A nonlinear analysis of the temporal characteristics of handwriting

Motor skills provide us with an almost infinite variety of ways in which we can interact with the world. This paper considers the problem of how the psychomotor system translates a stable motor memory into an invariant spatial output within an infinitely variable biomechanical and environmental context. Initially the validity of a novel methodology, based on the concatenation of handwriting velocity data over several trials to form long time series, combined with singular value decomposition to reduce noise, was confirmed. The data analyzed were the horizontal and vertical velocity of the stylus as eight participants wrote the pseudo-word madronal on a computer graphics tablet. Nonlinear dynamic analysis techniques such as examination of delay portraits, as well as calculation of the correlation dimension and Lyapunov spectra were applied to test the hypothesis that handwriting velocity profiles are chaotic. The findings that the largest Lyapunov exponents were positive, the sums of Lyapunov spectra components were negative and the correlation dimensions were low and fractional supported this hypothesis. We conclude by proposing that the psychomotor actions found in handwriting are a product of a chaotic dynamic process whose initial conditions depend on the environmental and biomechanical context. © 1999 Elsevier Science B.V.