COORDINATION OF SIDE-TO-SIDE HEAD MOVEMENTS AND WALKING IN AMPHETAMINE-TREATED RATS - A STEREOTYPED MOTOR PATTERN AS A STABLE EQUILIBRIUM IN A DYNAMICAL SYSTEM

Rats injected with 5.0 mg/kg(+)-amphetamine perform, at one stage of t he drug's influence, rhythmic side-to-side head movements while walkin g. This makes them an interesting preparation for investigating how st ereotyped motor patterns emerge from the coordination of periodic move ments. We report here such a pattern we have isolated: the left forele g and the right hindleg land on the ground as the head reaches the pea k of its movement to the right, and vice versa (contra-lateral pattern ). We show that this pattern can be explained as a stable equilibrium in a simple, nonlinear dynamical model, similar to models developed fo r tapping with both hands in human subjects. The model also accounts f or sequences of behavior that are not in the contra-lateral pattern, e xplaining them as a flow of the system back towards the stable equilib rium after a disturbance. Motor patterns that constitute the building blocks of unconstrained behavior are often defined as fixed phase rela tions between movements of the parts of the body. This study applies t he paradigm of Dynamic Pattern Generation to free (unconstrained) beha vior: motor patterns are defined as stable equilibria in dynamical sys tems, assembled by mutual influence of concurrent movements. Our findi ngs suggest that this definition is more powerful for the description of free behavior. The amphetamine-treated rat is a useful preparation for investigating this notion in an unconstrained animal whose behavio r is still not as complex and variable as that of the normal animal.