Control of chaos in biological systems - hearts and brains
William L. Ditto
Applied Chaos Lab, School of Physics, Georgia Institute of Technology,
Atlanta GA 30332-0430
Before its discovery chaos was unevitably confused with randomness and indeterminacy. Because many systems appeared random, they were actually thought to be random. This was true despite the fact that many of these systems seemed to display intermittent almost periodic behavior before returning to more ``random'' or irregular motion. Indeed this observation leads to one of the defining features of chaos: the superposition of a very large number of unstable periodic motions. Thus the identification in biological systems of unstable periodic or fixed point behavior consistent with chaos makes new therapeutic strategies possible. Recently we were able to exploit such unstable periodic fixed points to achieve control in two experimental systems: in cardiac tissue [1] and hippocampal brain tissue [2].
Our cardiac preparation consisted of an isolated well-perfused portion of the interventricular septum from a rabbit heart induced to beat chaotically by the injection of the drug ouabain. The effect of the drug is to accelerate the heartbeat and to cause the interbeat intervals to vary chaostically. We were able to indentify and control the subsequent unstable fixed point behavior. New studies on human atrial and animal ventricular fibrillation indicate that the presence of unstable fixed points used for chaoscontrol are present and experimentally accessible.
Our brain model consists of a rat hippocampal slice perfused in physiologic artificial cerebrospinal fluid. The presence of high potassium in the CA3 region of the slice organizes irregular synchronous bursting behavior which displays unstable periodic fixed points about which control has been achived. Intriguingly we have also been able to make the timing of the burst ``more chaotic'' through the use of anti-chaos control. While it is not yet clear whether chaos or anti-chaos control will be beneficial for the suppression of seizure activity in the brain it is intriguing that brain tissue responds to chaos control techniques. The identification of unstable fixed points and their control in biological systems may very well help us to understand and possibly control previously intractable physiological problems such as fibrillation and epilepsy.
Supported by the Office of Naval Research Physics Division.