Hierarchical Control in the Execution of Action Sequences: Tests of Two Invariance Properties

What might it mean for execution of an action sequence to be controlled hierarchically? We argue that if production of a sequence consists of the execution of nested constituent subsequences, then it should be characterized by two invariance properties-properties that limit the effects of one part of the sequence on another. Because one such constituent structure merely partitions the stream of action into action units, these properties have wide applicability. According to low-level invariance , the process that executes a constituent should not be influenced by changes in any higher level constit­ uent. According to high-level invariance , changes in a constituent should have at most limited and local effects on higher level constituents. We report on tests of these two properties in the rapid production of brief utterances and short strings of keystrokes, in which we examine the effects of sequence length, serial position, and unit size on measures of timing. The tests support the existence of hierarchical constituents at the level of the stroke in typing and the stress group in speech, but provide only limited evidence for deeper hierarchical structure.