This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.

Abstract — The current study addressed the degree to which maternal speech and action are synchronous in interactions with infants. English-speaking mothers demonstrated the function of two toys, stacking rings and nesting cups, to younger infants (6-9.5 mo.) and older infants (9.5-13 mo.). Action and speech units were identified, and speech units were coded as being ongoing action descriptions or non-action descriptions (examples of non-action descriptions include attention-getting utterances such as 'Look! ' or statements of action completion such as 'Yay, we did it!'). Descriptions of ongoing actions were found to be more synchronous with the actions themselves in comparison to other types of utterances, suggesting that 1) mothers align speech and action to provide synchronous "acoustic packaging " during action demonstrations and 2) mothers selectively pair utterances directly related to actions with the action units themselves rather than simply aligning speech in general with actions. Our results complement past studies of acoustic packaging in two ways. First, we provide a quantitative temporal measure of the degree to which speech and action onsets and offsets are aligned. Second, we offer a semantically-based analysis of the phenomenon, which we argue may be meaningful to infants known to process global semantic messages in infant-directed speech. In support of this possibility, we determined that adults were capable of classifying low-pass filtered action- and non-action-describing utterances at rates above chance. Index Terms—Action processing, pediatrics, speech processing, multimodal communication I