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The Sensori-Actuator Level

The Sensori-Actuator level is the level of primitive motor and sensory actions, for instance ``move from to or ``look at ''. At this level, there are no object representations as there are at the Knowledge level and the Perceptuo-Motor level. There are no explicit declarative representations of any kind, only procedural representations (on the actuator side) and sensor data (on the sensory side). Primitive motor actions may typically be implemented in a robot control language like VAL, and some elementary data processing routines may be implemented in a sensory sub-system, like dedicated vision hardware. At this level, we also situate reflexes, which we consider to be low-level loops from sensors to actuators, controlled by simple thresholding devices, operating independently of higher-level mechanisms, and able to pre-empt the latter. We see reflexes as primitive mechanisms whose main purpose is prevention of damage to the hardware, or to put it in anthropomorphic terms, survival of the organism. As such they take precedence over any other behavior. When reflexes are triggered, the higher levels are made ``aware'' of this by the propagation of a signal, but they have no control over the reflex's execution, which is brief and simple (like a withdrawal reflex seen in people when they unintentionally stick their hand into a fire). After the completion of a reflex, the higher levels regain control and must decide on how to continue or discontinue the activity that was interrupted by the reflex. Reflex-like processes may also be used to shift the focus of attention of the Knowledge level.