However, although SSVEP can be an objective method to assess visual acuity, mental fatigue caused by uncomfortable light twinkling and contrast changes of prolonged visual stimulus can decrease arousal level and attention, worsening the SSVEP signal quality and consequently degrading the practical performance ( Lee et al., 2010 Zhu et al., 2010 Cao et al., 2014 Chen et al., 2015). As for stimulus paradigms used in SSVEP visual acuity assessment, previous studies have compared some performance, such as sensitive electrodes, harmonic components of SSVEP response, correlation, and agreement between objective SSVEP and subjective psychophysical visual acuity, of six paradigms (reverse vertical sinusoidal gratings, reverse horizontal sinusoidal gratings, reverse vertical square-wave gratings, brief-onset vertical sinusoidal gratings, reversal checkerboards, and oscillating expansion–contraction concentric rings) ( Tobimatsu et al., 1993 Chen et al., 2019 Zheng et al., 2019a, 2020 Hamilton et al., 2020). There are some parameters, such as electrode placement, temporal frequency, stimulus area, and sweep duration, related to SSVEP visual acuity assessment, and some studies have given their recommended parameter settings ( Yadav et al., 2009 Almoqbel et al., 2011 Hemptinne et al., 2018). Within 40 years, the SSVEP technique has been used for measuring visual acuity in some studies, demonstrating that SSVEP provides an objective and quantitative method in visual acuity assessment, especially for infants or individuals with intellectual disabilities, hysteria, or malingering ( Tyler et al., 1979 Norcia and Tyler, 1985a, b Hemptinne et al., 2018). As an essential part of any ophthalmological or optometric examination, visual acuity is the most commonly measured visual function ( Fahad et al., 2008). Recently, there have been some research findings across a range of applications in vision science based on steady-state visual evoked potential (SSVEP) ( Norcia et al., 2015 Odom et al., 2016 Zheng et al., 2019b).
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