Tonustal
“Swiftly the brain becomes an enchanted loom, where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of sub-patterns.” — Charles Scott Sherrington
The early 20th century theoretical biologist Jacob Johan von Uexküll noticed the decidedly ambiguous relationship between a stimulus and the excitation patterns that ensued. He realized a reaction was not simply triggered or fired by one fixed center of coordination. Instead, internally generated rhythmic activity, like tiny pulses, seemed to indicate that each small part of a nervous system was itself a mini reflex center. Coordination appeared to be located everywhere and nowhere at once. For von Uexküll, coordinated behavior was a consequence of certain regular, distributed criteria. It was variable, plastic, and flowing; something realizing itself over time, under certain conditions.
Yet, precisely how these fluid processes were regulated remained a mystery. Von Uexküll proposed the topographic concept of the Tonustal, or the “tonus valley,” a model of displaceable fluids using gradients as a form of regulation. If nervous excitation is prevented from spreading in one part of an organism, it moves to another location as if in a valleyed landscape along which it naturally flows. In the case of the Tonustal, the plastic distribution system comprises a variable nerve net across which impulses move, are caught and take form rather than being transmitted in a linear chain-reaction manner along a prescribed path as Sherrington earlier had thought.
Beharringstendenz and Magneteffekt
Von Uexküll’s Tonustal remains a conductive model. Though able to explain, if vaguely, why a single stimulus can result in a range of response forms, it fails to address those strangely spontaneous, rhythmic activities that unfailingly suggest generative processes at play. In the 1930s, after an extensive comparative study of animal locomotion producing two miles of tracings, the systems physiologist Erich von Holst identified a neural oscillator and defined it as a system effecting periodic behavior. Von Holst may be said to have done for the neural oscillator what Sherrington did for the reflex. Examining the multiple ways in which Labus, a fish distinguished by the fact that it swims using rhythmic fin motions while keeping its body immobile, synchronized its fin movements, he arrived at two basic principles that characterize the coordinative properties of oscillators: the Baharrungstendenz and the Magneteffect. Beharrungstendenz, or the tendency of an oscillator to maintain its rhythm, leads to totally synchronized movements like chewing, breathing, and running, which von Holst referred to as states of absolute coordination. These steady, rhythmic oscillations work in clear contrast to the Magneteffect, which is the effect one oscillator exercises over another of different frequency so that it seems magnetically to draw and couple it to its own frequency. Phase slippages and temporal drifts, the outcome of a latent and perpetual struggle between Beharrungstendenz and Mageneteffect, render infinitely variable couplings, easily forming larger compositions with smoothly altering tempos. Accelerated and decelerated running are states of relative coordination. Plastic forms such as dance are also manifestations of this phenomenon where oscillatory motions combine, forming molar ensembles moving fluidly from one mode to another. Sherrington’s reflex arc intervenes here as an adaptive agent in these fields of oscillators. By introducing information from the outside into this highly tuned but otherwise hermetic ensemble, the reflex arc sensitizes ensuing activities to changing conditions in the environment. Here coordination is clearly plastic, variable, and adaptive, a versatile tiling of many scales of activity in space and time. Neural oscillators are an elementary unit of a nervous system. Coupled oscillators are prototypes of a time-dependent nervous geometry.
Plasticity
In everyday usage, when we say something shows plasticity we imply that it can be molded or readily made to assume (and to retain) a new shape. Probably the term should not be used in relation to living systems, since plasticity is something found in inert, inanimate material. The nervous system is living and changing all the time; and while it can be induced to change shape by surgical or other traumatic means, it cannot be induced to maintain a new shape without being killed. Yet, the concept of plasticity is used in relation to other growing systems; one talks about molding the character of a young person, or molding the shape of a tree by selective pruning. What is meant by such cases is that constraints are applied so that the form of the organism changes and future growth is differently channeled. Plasticity in the nervous system means an alteration in structure or function brought about by development or experience.1 But not just any alteration, to qualify for term plasticity, an alteration has to show pattern or order. Plasticity here means patterned alteration of organization.