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John P Kelly, Felix Darvis, James O Phillips, Avery H Weiss; Infantile nystagmus syndrome in childhood: Isolation of the visual cortical signal.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3506.
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© ARVO (1962-2015); The Authors (2016-present)
Visual cortical responses are selectively reduced to checkerboard reversal stimulation when recorded from patients with infantile nystagmus syndrome (INS). This study examined if visual evoked potential (VEP) responses could be improved by signal processing and if these improvements were related to presumed foveation periods.
Subjects were 12 children (0.4 - 14 yrs age) with INS (normal retina without albinism or optic nerve hypoplasia). VEP amplitude, latency, and signal-to-noise ratios (SNR) were recorded to contrast-reversing (1.4 Hz) checkerboards of 163 arc minutes and compared to brief onset of horizontal gratings that reduced retinal image motion (0.5 cycle/degrees, 150/500 ms on/off period; constant mean luminance). Individual VEP epochs underwent 1) latency correction using phase cross correlation, or 2) selection of epochs based on phase consistency at 13 frequencies (5.5-21.9 Hz) in the Fourier domain. The probability of foveation in 9 subjects was estimated by the percentage of time eye velocity was ≤ 3 degrees/second from video-oculography.
VEP amplitude and SNR was significantly correlated with the probability of foveation for check reversal only (amplitude, r = 0.69; p = 0.026; SNR, r = 0.76; p = 0.011). VEP amplitude increased up to 2.5 fold after latency correction (p < 0.0001) and increased up to 5.4 fold after selective averaging based on phase consistency (p < 0.0001). Improvement in amplitude was always greater for check reversal compared to pattern-onset. The changes in VEP amplitude were confirmed by similar improvements in SNR (p < 0.0001). Phase correction had no effect on latency with either stimulus condition (p ≥ 0.06). Improvements in VEP amplitude were accounted for by a significant relationship between SNR and the probability of foveation (r = 0.66; p = 0.038).
The reduction in visual cortical signal in patients with INS is significantly related to a reduction in SNR due to reduced foveation periods. This reduction in VEP is associated with loss of VEP coherency or an absence of cortical phase-locking. Our findings were specific to stimuli undergoing retinal image motion, since there was no significant relationship to transient horizontally oriented stimuli. Our VEP analysis likely extracts brief epochs in the visual cortical signal in which the retinal image motion is minimal thus allowing for better visual sampling.
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