Entrainment and the Motor System

What is Entrainment?In 1666, the Dutch physicist Christian Huygens, the inventor of the pendulum clock, discovered that the pendulum frequencies of two clocks mounted on the same wall or board became synchronized to each other. He surmised that the vibrations of air molecules would transmit small amounts of energy from one pendulum to the other and synchronize them to a common frequency. However, when set on different surfaces, the synchronization effect disappeared. As it turned out, the transmitting medium was actually the vibrating board or wall. For air molecule vibrations there would have been too much dampening of the energy transmission, as was later discovered. The effect he observed was subsequently confirmed by many other experiments and was called entrainment. In entrainment the different amounts of energy transferred between the moving bodies due to the asynchronous movement periods cause negative feedback. This feedback drives an adjustment process in which the energy is gradually eliminated to zero until both moving bodies move in resonant frequency or synchrony. The stronger 'oscillator' locks the weaker into its frequency. When both are equally strong, the faster system slows down and the slower system speeds up until they lock into a common movement period (Pantaleone, 2002).Technically, entrainment in physics refers to the frequency locking of two oscillating bodies, that is, bodies that can move in stable periodic or rhythmic cycles. They have different frequencies or movement periods when moving independently, but when interacting they assume a common period. Incidentally, Huygens' pendulums assumed a common period 180 degrees out of phase, which he humorously called 'odd sympathy.' It is now known that entrainment can occur in various phase relationships of the movement onsets of the oscillating bodies-often one can observe a stable phase relationship between the two bodies. A stable phase relationship is achieved when both bodies start and stop their movement period at the same time. However, this is not a necessary prerequisite for entrainment to occur. The deciding factor for entrainment is the common period of the oscillating movements of the two bodies. This phenomenon is of considerable importance for clinical applications of rhythmic entrainment in motor rehabilitation (Kugler & Turvey, 1987; Thaut, Bin, & Azimi-Sadjadi, 1998).Entrainment is a common phenomenon in the physical world-for example, coupled oscillators, fluid waves, and so on. Entrainment also occurs in nature, for example, fireflies flashing their light signal, or circadian rhythms entraining to light-dark cycle within the 24-hour day period. Unfortunately, the term entrainment is often also used loosely connected to many unrelated claims-usually in the context of claims for health or healing-with little or no scientific evidence, for example, brain wave entrainment, altered states of consciousness, trance, drum circles, or binaural beat entrainment. Therefore, it is important for the clinician who follows an evidence-based intervention model to check the scientific validity for therapeutic claims.The Auditory System and Rhythm PerceptionThe ability of the auditory system to rapidly construct stable temporal templates is well known (Thaut & Kenyon, 2003). The auditory system is superbly constructed to detect temporal patterns in auditory signals with extreme precision and speed, as required by the nature of sound as only existing in temporal vibration patterns (Moore, 2003). The auditory system is faster and more precise than the visual and tactile systems (Shelton & Kumar, 2010). Since sound waves that are most important for speech and music and other perceptual tasks are based on periodic motions that repeat themselves in regularly recurring cycles, the auditory system is also perceptually geared towards detecting and constructing rhythmic sound patterns. However, the observation of timing in the auditory system does not stop its function there. …