Effects of digital low pass filtering on the parameters of nystagmus eye movements

We examined how lowpass filtering has an influence on nystagmus parameters: durations, amplitudes, and mean velocities of its slow and fast phases. We experimented with two types of common linear digital filters by using both nonrecursive and recursive filters, and with two types of nonlinear digital filters by using so-called standard median and hybrid median filters. We concluded that 70 Hz as strict cutoff frequency is high enough not to cause errors in the nystagmus parameters when information content in nystagmus signals is filtered above that cutoff frequency with linear filters. Using nonlinear filters good window lengths of filtering are equal to or less than about nine samples for a sampling frequency of 400 Hz which we applied.