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In Berthouze, L., Kozima, H., Prince, C. G., Sandini, G., Stojanov, G., Metta, G., and Balkenius, C. (Eds.)
Proceedings of the Fourth International Workshop on Epigenetic Robotics
Lund University Cognitive Studies, 117, ISBN 91-974741-3-4
Synchronisation and Differentiation: Two Stages of
Coordinative Structure
Tomoyuki Yamamoto and Tsutomu Fujinami
School of Knowledge Science, Japan Advanced Institute of Science and Technology (JAIST)
Tasunokuchi, Ishikawa, 923-1292 Japan
e-mail: (t-yama,fuji)@jaist.ac.jp
Abstract
While motor skill acquisition process is re-
garded as development of coordination, typically
regarded as synchronisation among joint move-
ments, we found another phenomenon which we
call differentiation as a consequence of synchroni-
sation. The synchronised movement established
is decomposed into several sections or modulated
to be executed on different timings without break-
ing the coordination among them, resulting in the
gain of efficiency or flexibility. In the acquisi-
tion of skills, the coordinative structure thus goes
through two stages: synchronisation and differen-
tiation. We verify in this paper our observation
through our experiments and dynamical analysis
of the kneading of ceramic art and playing the
shaker in samba.
1 Introduction
The ability of the bodily movements such as walking or
throwing has been thought to be acquired by decreasing
the degrees of freedom inherent in the structure of our
body. Our body consists of many parts and the number
of possibilities in combining them is enormous. In his
pioneering work, Bernstein found that the combination
among the body parts has to be restricted to make our
body controllable (Bernstein, 1967). Then coordinating
joints show freezing, coupling, synergy or synchronisa-
tion. In this paper, we denote this phenomenon simply
as synchronisation since we focus on the temporal rela-
tionship among joint movements.
Is the same principle governing the basic bodily move-
ments applicable to skills, too? In this paper, we regard
the skill as a highly sophisticated movement that fulfills
the task such as seen in craftwork. To obtain a skill,
it usually takes years of training. What makes skills
so distinctive from novice’s (or even experienced per-
son’s) movements? We have studied several skills such
as kneading in the ceramic art to identify the character-
istics of skills.
We have so far examined two skills, kneading clay in
the ceramic art and playing the shaker on the samba
rhythm, both of which are repeated, cyclic movements.
Through our examination of kneading, we found that the
expert organizes his body into two sections, namely, the
torso and arm sections, and correlates them by moving
them at different timings, while unskilled persons tend to
move their bodies on a single timing (Abe et al., 2003).
Our finding reported in (Abe et al., 2003) was to ex-
tend the conception of motion. Previous studies of mo-
tion presuppose the coordination among the body parts
to be fallen into smaller number of degrees of freedom
(i.e., synchronisation). Haken, Kelso, and Bunz, for ex-
ample, found such a decrease in the degree of freedom
(Haken et al., 1985), where they studied human hand
movements. We found, however, it is a half of the story
and there comes the next stage of development, where
the degree of freedom increases to obtain flexibility and
adaptability.
The first stage of development is characterised with
the notion of synchronisation, which indicates that the
movements of each body part are synchronised on a sin-
gle timing due to the loss of degrees of freedom (DOF).
The second stage is characterised with the notion of
differentiation, which indicates that the synchronised
movement established is decomposed into several sec-
tions to be executed on different phases, maintaining co-
ordination of the whole body. The motion capture device
is necessary to investigate these features. By direct mea-
suring of coordinates in three dimension, the joint angle
is obtained and by high temporal resolution (86.1 Hz),
temporal relationship is accurately observed. Without
the motion capture device, we could not have found the
characteristics of the expert’s motion when we studied
the kneading.
We explain in this paper the data we measured
through our experiments and present an analysis to
distinguish experts from novices with the notions of
synchronisation and differentiation. As for measure-
ment, we carried out two sets of experiments. In the
first set of experiment, we examined the motions for
kneading clay as a follow-up to our previous experiment
(Abe et al., 2003). In the second set, we examined the
motions for playing the shaker on the samba rhythm. As
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