Abstract
Purpose: :
To investigate the effect of the sweep direction, stimulus area, the presence of a fixation target, and the range of regression line fitting on the visual acuity and contrast threshold, measured with the sweep visual evoked potential (sVEP), in children between 6 and 8 years of age.
Methods: :
Sweep VEPs were measured with horizontal sine-wave gratings swept from both low to high and high to low contrast or spatial frequency, with and without the presence of a fixation target. Contrast threshold was measured for 1 and 8 cpd and three stimulus areas were used. The regression line to determine threshold was fitted with the following criteria: criterion 0 (C0) - by the software, criterion 1 (C1) - by eye, criterion 2 (C2) - from the peak amplitude to the last data point with a signal to noise ratio (SNR) > or = 1, criterion 3 (C3) - from the peak amplitude to the last data point with an SNR > or = 1 giving a threshold that is within the swept range, criterion 4 (C4) - from peak amplitude to the last data point with an SNR > or = 1 not including any data point with SNR is < 1.
Results: :
Repeated measures ANOVAs showed that, for all measures there was a significant effect of criterion (p<0.05), but no higher order interactions. There was a significant effect of the presence of fixation target for contrast threshold at 1 cpd, with lower threshold values when the fixation target was present (p = 0.025). There were no significant effects of the sweep direction or stimulus area. Post-hoc analysis revealed that, considering all the measures, C2 and C3 tended to be significantly different from C0, C1, and C4, giving higher visual acuity and lower contrast threshold values. C2 and C3 also tended to give more viable readings. Overall, of all the criteria, C2 gave higher VA and CS and more viable readings.
Conclusions: :
This study demonstrated that the criteria for regression line fitting had more effect on threshold values and the number of viable readings than the other parameters tested.
Keywords: visual acuity • contrast sensitivity • electrophysiology: non-clinical