Conversational spoken dialogue systems that interact with the user rather than merely reading the text can be equipped with hesitations to manage dialogue flow and user attention. Based on a series of empirical studies, we elaborated a hesitation synthesis strategy for dialogue systems, which inserts hesitations of a scalable extent wherever needed in the ongoing utterance. Previously, evaluations of hesitation systems have shown that synthesis quality is affected negatively by hesitations, but that they result in improvements of interaction quality. We argue that due to its conversational nature, hesitation synthesis needs interactive evaluation rather than traditional mean opinion score (MOS)-based questionnaires. To validate this claim, we dually evaluate our system’s speech synthesis component, on the one hand, linked to the dialogue system evaluation, and on the other hand, in a traditional MOS way. We are thus able to analyze and discuss differences that arise due to the evaluation methodology. Our results suggest that MOS scales are not sufficient to assess speech synthesis quality, leading to implications for future research that are discussed in this paper. Furthermore, our results indicate that synthetic hesitations are able to increase task performance and that an elaborated hesitation strategy is necessary to avoid likability issues.

This study investigates the interplay of spoken and gestural hesitations under varying amounts of cognitive load. We argue that not only fillers and silences, as the most common hesitations, are directly related to speech pausing behavior, but that hesitation lengthening is as well. We designed a resource-management card game as a method to elicit ecologically valid pausing behavior while being able to finely control cognitive load via card complexity. The method very successfully elicits large amounts of hesitations. Hesitation frequency increases as a function of cognitive load. This is true for both spoken and gestural hesitations. We conclude that the method presented here is a versatile tool for future research and we present foundational research on the speech-gesture link related to hesitations induced by controllable cognitive load.