Page 159 - Designing Sociable Robots

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140 Chapter 9

whether the robot should engage people and satiate the social-drive, engage toys and
satiate the stimulation-drive, or rest and satiate the fatigue-drive.
To make this decision, each behavior receives input from its affiliated drive. The larger the
magnitude of the drive, the more urgently that need must be addressed, and the greater the
contribution the drive makes to the activation of the behavior. The satiate-social be-
havior receives input from the people-present releaser, and the satiate-stimulation
behavior receives input from the toy-present releaser. The value of each of these releasers
is proportional to the intensity of the associated stimulus (for instance, closer objects ap-
pear larger in the visual field and have a higher releaser value). The fatigue-drive is
somewhat different; it receives input from the activation of the sleep behavior.
The winning behavior at this level performs two functions. First, it spreads activation
downward to the next level of the hierarchy. Thus, behavior becomes organized around
satisfying the affiliated drive. This establishes the motivational context that determines
whether a given type of stimulus is desirable (whether it satiates the affiliated drive of the
active behavior).
Second, the top-level behaviors act to satiate their affiliated drives. Each satiates its
drive when the robot encounters a good-intensity stimulus (neither under-stimulating nor
overwhelming). “Satiation” moves the drive to the homeostatic regime. If the stimulus
is too intense, the drive moves to the overwhelmed regime. If the stimulus is not intense
enough, the drive moves to the under-stimulated regime. These conditions are addressed by
Level One behaviors.
Level One: The Environment-Regulation Level
The behaviors at this level are responsible for establishing a good intensity of interaction
with the environment (see figure 9.5). The behaviors satiate-social and satiate-
stimulation each pass activation to their Level One behavior group below. The behavior
group consists of three types of behaviors: searching behaviors set the current task to explore
the environment and to bring the robot into contact with the desired stimulus; avoidance
behaviors set the task to move the robot away from stimuli that are too intense, undesirable,
or threatening; and engagement behaviors set the task of interacting with desirable, good-
intensity stimuli.
Search behavior establishes the goal of finding the desired stimuli. Thus, the goal of the
seek-people behavior is to seek out skin-toned stimuli, and the goal of the seek-toys
behavior is to seek out colorful stimuli. As described in chapter 6, an active behavior
adjusts the gains of the attention system to facilitate these goals. Each search behavior
receives contributions from releasers (signaling the absence of the desired stimulus) or low
arousal affective states (such as boredom and sorrow) that signal a prolonged absence of
the sought-after stimulus.