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216 Chapter 12

12.3 The Oculo-Motor System

The implementation of an oculo-motor system is an approximation of the human system.
The system has been a large-scale engineering effort with substantial contributions by Brian
Scassellati and Paul Fitzpatrick (Breazeal & Scassellati, 1999a; Breazeal et al., 2000). The
motor primitives are organized around the needs of higher levels, such as maintaining and
breaking mutual regard, performing visual search, etc. Since our motor primitives are tightly
bound to visual attention, I will first briefly survey their sensory component.

Low-Level Visual Perception
Recall from chapter 5 and chapter 6, a variety of perceptual feature detectors have been
implemented that are particularly relevant to interacting with people and objects. These
include low-level feature detectors attuned to quickly moving objects, highly saturated
color, and colors representative of skin tones. Looming and threatening objects are also
detected pre-attentively, to facilitate a fast reflexive withdrawal (see chapter 6).
Visual Attention

Also presented in chapter 6, Wolfe’s model of human visual search has been implemented
and then supplemented to operate in conjunction with time-varying goals, with moving cam-
eras, and to address the issue of habituation. This combination of top-down and bottom-up
contributions allows the robot to select regions that are visually salient and behaviorally rel-
evant. It then directs its computational and behavioral resources towards those regions. The
attention system runs all the time, even when it is not controlling gaze, since it determines
the perceptual input to which the motivational and behavioral systems respond.
In the presence of objects of similar salience, it is useful be able to commit attention to
one object for a period of time. This gives time for post-attentive processing to be carried
out on the object, and for downstream processes to organize themselves around the object.
As soon as a decision is made that the object is not behaviorally relevant (for example, it
may lack eyes, which are searched for post-attentively), attention can be withdrawn from it
and visual search may continue. Committing to an object is also useful for behaviors that
need to be atomically applied to a target (for example, the calling behavior where the robot
needs to stay looking at the person it is trying to engage).
To allow such commitment, the attention system is augmented with a tracker. The tracker
follows a target in the wide visual field, using simple correlation between successive frames.
Changes in the tracker target are often reflected in movements of the robot’s eyes, unless this
is behaviorally inappropriate. If the tracker loses the target, it has a very good chance of being
able to reacquire it from the attention system. Figure 12.3 shows the tracker in operation,
which also can be seen in the CD-ROM’s sixth demonstration, “Visual Behaviors.”