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also becomes the principal mean used to show attention and interest.
Directing gaze toward the speaker is usually associated with focus
of attention (Heylen, 2005; Kendon, 1967; Argyle and Cook, 1976).
Turning even the head and the body in the direction of the speaker
signals an even higher level of interest and attention (Peters, 2000). To
make a conversation successful, creating a bond with one's interlocutor
is quite relevant. Building this type of relationship is linked to the
notion of
engagement
. According to Conte and Castelfranchi (1995) and
Poggi (2007), engagement is defined as “the value that a participant
in an interaction attributes to the goal of being together with the
other participant(s) and of continuing the interaction”. Another
definition of engagement is provided by Sidner et al. (2004); they
defined the engagement as “the process by which two (or more)
participants establish, maintain and end their perceived connection
during interactions they jointly undertake”. In terms of engagement,
gaze behavior represents a useful way in which the listener may let
the speaker know her/his level of intention to maintain engagement
without the need to be explicit and interrupt the flow of conversation.
An example of gaze behavior that communicates a high level of
engagement is “mutual gaze”, that is looking at each other or looking
at a common object (Sidner et al., 2004).
3. Evaluating Backchannel Signals
The previous section has provided an overview on the behavioral
signals that people can display while listening to a speaker. A similar
behavior must be generated for an ECA when it interacts with a
user. In fact, providing the agent with the capability of displaying
backchannel signals, that users can interpret, will make the agent
more realistic and human-like. The user will feel more at ease since
the ECA shows the appropriate level of understanding and actively
participates in the conversation. Although a lot has been written about
human behavior expressed while listening, the existing literature does
not cover all possible signals and all the meanings that these signals
can convey, particularly when several different signals are displayed at
the same time. For such a reason, several perceptual studies have been
performed. We will describe some of them in detail in the following
two sections. The main aim of these studies was two-fold: on the
one hand, they wanted to investigate the relation between the form
of a signal (or combination of signals) and the possible transmitted
meanings and on the other hand, they aimed to understand how
people interpret backchannels performed by virtual agents. Several
tests to study the listener's behavior have already been performed
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