SSVEP at Single and Beating Frequencies Utilizing Perceptual Insights

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Steady state visual evoked potentials (SSVEP) are widely used in EEG research as they offer a relatively high signal to noise ratio allowing the investigation of visual processing at the cortical level. When the eyes are presented with a flickering stimulus (up to 100Hz), the visual cortex generates a response oscillating at the same fundamental frequency as the stimulus. For research purposes, these stimuli always had the goal of eliciting a sufficiently strong EEG response. As a negative result, these stimuli are annoying to most users and even become dangerous in regard to photosensitive epilepsy. Results from psychophysical research were used to generate flickering stimuli which are around the human perception threshold for flickering lights. It is shown for multiple frequencies that eliciting an SSVEP response with stimuli below the perception threshold is possible. The human sensitivity to low frequency flicker makes it difficult to elicit an SSVEP response around the perception threshold. Therefore, we tried generating waves combining two higher frequencies and using the non-linearity of the human visual system to generate a sub-harmonic beating frequency. We attempted to elicit an 8Hz response by using the sum wave of n*8Hz and (n+1)*8Hz (where n is a positive integer). Keywords: SSVEP, modulation depth, frequency, repetitive visual stimuli, EEG, beating frequency
Faculteit der Sociale Wetenschappen