Cybernetics elegantly describes some existing phenomena in motor control. In speech, pointing and handwriting movements the existence of feedback loops such as the mono-synaptic reflex arc and the cortico-cerebellar loops introduce a self-organizing autonomy into the effector system. At the same time however, the concept of feedback is insufficient to explain the corrective properties of motor control in case of absent or delayed sensory information. Also, the origin of complex patterns like writing is left implicit in a pure cybernetical theory.
Experiments by [23,24] played an essential role in the paradigmatic shift in which feedback as such was increasingly considered to be inadequate as an general explanation of motor control. It was shown that in fast aiming movements of the head or the arm , final targets could be reached in the absence of essential feedback information (visual, vestibular, or proprioceptive feedback). The explanation for this phenomenon that was put forward, and that is still accepted for the greater part today, is that the central nervous system determines in advance of such an aiming movement, the ratios of muscle activation (co-contraction) levels. In this view, the motor apparatus is a combination of tunable mass-spring systems. The role of the existing feedback loops was consequently limited to (1) slow and fine adjustment as in posture control, to (2) adaptation to new or strange postures , not internalized by the "programming" system, and (3) and to learning.
More cognitively oriented researchers interpret the ``Open loop model'' in terms of Computer Science and Artificial Intelligence concepts (like the Turing machine concept) and utilize the metaphor of ''brain like a computer''. It is assumed that each articulatory activity is controlled through a sequence of instructions [106,107] (Magno-Caldognetto-and-Croatto-1986,Harris-1987,Keller-1987, Wilson-1987,Wilson-and-Morton-1990). In the extreme (purist) case, the articulatory trajectories are pre-programmed (off-line) point by point and are executed deterministically. However, it is well known that only a limited number of parameters is needed to produce curved trajectories. The impulse response of the biomechanical output system, and the well-timed production of intrinsically primitive electro-myographical burst patterns, leads to smooth trajectories by itself. The details of such a movement do not have to be stored explicitly in the brain. What the neural control system does, in fact, is to represent the inverse transform of the non-linear output system in such a way that (1) non-linearities may be counteracted, and (2) useful properties, like the energy present in a mechanical oscillation, may be exploited. The third group of theories elaborates on this latter idea.