Untitled

Abstract

Fourier analysis is a powerful tool in signal analysis that can be very fruitfully applied to steady-state evoked potentials (flicker ERG, pattern ERG, VEP, etc.). However, there are some inherent assumptions in the underlying discrete Fourier transform (DFT) that are not necessarily fulfilled in typical electrophysiological recording and analysis conditions. Furthermore, engineering software-packages may be ill-suited and/or may not fully exploit the information of steady-state recordings. Specifically: * In the case of steady-state stimulation we know more about the stimulus than in standard textbook situations (exact frequency, phase stability), so 'windowing' and calculation of the 'periodogram' are not necessary. * It is mandatory to choose an integer relationship between sampling rate and frame rate when employing a raster-based CRT stimulator. * The analysis interval must comprise an exact integer number (e.g., 10) of stimulus periods. * The choice of the number of stimulus periods per analysis interval needs a wise compromise: A high number increases the frequency resolution, but makes artifact removal difficult; a low number 'spills' noise into the response frequency. * There is no need to feel tied to a power-of-two number of data points as required by standard FFT, 'resampling' is an easy and efficient alternative. * Proper estimates of noise-corrected Fourier magnitude and statistical significance can be calculated that take into account the non-linear superposition of signal and noise. These aspects are developed in an intuitive approach with examples using both simulations and recordings. Proper use of Fourier analysis of our electrophysiological records will reduce recording time and/or increase the reliability of physiologic or pathologic interpretations.

Tradeoff between User Experience and BCI Classification Accuracy with Frequency Modulated Steady-State Visual Evoked Potentials

“…As SSVEPs typically have a high SNR, we used a short reasonable epoch-length (1 s) to compare SSVEP amplitudes to identify such non-responders. When comparing SSVEP-amplitudes for different frequencies, it is important that the spectra are based on time-windows that contain an integer number of stimulation cycles (Bach and Meigen, 1999). Subjects that showed no apparent SSVEP peaks at the stimulation frequencies will be referred to as non-responders.…”

Steady‐state visual evoked potentials as a research tool in social affective neuroscience

“…When the epoch is too short in duration, frequency resolution may not suffice for discriminating between the two or more frequencies used for tagging. It is also advisable to use epoch durations that hold integer numbers of cycles for each frequency, resulting in a spectrum with bins at the exact stimulation frequency (for an extensive discussion of these points, see Bach & Meigen, 1999). This serves to minimize distortions related to so-called "spectral leaking" or smearing of oscillatory responses across two or more bins of the spectrum that are equally distant from the actual frequency.…”

“…Such leaking may lead to misinterpretation of competition effects, as well as condition differences, especially when the mapping of tagging frequencies to stimuli and experimental condition is not counter balanced across the experiment. The interested reader is directed to reviews and guidelines regarding the technical aspects of ssVEP procedures (Bach & Meigen, 1999; Keil et al, 2014; Norcia et al, 2015; Vialatte, Maurice, Dauwels, & Cichocki, 2010). …”

“…The modulus of the resulting complex number represents an intuitive measure of the stability of the phase at the ssVEP frequency across the duration of the trial. Note that application of the method requires that integer numbers of EEG sample points are available for the window length k , as well as for the step size between windows t 1 (w+1)− t 1 (w), achieved by following the recommendations by Bach and Meigen (1999). It should be noted that the resulting measure reflects the regular and stable alignment of the oscillatory waveform to the driving stimulus over time, but not the phase-consistency within the ssVEP signal.…”

Evaluation of potential retinal toxicity of adalimumab (Humira)

“…ERG analysis for flash responses was based on measurements of the b-wave amplitude, from the trough of a-wave to the peak of b-wave. Recordings to 30 Hz steadystate stimulation were subjected to analysis in the frequency domain using discrete Fourier transformation (DFT) [22]; the magnitude at 30 Hz frequency was taken as response measure.…”