A Survey of Listener Behavior and Listener Models for Embodied Conversational Agents - Coverbal Synchrony in Human-Machine Interaction - page 246

understandable any more. These negative signals help her/him to
apply strategies to make the conversation successful again. Like
Allwood et al., Poggi sees feedback as a means used by listeners to
provide information about some conditions of the communicative
process (Poggi, 2007): the functions of attention, comprehension,
believability, interest and agreement. In addition, she defined feedback
signals as a mechanism used by interlocutors to fulfill the other party's
“control goals” during an interaction (Castelfranchi and Parisi, 1980;
Poggi, 2007). While talking, people aim to obtain or give information,
have the interlocutors do something, in short they have some “central
goals” they want to see fulfilled. But, at the same moment, people
have also some side goals, called “control goals”, that, even if not
explicitly mentioned, are quite relevant: the goals of knowing if their
interlocutor is listening, understanding, agreeing to do what they ask
for, and so on. In this idea, feedback signals provide information about
the fulfillment of these control goals, showing the interlocutor's level
of comprehension and personal reaction.
2.1 Verbal and non-verbal backchannels
Communication between humans takes place through several
modalities. Voice, head movements, facial expressions, gaze shifts,
gestures and changes in posture can be performed to convey a
communicative function both while speaking and listening. A
backchannel can be constituted by either a single verbal or non-
verbal signal or it can be a combination of several signals on different
modalities. In this chapter, we focus on backchannels performed on
the face, head, gaze and voice modalities.
2.1.1 Verbal and acoustic signals
Backchannels that received a great deal of attention are the listener's
linguistic signals that include both verbal and vocal signals. For
verbal signals, we mean single words like “yeah”, “ok”, “no” or short
sentences as “I see”, while vocal signals, also called paraverbals,
consist of brief sounds like “mm mh”, “aha”, “uhhh”. Allwood et
al. studied them from a semantic point of view in Allwood et al.
(1993) and, besides defining the communicative functions they can
convey, they discovered that the meaning of a linguistic backchannel
depends strongly on the polarity of the previous utterance, for
example a negative answer that follows a negative phrase shows
acceptance, whereas a negative answer after a positive utterance
shows disagreement. Poggi and Gardner noticed that the canonical
use and value of backchannels like “yeah”, “mm hm” and “mm”