The effect of stimulus contrast on the harmonic components of steady state visual evoked potentials

Abstract

AbstractPurposeSteady state visual evoked potentials (ssVEP) are widely used for objective functional diagnostics. Due to stimulation with intermittent stimuli, the evoked response contains frequency components related to the stimulation frequency and its harmonics. In general, the evoked amplitude at stimulation frequency is used for functional diagnostics. However, clinicals trails also reported main components at harmonic frequencies. The origin of the harmonic frequency components is not fully understood. This work aims to analyze the influence of the stimulation contrast on harmonic components of ssVEP.MethodsWe studied 10 healthy subjects (mean age 27.2 ± 5.8 years) using checkerboard stimuli (d = 7.2°) with a check size of 0.35°. Stimuli were presented in pattern‐reversal mode (7.5 changes per second) for 5 contrast levels (98%, 50%, 25%, 12.5%, 10%) at mean luminance of 50 cd/m2 and 20 cd/m2 in a random order. The active electrode was placed at Oz, reference electrode at FCz and ground electrode at FPz. The stimulations were performed monocularly in a dimmed room. The recorded ssVEP were transformed using Fourier Analysis. The normalized ratio of the amplitudes at stimulation frequency (7.5 Hz) and first harmonic (15 Hz) were calculated. For statistical analysis Wilcoxon test for paired samples were used.ResultsIn our investigation, all 10 subjects exhibited the main component at stimulation frequency (7.5 Hz). The normalized ratio of the amplitudes showed no significant differences between the 5 contrast levels for the mean luminance of 50 cd/m2 and 20 cd/m2.ConclusionThe presented work revealed, that main components at harmonic frequencies are not dependent on the stimulation contrast. The stimulation contrast has no significant effect on the harmonic responses of ssVEP.