Artificial gravity reveals that economy of action determines the stability of sensorimotor coordination.
BACKGROUND
When we move along in time with a piece of music, we synchronise the downward phase of our gesture with the beat. While it is easy to demonstrate this tendency, there is considerable debate as to its neural origins. It may have a structural basis, whereby the gravitational field acts as an orientation reference that biases the formulation of motor commands. Alternatively, it may be functional, and related to the economy with which motion assisted by gravity can be generated by the motor system.


RESULTS
We used a robotic system to generate a mathematical model of the gravitational forces acting upon the hand, and then to reverse the effect of gravity, and invert the weight of the limb. In these circumstances, patterns of coordination in which the upward phase of rhythmic hand movements coincided with the beat of a metronome were more stable than those in which downward movements were made on the beat. When a normal gravitational force was present, movements made down-on-the-beat were more stable than those made up-on-the-beat.


CONCLUSIONS
The ubiquitous tendency to make a downward movement on a musical beat arises not from the perception of gravity, but as a result of the economy of action that derives from its exploitation.
School of Psychology, Queen's University Belfast, Belfast, United Kingdom. r.g.carson@qub.ac.uk
Richard G
RG
Carson
Yalchin
Y
Oytam
Stephan
S
Riek
D000328:Adult; D001132:Arm; D005260:Female; D018470:Gravity, Altered; D006225:Hand; D006801:Humans; D008297:Male; D008962:Models, Theoretical; D009043:Motor Activity / Q000502:physiology*; D009146:Music; D009949:Orientation / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D012371:Robotics; D055815:Young Adult