What might it mean for execution of an action sequence to be controlled
hierarchically? We argue that if production of a sequence can be
decomposed into the execution of nested constituent subsequences
then it should be characterized by two invariance principles that entail
limits on the effects of context. Since the most primitive such
decomposition merely partitions the stream of action into action units,
these principles have wide applicability. According to low-level
invariance the process that executes a constituent should not be
influenced by changes in the larger constituents to which it belongs.
According to high-level invariance changes in the content of a
given constituent should have only local effects on these larger
constituents. We report on tests of the
two principles in the rapid production of brief utterances and short
strings of keystrokes prescribed in advance, in which we examine
the effects of sequence length, serial position, and unit size
on measures of timing. The tests, which we intend to be illustrative
rather than definitive, give some support to the existence of
hierarchical constituents at the level of the individual stroke
in typing and the individual stress group in speech, but provide
little evidence for deeper hierarchical structure.