| Summary: | Purpose and hypothesis:
The purpose of this study was to optimize and standardize the following parameters of sweep Visually Evoked Potential (sVEP) in adults: criteria for fitting the regression line to estimate threshold, luminance, electrode placement, temporal frequency, sweep direction, presence of fixation target and stimulus area. The hypothesis is that the parameters chosen will have an impact on the measured visual acuity, contrast threshold and on the number of viable sVEP plots.
Methods:
The Power Diva software, Version 1.9 was used for this study. Five gold cup active electrodes, one reference electrode and one ground electrode were used to measure the Electroencephalography (EEG) signals. Six adult participants (aged 17 to 35 years), with corrected to normal visual acuity and no history of ocular disease took part in each experiment, except for the repeatability experiment in which 3 subjects participated. Four criteria for regression line fitting were compared. Psychophysical thresholds were used to validate the sVEP measures for the different criterion and repeatability of sVEP was estimated for 10 sessions. The effect of luminance (25 cd/m2, 50 cd/m2, 100 cd/m2), electrode placement (Power Diva and ISCEV), temporal frequency (6 Hz, 7.5 Hz, 10 Hz), sweep direction, fixation target and stimulus area were investigated. A repeated measure ANOVA statistical method was used to analyze the average threshold and the number of viable plots out of five active channels for all subjects.
Results:
Criterion 2 and 3 gave better visual acuity, higher contrast sensitivity, better repeatability and gave results that were closer to the psychophysical threshold than criterion 0 and 1. Luminance of 25 cd/m2 gave significantly fewer viable readings than 50 and 100 cd/m2 while measuring visual acuity (F = 5.11, df = 2, p = 0.0295). Temporal frequency of 7.5 Hz gave significantly more viable readings than 6 and 10 Hz while measuring visual acuity (F = 50.53, df = 2, p < 0.0001) and contrast threshold (F = 9.87, df = 2,p = 0.0043). There was a highly significant interaction of criterion with temporal frequency (F = 1536.98, df = 6, p < 0.0001) while measuring contrast threshold. There was a significant interaction of criterion with sweep direction (F = 4.26, df = 3, p = 0.0231) and for the number of readings (F = 3.75, df = 3, p = 0.0343) while measuring visual acuity. There was an interaction of criterion with sweep direction (F = 4.97, df = 3, p = 0.0136) while measuring contrast threshold at a spatial frequency of 1 cpd. There was a significant effect of fixation target (F = 7.64, df = 1, p = 0.0396) while measuring visual acuity. There was a significant effect of stimulus area (F = 11.78, df = 4, p < 0.0001) on the number of readings while measuring contrast threshold.
Conclusion:
The sVEP parameters chosen do have a significant effect on visual acuity, contrast threshold and on the number of viable readings. The following parameters are recommended in adults on the basis of results; Criterion 2 or 3 for fitting regression line (C2 - regression line fitted from the signal peak amplitude to the last data point with a signal to noise ratio (SNR) >1; C3 – similar to criterion 2, but the threshold should be within sweep range used), luminance of 50 or 100 cd/m2 , either Power Diva (PD) or International Society for Clinical Electrophysiology of Vision (ISCEV) electrode placement, temporal frequency of 7.5 Hz, either sweep direction, measurement with the central fixation target, larger stimulus area.
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