## 3 Question-Answer-Response Dialogue Model
### Utterance Selection in Each State
However, when the topic changes from the previous question, the side-participant of the previous question is selected as a speaker. This approach is based on the concept of common ground [55] among the participants. When a speaker asks a question on a certain topic, the addressee and the side-participant would have common ground that the speaker appears to be interested in the topic. Such common ground would allow the speaker to continue asking questions on the same topic. Therefore, it is reasonable for the speaker of the previous question to continue questions. However, when the topic of a question changes, it is not easy to interpret the sudden topic shift on the common ground. [PERSON] et al. [46] reported that the unnaturalness of the sudden topic shift would be alleviated by changing the speaker. Therefore, it is reasonable for the side-participant of the previous question to become a speaker when the topic changes from the previous question. In the answer state, the user becomes a speaker. The speaker of the previous question would be regarded as an addressee from the viewpoint of the concept of adjacency pair [56]. In the backchannel state, the speaker of the previous question is selected as a speaker again. This is grounded on the concept of the sequence-closing third [57]. Since the backchannel state is regarded as post-expansion of the question-answer adjacency pair, it is reasonable that the speaker of the previous question, which were addressed in the previous answer state, become a speaker. In the comment state, a speaker depends on a speech recognition result of the previous answer state. Although the speaker of the previous question is basically selected as a speaker, the side-participant of the previous question is selected when the result has a kind of negative expression (including \"No\", \"Nothing\", \"Never\", etc.) or timeout. When the side-participant is selected, the side-participant speaks to the speaker; in other words, the speaker is selected as an addressee. In this manner, the speaker can easily continue asking a question in the next question state. A user's answer with negative expressions might indicate low interest in the topic. Here, if the side-participant expresses interest in the topic by commenting on the previous question, it appears to be reasonable for the speaker to continue asking a question on the same topic from the viewpoint of common ground [55] because at least one participant shows the interest in the topic. ## 5 System

We developed a twin-robot dialogue system including the two features: the question-answer-response dialogue model and the participation of two robots in a dialogue. The hardware components of the system are shown in Figure 3 and the system architecture is shown in Figure 4.

A microphone array collects sounds. The sounds are integrated though noise a reduction process by a microphone array. The integrated sound is then sent to the automatic speech recognition module, which recognizes the user utterance. We used a cloud speech recognition service provided by NTTDocomo. The service receives a voice and returns the voice recognition results, which are sent to the utterance selection module. According to the selection rules (see Section 3.3), the utterance selection module selects an utterance from the database. The selected utterance is sent to the robot controllers. The voice recognition results are also sent to the nodding generation module as a signal of user speech. The nodding generation module sends a nodding motion to the robot controllers. Nodding is a motion for expressing that the robot is listening to the user in the nodding generation module. This motion is always executed in the answer state whenever the system received a speech recognition result. The robot controllers interpret the utterance with motions and execute them. After the execution is completed, the completion signal is sent to the utterance selection module. The utterance selection module selects a next utterance.