A Survey of Listener Behavior and Listener Models for Embodied Conversational Agents - Coverbal Synchrony in Human-Machine Interaction - page 248

2.1.3 Facial expressions
The face is an important channel of non-verbal communication. During
interaction, both while speaking and listening, facial expressions
are used to accomplish several fundamental tasks. They can help to
regulate the flow of the conversation (Duncan, 1972), for example by
smiling to the speaker, the listener can encourage her/him to go on in
the conversation (Poggi and Pelachaud, 2000). Facial expressions have
been found to convey one's opinion about events, people or objects
(Chovil, 1992), through this kind of facial displays, called personal
reactions , a person shows what she feels or thinks, in particular a
listener can show disgust and disliking by wrinkling her/his nose, or
liking and approval by smiling and raising her/his eyebrows. Facial
expressions provide both consciously and non-consciously information
about one's moods and emotional states (Argyle, 1988; Ekman and
Friesen, 1969). Not displaying the correct facial expression at the right
moment can be a source of misunderstanding and convey the wrong
message (Poggi and Pelachaud, 2000). From the listener's point of view,
through facial expressions, a person can show, in the first place, if she
is following the conversation or not. Lowering the eyelids and yawning
could be a clear sign of boredom and lack of attention, while keeping
the eyes wide open and raising the eyebrows show that the listener
is keeping up with the conversation and that she/he is interested.
One of the most frequent and studied listener's facial displays is the
smile. Brunner (1979) conducted several tests to understand the role
of smiles produced by a listener during conversation. He found out
that placement of smiles in conversations is very similar to that of
other typical backchannel signals like paraverbals and head nods, so
smiles can be considered as backchannels.
2.1.4 Gaze behavior
Gaze behavior in human-human interaction is one of the most studied
aspects of non-verbal activity during conversation. It has been found
to serve many functions as, for example, helping to regulate the flow
of conversation, ensuring smooth turn-taking behavior and defining
(through duration, direction and type of gaze) the relationship
between participants (Kendon, 1967). In listener's behavior, gaze
fulfills a quite important role. At a perception level, turning the gaze
toward the speaker allows the listener to see better and so to perceive
visual input better; such perception is even more optimized if also
the head turns in the direction of the speaker, enhancing the eyes'
view and allowing the listener to hear better (Heylen, 2005). So, gaze