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change by varying the intonation of the voice (Poggi, 1981; Gardner,
1998). Campbell (2007) studied the importance of non-verbal speech
utterances emitted both by the speaker and by the listener during
an interaction. He found out that components, like laughs, grunts
or sound as “umms” and “ahhs”, can be easily recognized and that
they help to identify the status of each member of the conversation.
This constant stream allows participants to determine the progress of
the interaction. Especially, it helps the speaker to be informed of the
successfulness of the communication and, consequently, to transmit the
content more efficiently. For example, laughs, grunts, etc. can indicate
the degree of attentiveness of the listener. Campbell's work shows how
this information can be automatically annotated and can provide an
estimation of the participants' status. Moreover, the prosodic variation
of these utterances can be quite relevant. This variation may indicate
the level of the relationship between the speaker and the listener
(Campbell, 2006).
2.1.2 Head movements
Head movements are the communicative signals most employed by the
listeners to provide backchannels, in particular head nods and shakes
(Rosenfeld, 1978). In fact, during conversation participants use their
head a lot; Heylen (2005) studied several communicative functions
transmitted by participants through head movements. In particular,
listeners perform head signals to provide continuous backchannels to
the speaker. They respond almost instantly to speaker's non-verbal
request for backchannel in the form of head movements. Head nod
and head shake are produced to silently say “yes” and “no”; that is
why they are called symbolic gestures. Allwood and Cerrato (2003)
and Cerrato and Skhiri (2003) studied how people use specific head
movements as backchannel signals, in particular when they co-occur
with vocal expressions. Among all possible movements they decided
to focus on nods, jerks (a sudden abrupt motion), shakes, waggles (a
back and forth, left to right movement) and side-way turns (a single
turn on the left or on the right). They saw, for example, that head nods
and jerks produced with short facial expressions showed continuation
of contact, perception and understanding of the message. Shakes
were performed with short negative answers to show disagreement
and waggles where often accompanied by statements of doubt and
hesitation. While Cerrato showed that listener's head movements are
synchronized with her/his vocal expressions, Hadar and colleagues
(Hadar et al., 1985) showed that about a quarter of the head movements
performed by listeners are also in synchrony with the speaker's speech.
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