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In-Depth Information
In the virtual world, an agent tries to cater for its needs, its motivations, through spe-
cific actions and interacting with different objects. Five basic needs have been identified
for the agent, which are easily identifiable in human beings: hunger, thirst, tiredness,
boredom and dirtiness. Along with the first three, widely used in many systems, we
have added dirtiness and boredom, which are more domain-specific to add a wider va-
riety of actions and get richer behaviors. These basic needs increase over time, so their
values increase as time goes by. Thus, the agent always needs to carry out actions to
maintain its basic needs values within reasonable limits.
To cater for each of these basic needs, the agent must perform actions. For example,
it can drink to satisfy its thirst or sleep to recover from fatigue. There are different
actions to cater for the same need, and the agent prefers some actions over others. Thus,
the agent may choose to read a topic or play a game to reduce boredom. Besides, the
effects of those actions can be different depending on its emotional state. It will receive
more benefit from applying more active actions when its emotional state is more aroused
and more passive or relaxed actions when it is calm.
To carry out each of these actions, the agent needs to use objects of specific types.
Thus, it will need food to eat, a ball to play or a topic to read. There are different objects
of each type in its environment and the agent has preferences over them. When an agent
executes an action with an object, its emotional state is modified depending on the agent
personality, and preferences and activations for this object.
We have chosen to implement a model widely-accepted in psychology that represents
the emotional state of an agent as a two-dimensional space of two qualities: valence and
arousal [13]. Valence ranges from highly positive to highly negative, whereas arousal
ranges from calming or soothing to exciting or agitating. The first one is a measure of
the pleasantness or hedonic value, and the second one represents the bodily activation.
Other models use a set of independent emotions, which requires defining a group of
basic emotions. However, not all combinations of values for these emotions are a valid
emotional state (e.g. the combination of maximum values in the emotions of joy and
anger is not a realistic emotional state). In general, the valence and arousal model can
be shown to be equivalent to the explicit representation of the usual set of emotions of
other computational cognitive simulations, though it requires a simpler representation
and reasoning. For instance, an emotion such as happiness can be represented as high
valence and high arousal. Both models are recognized and defended by experts in psy-
chology, but we prefer the second alternative because it makes processing easier and
prevent invalid states. In our model, the valence and the arousal are modified by the
execution of actions, so both values are modified when an agent executes an action with
an object, depending on the agent preference and activation for this object, the person-
ality traits and the emotional state. Our goal is that the agent generates plans to satisfy
its needs and to achieve the most positive value of valence.
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Domain Description
In order to use domain-independent planning techniques, we have to define a domain
model described in the standard language PDDL [16]. This domain should contain all
