  This squib examines the role of limited attention in a theory of discourse
structure and proposes a model of attentional state that relates current
hierarchical theories of discourse structure to empirical evidence about human
discourse processing capabilities. First, I present examples that are not
predicted by Grosz and Sidner's stack model of attentional state. Then I
consider an alternative model of attentional state, the cache model, which
accounts for the examples, and which makes particular processing predictions.
Finally I suggest a number of ways that future research could distinguish the
predictions of the cache model and the stack model.
