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Designing Artificial Emotions

The design phase must decide which emotions are experienced by the animat, which sensations trigger changes in emotions, and how they are portrayed in the behavior. This prototype will be extended later in Chapter 42 with additional primary emotions, secondary emotions, and moods.

Primary Emotions

Because this chapter builds the first emotional prototype, it seems appropriate to include the most important emotions only. As such, the animats have only four of the primary emotions defined by Robert Plutchik in his psychoevolutionary theory [Plutchik80]:

  • Fear and anger

  • Surprise and anticipation

As complementary emotions, only two of four may be observable at any point in time. Each of the emotions is defined as a binary value: either fully active or inactive.

Sensations and Stimuli

The sensations are patterns that cause changes in the emotions. These patterns can be present in the current state, but also over periods of time. In this prototype, the sensations are based directly on the stimuli from the environment. Sensations are identified as sequential combinations of stimuli (see Table 39.1).

Table 39.1. Stimuli from the Environment Triggering Sensations

Player appears

Object respawns

Unexpected sound

Item picked up

Low health

Small/large damage



Switch pressed

Door/platform ready

Blood splat

Body part

These sensations are considered as Boolean triggers that cause the emotions to change. Specifically, a sensation of fear will toggle the corresponding emotion fear on and anger off.

Behavioral Response

The emotions need to be portrayed by the behaviors for them to have any purpose. Selecting the most obvious manifestations of emotions improves the realism, and keeping their number to a minimum keeps the system simple. In this chapter, we'll use the following parameters—applied via the interfaces to the body:

  • Action precision and power— Depending on its mood, an animat will run quicker or slower and have varied control over each step. It will also turn with different speed, sometimes with additional precision when aiming.

  • Sensing delay and accuracy— The entities present in the nearby world may take a short amount of time to be perceived by the animat, modeling human reaction times. Animats also have trouble estimating the distance of objects in some cases.

These are restrictive modifications, because they decrease the capabilities of the animats. In the best case, the animat will not be affected by its emotions. Chapter 42 extends upon these ideas by influencing the decision making with moods, too.

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