The case
Bigand, Bianco, Abalde and Novembre (LIT-1424, read in NOTE-1093).
- Participants. 80 recruited, 40 dyads, 5 dyads lost to recording failure: 70 participants. Laypersons “without dance training”; “All pairs of participants were familiar with each other.”
- Setting. A “silent disco”: the two stood facing each other 2.5 m apart, each in a marked 0.5 × 0.7 m space, free to dance, not to touch, speak or sing.
- Manipulations. “We manipulated visual contact by placing (or not placing) a curtain in between the two participants. We manipulated musical input by presenting either the same song or different songs to the participants (through earphones).” Different songs were 8.5% apart in tempo. 32 trials of about 40 s, two per condition per block.
- Measure. Motion capture of 22 markers per person, decomposed by PCA into 15 principal movements; synchrony of each movement between the partners by cross-wavelet transform; time-resolved 2 × 2 ANOVAs, Bonferroni- and cluster-corrected.
- Result. Seeing the partner synchronized side sway, twist with hands, side hands and side shuffle. Shared music synchronized vertical hands, front-back sway, upper-body rock, twist and head bob. Bounce synchronized through both. “we found no evidence of an interaction between musical input and visual contact, and this was so for all PMs.” A classifier on the movements’ spatial directions predicted the category “with 100% accuracy”; one on the periodicity of their synchrony “confused 48% of the data associated with music-driven synchrony with partner-driven synchrony.”
What it shows
A pair’s synchrony can be apportioned between a shared external drive and coupling between the members, by intervening on each separately, and both can run at once in different components. The authors: “even when participants were presented with non-synchronous music, some PMs would still synchronize with the movements of the dancing partner, just as when participants were presented with synchronous music. Conversely, another group of PMs would keep synchrony with music, even if the partner was visible and moving at a different pace.”
And a measure that sees only timing would not have found it: “when (unlike where) participants move does not reflect whether synchrony is established through music or partner vision.” This is the result stated in THEORY-891, and it answers QUESTION-063 in part.
For the proposed study CASE-045, which fits a model to a dancing couple’s joint kinematics with the music’s beat as an exogenous input, it says the input is needed and is not uniform: the music drives some components, the partner others. A block criterion (CLAIM-157) applied to the whole body could mix the two.
What it does not show
- The direction of the coupling. Synchrony here is symmetric period locking. CASE-045’s lead→follow and follow→lead estimate, and the peer-or-hierarchy reading of CLAIM-167 and CLAIM-172, are untouched.
- A perturbation response. No member was perturbed; CASE-045’s compensation test is not run.
- Partner dancing. No touch, no lead and follow, no training: the couple CASE-045 proposes is coupled mainly through the hands and frame, a channel this study excluded.
- That the components are fully independent. No interaction was found; none was bounded.
- Why the geometry falls as it does. Visibility to a facing partner is the authors’ suggestion, untested.