Abstract
Steady-state visual evoked potentials (ssVEPs) are commonly used for functional objective diagnostics. In general, the main response at the stimulation frequency is used. However, some studies reported the main response at the second harmonic of the stimulation frequency. The aim of our study was to analyze the influence of the stimulus design on the harmonic components of ssVEPs. We studied 22 subjects (8 males, mean age ± SD = 27 ± 4.8 years) using a circular layout (r1 = 0–1.6°, r2 = 1.6–3.5°, r3 = 3.5–6.4°, r4 = 6.4–10.9°, and r5 = 10.9–18°). At a given eccentricity, the stimulus was presented according to a 7.5 Hz square wave with 50% duty cycle. To analyze the influence of the stimulus eccentricity, a background luminance of 30 cd/m2 was added to suppress foveal stray light effects; to analyze the influence of simultaneous foveal and peripheral stimulations, stimulations are performed without stray light suppression. For statistical analysis, medians M of the amplitude ratios for amplitudes at the second harmonic to the first harmonic and the probability of the occurrence of the main response at the second harmonic P(MCSH) are calculated. For stimulations with foveal stray light suppression, the medians were M0–1.6° = 0.45, M1.6–3.5° = 0.45, M3.5–6.4° = 0.76, M6.4–10.9° = 0.72, and M10.9–18° = 0.48, and the probabilities were P0–1.6°(MCSH) = 0.05, P1.6–3.5°(MCSH) = 0.05, P3.5–6.4°(MCSH) = 0.32, P6.4–10.9°(MCSH) = 0.29, and P10.9–18°(MCSH) = 0.30. For stimulations without foveal stray light suppression, the medians M were M0–1.6° = 0.29, M1.6–3.5° = 0.37, M3.5–6.4° = 0.98, M6.4–10.9° = 1.08, and M10.9–18° = 1.24, and the probabilities were P0–1.6°(MCSH) = 0.09, P1.6–3.5°(MCSH) = 0.05, P3.5–6.4°(MCSH) = 0.50, P6.4–10.9°(MCSH) = 0.55, and P10.9–18°(MCSH) = 0.55. In conclusion, the stimulus design has an influence on the harmonic components of ssVEPs. An increase in stimulation eccentricity during extrafoveal stimulation leads to a transition of the main response to the second harmonic. The effect is enhanced by a simultaneous foveal stimulation.
Highlights
In clinical practice, visual evoked potentials (VEPs) are commonly used for ophthalmologic diagnostics
The recorded VEPs of the 22 subjects are shown in Figure 3 for the stimulations with foveal stray light suppression and in Figure 4 for the stimulations without foveal stray light suppression
The aim of this paper was to investigate the influence of stimulus design on the occurrence of the main response in steady-state VEPs (ssVEPs)
Summary
Visual evoked potentials (VEPs) are commonly used for ophthalmologic diagnostics. A distinction is made between transient VEPs, as a response to a single stimulus event, and steady-state VEPs (ssVEPs), as a response to intermittent stimulation with short stimulus interval time (Regan, 1966). Due to the advantages of short recording time with a high number of responses, ssVEPs are used for objective functional diagnostics, e.g., in the determination of visual acuity (Bach et al, 2008) or contrast threshold (Norcia et al, 1989). Visual stimuli are presented to the subject, and one stimulation parameter, e.g., the stimulation contrast, is swept over the stimulation time (Regan, 1973; Tyler et al, 1979; Norcia and Tyler, 1985). A review of the different sweep VEP techniques and use cases is given in Almoqbel et al (2008)
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