Abstract
In the study of steady-state visual evoked potentials (SSVEPs), it remains a challenge to present visual flickers at flexible frequencies using monitor refresh rate. For example, in an SSVEP-based brain-computer interface (BCI), it is difficult to present a large number of visual flickers simultaneously on a monitor. This study aims to explore whether or how a newly proposed frequency approximation approach changes signal characteristics of SSVEPs. At 10 Hz and 12 Hz, the SSVEPs elicited using two refresh rates (75 Hz and 120 Hz) were measured separately to represent the approximation and constant-period approaches. This study compared amplitude, signal-to-noise ratio (SNR), phase, latency, scalp distribution, and frequency detection accuracy of SSVEPs elicited using the two approaches. To further prove the efficacy of the approximation approach, this study implemented an eight-target BCI using frequencies from 8–15 Hz. The SSVEPs elicited by the two approaches were found comparable with regard to all parameters except amplitude and SNR of SSVEPs at 12 Hz. The BCI obtained an averaged information transfer rate (ITR) of 95.0 bits/min across 10 subjects with a maximum ITR of 120 bits/min on two subjects, the highest ITR reported in the SSVEP-based BCIs. This study clearly showed that the frequency approximation approach can elicit robust SSVEPs at flexible frequencies using monitor refresh rate and thereby can largely facilitate various SSVEP-related studies in neural engineering and visual neuroscience.
Highlights
Steady-state visual evoked potential (SSVEP) is the brain’s electrical response to repetitive visual stimulation, which can be recorded from the scalp over the visual cortex, with maximum amplitude at the occipital region
The fast Fourier transform (FFT)-based method has been widely used in SSVEP-based brain-computer interface (BCI), recent studies reported that the canonical correlation analysis (CCA) method can significantly improve the signal-to-noise ratio (SNR) of the SSVEP signals [29,30]
The amplitudes of SSVEPs at the other frequencies using the approximation approach were comparable under the two refresh rates (75 Hz vs. 120 Hz, 9 Hz: 4.79 mV vs. 4.83 mV, 11 Hz: 3.10 mV vs. 2.99 mV, 13 Hz: 2.77 mV vs. 2.95 mV), and there was no significant difference between the two refresh rates
Summary
Steady-state visual evoked potential (SSVEP) is the brain’s electrical response to repetitive visual stimulation, which can be recorded from the scalp over the visual cortex, with maximum amplitude at the occipital region. In visual neuroscience research, there are many situations that require accurate measurement of signal characters of SSVEPs at multiple frequencies [3,4] In these circumstances, the feasibility of the approximation approach highly depends on the stability and robustness of the elicited SSVEPs. In these circumstances, the feasibility of the approximation approach highly depends on the stability and robustness of the elicited SSVEPs To answer these questions, this study proposes to compare the amplitude, SNR, phase and latency, scalp distribution, and frequency detection accuracy of SSVEPs elicited using the two approaches with a CRT monitor at two refresh rates (75 Hz and 120 Hz). The amplitude spectrum F (f ) was calculated by taking the absolute value of the fast Fourier transform (FFT):
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