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The Behavior System 147

visual processes, such as the current target from the attention system, to relatively high-level
multi-modal percepts generated by the behavioral releasers.
The interaction constraints pertain to the arbitration of units that compose each layer. This
can range from low-level oculo-motor primitives (such as saccades and smooth pursuit) to
using visual behavior to regulate turn-taking.
Each level serves a particular purpose for generating the overall observed behavior. As
such, each level must address a specific set of issues. The levels of abstraction help simplify
the overall control of behavior by restricting each level to address those core issues that
are best managed at that level. By doing so, the coordination of behavior at each level (i.e.,
arbitration), between the levels (i.e., top-down and bottom-up), and through the world is
maintained in a principled way.
The social level explicitly deals with issues pertaining to having a human in the interaction
loop. This requires careful consideration of how the human interprets and responds to the
robot’s behavior in a social context. Using visual behavior (making eye contact and breaking
eye contact) to help regulate the transition of speaker turns during vocal turn-taking is
an example presented in chapter 9. Chapter 7 discusses examples with respect to affect-
based interactions during “emotive” vocal exchanges. Chapter 12 discusses the relationship
between animate visual behavior and social interaction. A summary of these findings is
presented in chapter 13.
The behavior level deals with issues related to producing relevant, appropriately per-
sistent, and opportunistic behavior. This involves arbitrating between the many possible
goal-achieving behaviors that Kismet could perform to establish the current task. Actively
seeking out a desired stimulus and then visually engaging it is an example. Other behavior
examples are described in chapter 9.
Themotorskillslevelisresponsibleforfiguringouthowtomovethemotorstoaccomplish
the task specified by the behavior system. Fundamentally, this level deals with the blending
of and sequencing between coordinated ensembles of motor primitives (each ensemble is
a distinct motor skill). The skills level must also deal with coordinating multi-modal motor
skills (e.g., those motor skills that combine speech, facial expression, and body posture).
Kismet’s searching behavior is an example where the robot alternately performs ballistic
eye-neck orientation movements with gaze fixation to the most salient target. The ballistic
movements are important for scanning the scene, and the fixation periods are important for
locking on the desired type of stimulus. I elaborate upon this system at the end of this chapter.
The motor primitives level implements the building blocks of motor action. This level
must deal with motor resource allocation and tightly coupled sensori-motor loops. Kismet
actually has three distinct motor systems at the primitives level: the expressive vocal system
(see chapter 11), the facial animation system (see chapter 10), the oculo-motor system (see
chapter 12). Aspects of controlling the robot’s body posture are described in chapters 10
and 12.