Semiparametric curve alignment and shift density estimation with application to neuronal data

Suppose we observe a large number of curves, all with identical, although unknown, shape, but with a different random shift. The objective is to estimate the individual time shifts and their distribution. Such an objective appears in several biological applications, in which the interest is in the estimation of the distribution of the elapsed time between repetitive pulses with a possibly low signalnoise ratio, and without a knowledge of the pulse shape. We suggest an M-estimator leading to a threestage algorithm: we split our data set in blocks, on which the estimation of the shifts is done by minimizing a cost criterion based on a functional of the periodogram; the estimated shifts are then plugged into a standard density estimator. We show that under mild regularity assumptions the density estimate converges weakly to the true shift distribution. The theory is applied both to simulations, as well as to alignment of real ECG signals. The estimator of the shift distribution performs well, even in the case of low signal-to-noise ratio, and it outperforms the standard methods for curve alignment.