The Russian physiologist Bernstein (1967) defined coordination as a problem of mastering the very many degrees of freedom involved in a particular movement--of reducing the number of independent variables to be controlled. The initial theorizing and experimentation on "Bernstein's problem" was conducted largely in terms of how a device of very many independent variables might be regulated without ascribing excessive responsibility to an executive subsystem. A second round of theory and research on Bernstein's problem is now under way. This second round is motivated by similarities between coordination and physical processes in which multiple components become collectively self-organized; it is directed at an explanation of coordination in terms of very general laws and principles. The major achievements of the first round of efforts to address Bernstein's problem are summarized, and six examples of the theory and research typifying the second round are presented.

By watching each other's lower oscillating leg, 2 seated Ss kept a common tempo and a particular phase relation of either 0° (symmetric mode) or 180° (alternate mode). This study investigated the differential stability of the 2 phase modes. In Experiment 1, in which Ss were instructed to remain in the initial phase mode, the alternate phase mode was found to be less stable as the frequency of oscillation increased. In addition, analysis of the nonsteady state cycles revealed evidence of a switching to the symmetric phase mode for the initial alternate phase mode trials. In Experiments 2 and 3, Ss were instructed to remain at a noninitial phase angle if it was found to be more comfortable. The transition observed between the 2 phase modes satisfies the criteria of a physical bifurcation-hysteresis, critical fluctuations, and divergence-and is consonant with previous findings on transitions in limb coordination within a person.The coordination of movements between people is an omnipresent aspect of daily life. Such coordinations consist of the very natural and commonplace coordinations exhibited by people walking and talking together and the very practiced and refined coordinations exhibited by people playing sports or music, or dancing. The degree of coordination between basketball players moving downcourt or the degree of coordination between two ballet dancers is quite obvious. The coordination of movements between speaker and listener (Kendon, 1970), however, or between mother and infant (Bernieri, Reznick, & Rosenthal, 1988) is more subtle and is apparent only through study. In all such cases a coordinative relationship is formed through an interaction of two individuals in order to produce some goal (e.g., score a basket, have a conversation). The unique challenge for an account of between-persons coordination-the durations, spacings, and phasings of movements and their components-resides in the fact that the two individuals share neither a common cognitive nor neural mechanism.Because of the cooperative nature of the relationship, the two individuals can be thought of as a single organism (Asch, 1952;Newtson, Hairfield, Bloomingdale, & Cutino, 1987 , 1987); that is, the spatial and temporal aspects of the two individuals' movements are related congruently-they are entrained-and the movements repeat (either periodically or stochastically)-they are rhythmic. Entrainment and sustained periodic behavior are properties of nonlinear dissipative systems, and it is from this dynamical perspective that we will attempt to understand between-persons coordination.The experiments reported in this article are directed at the questions of whether certain entrainment phenomena found in within-person coordination also hold for between-persons coordination, and whether the same very general dynamical principles govern both. There are two kinds of entrainment that two physically coupled oscillators can enter into. When the two oscillators are related stably in their timing, they are said to be frequency entrained. When they ...

Nine experiments are reported on the ability of people to perceive the distances reachable with hand-held rods that they could wield by movements about the wrist but not see. An observed linear relation between perceived and actual reaching distances with the rods held at one end was found to be unaffected by the density of the rods, the direction relative to the body in which they were wielded, and the frequency at which they were wielded. Manipulating (a) the position of an attached weight on an otherwise uniformly dense rod and (b) where a rod was grasped revealed that perceived reaching distance was governed by the principal moment(s) of inertia (I) of the hand-rod system about the axis of rotation. This dependency on moment of inertia (I) was found to hold even when the reaching distance was limited to the length of rod extending beyond an intermediate grasp. An account is given of the haptic subsystem (hand-muscles-joints-nerves) as a smart perceptual instrument in the Runeson (1977) sense, characterizable by an operator equation in which one operator functionally diagonalizes the inertia and strain tensors. Attunement to the invariants of the inertia tensor over major physical transformations may be the defining property of the haptic subsystem. This property is discussed from the Gibsonian (ecological) perspectives of information as invariants over transformations and of intentions as extraordinary constraints on natural law.

In 1:1 frequency locking, the interlimb phase difference phi is an order parameter quantifying the spatial-temporal organization of 2 rhythmic subsystems. Dynamical modeling and experimental analyses indicate that an intentional parameter phi psi (intended coordination mode, phi = 0 degrees or phi = 180 degrees) and 2 control parameters omega c (coupled frequency) and delta omega (difference between uncoupled eigen-frequencies) affect phi. An experiment was conducted on 1:1 frequency locking in which phi psi, omega c, and delta omega were manipulated using a paradigm in which a person swings hand-held pendulums. As delta omega deviated from 0, the observed phi deviated from the phi psi, indicating a displacement in the phi attractor point. The displacements were exaggerated by increasing omega c. The displacements were coordinated with a decrease in the stability of phi and with higher harmonics in power spectrum of phi. Implications of the results for modeling interlimb coordination are discussed.

The symmetrical dynamics of 1:1 rhythmic bimanual coordination may be specified by an order parameter equation involving the relative phase between rhythmic components, and an interlimb coupling which determines the relative attractiveness of in-phase and anti-phase patterns. Symmetry breaking of these dynamics can occur via the difference in the natural frequencies, delta omega, of the left and right rhythmic components, or by the intrinsic asymmetrical dynamics of the body. The latter is captured by additional terms that render the symmetrical coupling slightly anisotropic. A major prediction resulting from this step is that although delta omega = 0, as the frequency of coordination is increased, the asymmetrical coupling will increase and the symmetrical coupling will decrease. This results in a greater left-limb bias in left-handers and right-limb bias in right-handers. This "increased handedness" prediction was confirmed in an experiment in which 20 left-handed and 20 right-handed individuals performed 1:1 coordination with hand-held rigid pendulums. Manipulations of left and right pendulum lengths controlled delta omega, and the coupled frequency was determined by a metronome. Also confirmed was the prediction that the small shift in equilibria from in-phase and anti-phase due to the intrinsic asymmetry should be amplified in left-handers when delta omega > 0 and in right-handers when delta omega < 0. Further, the bias in left-handers was more consistent than the bias in right-handers, and a subgroup of right-handers was identified who performed similarly to left-handers. The coordination dynamics of functional asymmetry provides insights into the elementary synergy between the limbs, the dynamical mechanism that modulates it, and the nature of the asymmetry in left-handed and right-handed individuals.