
(e.g., 
) or cowards (e.g., 
).
| Xiaoyuan Tu | January 1996 |