Evaluation of magnitude and phase of sweep‐visual evoked potentials

Abstract

AbstractPurpose Sweep visual evoked potentials (SVEP) are frequently used for the assessment of visual acuity. Several studies have shown the presence in a significant number of subjects of a reduction of the response amplitude (“notch”) at intermediate spatial frequencies. The purpose of this study was to evaluate if this reduction of amplitude was related to a change in phase of the response.Methods SVEP records from 16 subjects presenting a notch were analyzed. The sweeps were performed with a checkerboard pattern reversing at a frequency of 12 Hz. The duration of the sweep was 12 seconds and a Fourier analysis was performed over a sliding time window. A vector average was calculated over the different sweeps recorded from each subject and the resulting magnitude and phase were plotted for the 2nd (12Hz) harmonic.Results The amplitude notch was found between 1.5 and 3 CPD. It was systematically correlated with a discontinuity of phase, typically constant for spatial frequency below the notch and increasing at a constant rate above that frequency. A numerical simulation pas performed to evaluate the effect of phase changes and demonstrated that a discontinuity of phase can produce a transient reduction in the measurement of magnitude similar to the notch found in the responses of some subjects.Conclusion This result gives a new possible interpretation for the notch found in SVEP responses of some subjects. It may help understand underlying physiological mechanisms. It also suggests that new algorithms taking into account phase changes may be more appropriate to analyze SVEP responses. Commercial interest