Robustness
to two-object images in human visual cortex
Figure W9: About notch filters
We applied a 5th order
Butterworth digital notch filter (58 to 62 Hz) to the raw data in order to eliminate
potential line noise artifacts. While this filter does not affect any of the
conclusions of the current study, we describe here how it distorts the original
data. We emphasize that our main results do not depend on ÒgammaÓ oscillations
in the vicinity of 60 Hz but rather on low-frequency amplitude changes in the
evoked responses. A-B. Power spectrum of the raw data (blue)
and the notch filtered data (red). Note the sharp peak at 60 Hz in the blue
curve. C. Impulse response of the notch filter. D. One-second
sample showing the raw data (blue) and the filtered data (red). The large
low-frequency amplitude fluctuations are not affected by the notch filter. The
inset zooms in on a 50 ms segment of the data to emphasize the differences
between the red and blue curves.
Last updated: 1/15/10