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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 ...

The pull to coordinate with other individuals is fundamental, serving as the basis for our social connectedness to others. Discussed is a dynamical and ecological perspective to joint action, an approach that embeds the individual's mind in a body and the body in a niche, a physical and social environment. Research on uninstructed coordination of simple incidental rhythmic movement, along with research on goal-directed, embodied cooperation, is reviewed. Finally, recent research is discussed that extends the coordination and cooperation studies, examining how synchronizing with another, and how emergent social units of perceiving and acting are reflected in people's feelings of connection to others.

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Previous research has demonstrated that people's movements can become unintentionally coordinated during interpersonal interaction. The current study sought to uncover the degree to which visual and verbal (conversation) interaction constrains and organizes the rhythmic limb movements of coactors. Two experiments were conducted in which pairs of participants completed an interpersonal puzzle task while swinging handheld pendulums with instructions that minimized intentional coordination but facilitated either visual or verbal interaction. Cross-spectral analysis revealed a higher degree of coordination for conditions in which the pairs were visually coupled. In contrast, verbal interaction alone was not found to provide a sufficient medium for unintentional coordination to occur, nor did it enhance the unintentional coordination that emerged during visual interaction. The results raise questions concerning differences between visual and verbal informational linkages during interaction and how these differences may affect interpersonal movement production and its coordination.

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Intra-and interpersonal inter]imb coordination of pendulums swung from the wrist was investigated. For both kinds of coordination, the steady state and breakdown of bimanual rhythmic coordination as indexed by the time series of the relative phase angle it were studied under the manipulation of coordination mode, frequency of oscillation, and the difference in the eigenfrequencies (preferred tempos) of the individual oscillating limbs. The properties observed for both intra-and interpersonal coordination were those predicted by a dynamical model of rhythmic coordination that considers the coordinated limbs coupled to be nonlinear oscillators. Using a regression method, the coupling strengths of the coupled system were recovered. As predicted by the dynamical model, the strength of the dynamic was generally greater for the in-phase than the anti-phase mode and decreased with increasing frequency. Further, the strength of the interpersonal interlimb coupling was weaker than that of intrapersonal interlimb coupling.Much research over the past decade has demonstrated that stable patterns of interlimb coordination and their dissolution can be modeled by assuming that the nervous system and concomitant metabolic machinery manifest dynamical processes of self-organization (Haken, Kelso, & Bunz, 1985;Kelso, 1990;Kugler, Kelso, & Turvey, 1980;Kugler & Turvey, 1987;Turvey, 1990). The methodological tactic of this research has been to demonstrate that the patterns of rhythmic coordination observed in single (Feldman, 1980;Kay, Saltzman, & Kelso, 1991) and multilimb (Beek, 1989;Kelso & Jeka, 1992;Schmidt, Shaw, & Turvey, 1993) movements are just those expected if the neuromuscular systems involved are governed by the dynamics of nonlinear oscillators. Interestingly, research has also demonstrated that patterns of coordination similar to those found in withinperson interlimb coordination occur in rhythmic movements optically coordinated across two people (Schmidt, Carello, & Turvey, 1990;Schmidt & Turvey, 1994). These latter findings demonstrate the generality and abstract incorporeal nature of the organizing principles involved in biological rhythmic coordination: The same control processes seem to be at work regardless of whether the coordination involves one nervous system or two or whether the coupling medium is the optic array or neural tissue. In two experiments we further compared the coordination dynamics found in intraand interpersonal interlimb rhythmic movements, particularly the strength of those dynamics and their patterns of coordination breakdown. We did this using a methodology that manipulated the frequency of oscillation and the dynamical similarity (i.e., the preferred frequency or eigenfrequency difference) of the rhythmic units being coordinated. We also present a new method of measuring the strength of the coupled oscillatory dynamic.The research on the dynamics underlying interlimb rhythmic coordination has used the relative phase angle (