# MATLAB appears to be applying my filter to the spectrum twice, once on each side

So it looks like MATLAB is applying the filter to the "negative" and "positive" frequencies separately and I end up with two band pass regions with a null around zero. I want the whole spectrum filtered once.

[b,a] = butter(2,[.18 .3739],'bandpass');
hm = filter(b,a,Y);
figure;
plot(20*log10(abs((fftshift(fft(hm))))))


Any ideas?