The Supervisory System model, in which there are two cognitive levels in the control action, is assessed. It is argued that there is a modulatory relation between the levels. It is further argued that standard connectionist variables, such as age of acquisition, familiarity, and frequency, are particularly useful for characterizing behavior produced by contention scheduling, the lower-level system, when Supervisory System function is impaired. By contrast, an analogy with symbolic Al models is used to theoretically motivate a fractionation of Supervisory System processing as created by a set of functionally selective and anatomically partially separable subsystems. It is argued that the systems for the Supervisory System's top-down selection of schemas in contention scheduling has a different lateralization of dorsolateral prefrontal cortex from the systems concerned with non-evident error detection and checking. The former are held to be the more left lateralized in comparison with the latter.
