Objectifying Social Presence: Evaluating Multimodal Degraders in ECAs Using the Heard Text Recall Paradigm

Embodied conversational agents (ECAs) are key social interaction partners in various virtual reality (VR) applications, with their perceived social presence significantly influencing the quality and effectiveness of user-ECA interactions. This paper investigates the potential of the Heard Text Recall (HTR) paradigm as an indirect objective proxy for evaluating social presence, which is traditionally assessed through subjective questionnaires. To this end, we use the HTR task, which was primarily designed to assess memory performance in listening tasks, in a dual-task paradigm to assess cognitive spare capacity and correlate the latter with subjectively-rated social presence. As a prerequisite for this investigation, we introduce various co-verbal gesture modification techniques and assess their impact on the perceived naturalness of the presenting ECA, a crucial aspect fostering social presence. The main study then explores the applicability of HTR as a proxy for social presence by examining its effectiveness under different multimodal degraders of ECA behavior, including degraded co-verbal gestures, omitted lip synchronization, and the use of synthetic voices. The findings suggest that while HTR shows potential as an objective measure of social presence, its effectiveness is primarily evident in response to substantial changes in ECA behavior. Additionally, the study also highlights the negative effects of synthetic voices and inadequate lip synchronization on perceived social presence, emphasizing the need for careful consideration of these elements in ECA design.