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How are band powers calculated?

EmotivPRO uses Fast fourier transform (FFT) to obtain band powers in µV2/Hz.

High-pass filter: The data is high-pass filtered before performing the fourier transform. We use the following coefficients:

filter_coeffs": {

"b": [0.96588528974407006000, -1.93177057948814010000, 0.96588528974407006000],          

"a": [1.00000000000000000000, -1.93060642721966810000, 0.93293473175661223000]}

DC Removal: We subtract the mean of the data for each epoch/window as the DC value is not usable and only distorts the Fourier transform of the lower frequencies.

eeg_norm = eeg - np.mean(eeg, axis=0)

**EmotivPRO process data in each epoch/window. Generally, 2 seconds epoch/window is used that consists of 256 eeg data samples.

Hanning Window: We use a hanning window size of 256 samples and we slide this window by 16 samples to create the new window.  The exact implementation of the Hanning window we use:

   hanning_window = []

    for i in range(256):

        hanning_window.append(0.5 * (1 - np.cos((2.0 * np.pi * (i + 1)) / (256 + 1))))

Band Powers: To compute band powers:

hanning_window = np.hanning(eeg_norm.shape[0]) * 2

eeg_norm_fft = (eeg_norm.T * hanning_window).T

fourier_transform = fft(eeg_norm_fft, axis=0)/eeg_norm_fft.shape[0]

eeg_fft_square_3d = np.absolute(fourier_transform)**2

band = list(range(16, 25))

band_power = np.sum(eeg_fft_square_3d[band, :, :], axis=0) / len(band)

Normalization: To normalize, EmotivPRO divides the output of the fft by the window length, i.e. 256

band_power_norm = band_power / hanning_window.shape[0]

Updated on 25 Mar 2024

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