Abstract
The purpose of this study was to enhance the performance of steady-state visual evoked potential (SSVEP)-based visual acuity assessment with spatial filtering methods. Using the vertical sinusoidal gratings at six spatial frequency steps as the visual stimuli for 11 subjects, SSVEPs were recorded from six occipital electrodes (O1, Oz, O2, PO3, POz, and PO4). Ten commonly used training-free spatial filtering methods, i.e., native combination (single-electrode), bipolar combination, Laplacian combination, average combination, common average reference (CAR), minimum energy combination (MEC), maximum contrast combination (MCC), canonical correlation analysis (CCA), multivariate synchronization index (MSI), and partial least squares (PLS), were compared for multielectrode signals combination in SSVEP visual acuity assessment by statistical analyses, e.g., Bland–Altman analysis and repeated-measures ANOVA. The SSVEP signal characteristics corresponding to each spatial filtering method were compared, determining the chosen spatial filtering methods of CCA and MSI with a higher performance than the native combination for further signal processing. After the visual acuity threshold estimation criterion, the agreement between the subjective Freiburg Visual Acuity and Contrast Test (FrACT) and SSVEP visual acuity for the native combination (0.253 logMAR), CCA (0.202 logMAR), and MSI (0.208 logMAR) was all good, and the difference between FrACT and SSVEP visual acuity was also all acceptable for the native combination (−0.095 logMAR), CCA (0.039 logMAR), and MSI (−0.080 logMAR), where CCA-based SSVEP visual acuity had the best performance and the native combination had the worst. The study proved that the performance of SSVEP-based visual acuity can be enhanced by spatial filtering methods of CCA and MSI and also recommended CCA as the spatial filtering method for multielectrode signals combination in SSVEP visual acuity assessment.
Highlights
Visual acuity, one of the most necessary parameters to test visual function, is a measure of the spatial resolution of the visual processing
The time-domain waveforms were obtained by averaging the 0.53-s nonoverlapping data segments subdivided by the 5-s single-channel state visual evoked potential (SSVEP) signals, with each segment containing four periods of the reversal process (Zheng et al, 2020a)
The time-domain waveforms in Figure 4A show that an obvious main periodicity was the fundamental reversal frequency of 7.5 Hz for all spatial filtering methods except for the twodimensional Laplacian combination, while some other periodic components existed in some waveforms, such as the native, bipolar, and one-dimensional Laplacian combination
Summary
One of the most necessary parameters to test visual function, is a measure of the spatial resolution of the visual processing. It is mainly tested by psychophysical methods, e.g., Sloan letters and tumbling E charts (Ricci et al, 1998). In SSVEP visual acuity assessment, SSVEPs are mainly collected at only one active electrode, e.g., Oz at the midline over the occiput (McBain et al, 2007; Odom et al, 2016; Ridder, 2019), except for some other electrode montages, e.g., the bipolar electrodes of Oz and O1 (Norcia and Tyler, 1985a,b; Skoczenski and Norcia, 1999), which was sometimes used to enhanced signal-to-noise-ratio (SNR), especially close to the threshold (Hamilton et al, 2021b)
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