Artefact cancellation in motor-sensory evoked potentials: two approaches using adaptive filtration and exponential approximation

Adaptive filtering for artefact cancellation in motor-sensory evoked potent ials using signals obtained by subtraction methods (double-stimulus, off-ne rve and subthreshold) is proposed. This is advantageous as inherent non-lin ear distortions can be overcome in an easier way by adaptive filtering. Eff iciency is assessed with reference signals synthesised by varying the shape and reducing the amplitude of a 'pure' evoked potential in the range from 10% to 50%. The experiments show virtually identical shapes of the 'pure' a nd the filtered signal. The time shift between them is insignificant if a c ausal filter and small number of Widrow coefficients, e.g. N=8, are used. F urther, two-exponential artefact approximation is applied with subsequent d irect subtraction from the contaminated signal by a specially designed PC-c ontrolled system for data acquisition and processing. For a fast procedure convergence, one-parametric optimisation of the time-constant tau is used, starting with tau=0.5 ms. The results obtained with artefact-corrupted evok ed potentials from several subjects prove the efficiency of the approach. I t has the substantial advantage of avoiding the need for reference signals. Both methods have advantages compared with other known software techniques .