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 “gamma” 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.