usage: y = filtfilt(b, a, x)

Forward and reverse filter the signal. This corrects for phase
distortion introduced by a one-pass filter, though it does square the
magnitude response in the process. That's the theory at least.  In
practice the phase correction is not perfect, and magnitude response
is distorted, particularly in the stop band.

Example
[b, a]=butter(3, 0.1);                   % 10 Hz low-pass filter
t = 0:0.01:1.0;                         % 1 second sample
x=sin(2*pi*t*2.3)+0.25*randn(size(t));  % 2.3 Hz sinusoid+noise
y = filtfilt(b,a,x); z = filter(b,a,x); % apply filter
plot(t,x,';data;',t,y,';filtfilt;',t,z,';filter;')