Measurements of spatial frequency threshold (grating acuity) and contrast sensitivity using the sVEP have been obtained previously in full-term, healthy infants and adults.
16–18 In the present study, we used the same technique as in previous studies,
16–18 but we selected a low spatial frequency (i.e., 1 c/deg) and a large contrast sweep range (e.g., 10%–80%) for contrast threshold measurement owing to the poor vision in children with CVI. We also used a low mean luminance (20 cd/m
2) for spatial frequency threshold measurement based on a previous result indicating that visual acuity of children with CVI is best under lower luminance-viewing conditions.
10,11 In brief, stimuli were presented on a multisynch video monitor (1600 × 1200 pixels; 60-Hz vertical refresh, video bandwidth, 150 MHz; MRHB2000, Richardson Electronics, Inc., Jasper, AL). The stimulus field was 18° × 25° for CVI patients at 70-cm viewing distance and 10° × 13° for healthy controls at 130-cm viewing distance. The contrast sweep stimulus (
Fig. 1A) was a 1-c/deg black vertical cosine-wave grating presented on a 109 cd/m
2 space-average luminance white background screen in a pattern on/off mode at a rate of 3.76 Hz. To compare contrast response functions (amplitudes) between groups, the same contrast sweep range (from 10%–80%) in 10-logarithmic steps over a 10-second period was used both for children with CVI and age-matched controls. Because this contrast sweep range (from 10%–80%) was a suitable range for children with CVI for estimating a threshold due to their poor vision, but was a supra-threshold for typically developing infants, it could not be used to estimate a threshold via extrapolation to zero amplitude. Thus, we used historical control data for contrast sensitivity thresholds from one of our previous studies,
17 in which contrast sweep range was used from 0.25% to 20.00% for typically developing infants. The spatial frequency sweep stimulus (
Fig. 1B) was a black vertical cosine-wave grating presented on 20-cd/m
2 space-average luminance white background screen at 80% contrast in a pattern on/off at 3.76 Hz. Spatial frequency was swept from 1 to 12 c/deg for CVI patients and 2 to 28 c/deg for age-matched controls in 10 linear steps over a 10-second period.