while spontaneity is often a desirable quality in music performance, the objective musical features that characterize genuinely spontaneous performances are unclear. We addressed this issue by recording pianists’ keystroke intensity and timing as they produced improvised jazz solos and rehearsed imitations of these solos. Results indicated that the entropy of keystroke intensity was reliably highest for improvisations. Corresponding effects for timing were less consistent. The effects observed for intensity may reflect irregularities in force control associated with relatively wide variations in the performer’s (un)certainty about upcoming actions during real-time musical invention. Related fluctuations in loudness may provide listeners with cues to musical spontaneity, thereby affecting aesthetic evaluations of performance quality.

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