Hands clapping at the concert hall, group firing of fire
flies, firing patterns of neurons in the brain,
24 hours rhythm of human life etc.
There exist many systems in nature that are composed of
many elements, each of which repeats rhythmic dynamical
patternx.
Through mutual interactions, timing of the pattern
generation can be entrained between such elements.
Such phenomenon is called ``synchronization.''
Synchronization constitutes a basis of living systems
on many levels.
For instance, adaptation to the cycle of sun rise and
sun set creates 24 hours rhythm of human life.
Synchronous firing induced by synaptic connection
between neurons enhances information processing inside
the brain.
Synchronization plays a key role also for acoustics.
Our research focuses on such synchronization phenomena as a universal view to understand biological systems as well as acoustical systems. Our theoretical framework is based on nonlinear dynamics and coupled nonlinear oscillators. Towards real-world applications, we develop various tools of nonlinear time series analysis.
[Simultaneous plots of 16 coupled electrical circuits. As the coupling strength is increased (left -> right), the outputs of the 16 circuits get synchronized with eatch other]
The vocal folds are composed of elastic muscle fibers situated on both left and right sides of the larynx. Induced by an air pressure from subglottis, the left and right vocal folds oscillate in a complete synchronous manner. For voiced sounds production, these vocal folds vibrations serve as the main source of sound. In normal subjects, the left and right vocal folds have a symmetric structure, which is indispensable to generate synchronous vibrations between both sides. Such synchrony is essential to produce a normal pitch of the sounds. However, in subjects with voice pathology, symmetry of the vocal folds is broken because of various reasons such as papoulis or atrophy. This asymmetry induces non-synchronous vibrations such as biphonation or chaos, which are perceived as abnormal voice. Hence, patients with voice diseases produce a pathological quality of voice.
[High-speed camera recording of the vocal fold vibration. Right and left vocal folds repeat closing and opening in a synchronous manner]
Asymmetric configuration between the left and right vocal folds is induced not only by voice diseases but also by aging that slowly changes the voice physiology. Some singing voice intentionally produces asymmetry to produce biphonic sounds. It is therefore important to detect the asymmetry as a indicator of non-synchronized vocal folds vibrations. Such asymmetry, however, is not easily detectable unless invasive technique is used such as a direct observation of the vocal folds by high-speed camera. We develop a non-invasive technique for estimating the asymmetry of the vocal folds from time series data. Such a technique should be of significant use for automatic diagnosis of voice pathology or aging.
Synchronization is related to various important problems of living biological systems such as neural systems or circadian clocks. Our research area includes analysis and comprehension of such biological systems. Another challenging subject is to make a bridge between science and art. For instance, synchronization plays a major role of resonance and synchronous play of musical instruments in orchestra. Evaluation of the artistic play in terms of synchronization may provide us with a novel link between science and art. We will deal with such a challenging problem in a future.