  A critical component of competence in language is being able to identify
relevant components of an utterance and reply appropriately. In this paper we
examine the extent of such dialogue response sensitivity in pre-trained
language models, conducting a series of experiments with a particular focus on
sensitivity to dynamics involving phenomena of at-issueness and ellipsis. We
find that models show clear sensitivity to a distinctive role of embedded
clauses, and a general preference for responses that target main clause content
of prior utterances. However, the results indicate mixed and generally weak
trends with respect to capturing the full range of dynamics involved in
targeting at-issue versus not-at-issue content. Additionally, models show
fundamental limitations in grasp of the dynamics governing ellipsis, and
response selections show clear interference from superficial factors that
outweigh the influence of principled discourse constraints.
