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John P. Kelly, Avery H. Weiss; Infantile nystagmus syndrome: Where is that visual cortical signal?. Invest. Ophthalmol. Vis. Sci. 2012;53(14):512.
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© ARVO (1962-2015); The Authors (2016-present)
Visual cortical responses are selectively reduced to checkerboard reversal stimulation when recorded in patients with infantile nystagmus syndrome (INS). This study performed a signal analysis on visual evoked potential (VEP) recorded in children with INS to determine if there is preservation of signals to check reversal (motion) relative to brief pattern onset of horizontal gratings.
Subjects were 9 children (0.4 - 8 yrs age) with INS without albinism, retinal, or optic nerve abnormalities. VEPs were recorded to contrast-reversing (1.4 Hz) checkerboards of 163 arc minutes and to pattern-onset of 0.5 cycle/deg horizontal sinewave gratings (on = 150 msec/off =500 msec). Contrast was 80% and there was no change in mean luminance. Individual VEP epochs underwent short-term-Fourier-transform (STFT) using a Hamming windowing (20-200 msec). VEPs were compared between raw averaging and selective averaging by optimizing epochs showing phase coherency at 5 high frequency harmonics (12 - 20 hz).
All subjects show robust improvement (up to 8.5 fold) in the check reversal VEP after optimization by STFT. The ratio of raw signal /STFT amplitude was on average 4.0. There was minimal change in latency (0.2 msec). Optimization by STFT had less effect on amplitude of pattern onset (ratio of raw signal /STFT amplitude = 2.1; latency difference = 1.6 msec). Selecting epochs with stronger coherency at one harmonic had similar effects across harmonics indicating coherency of across frequency bands.
There is a robust improvement to check reversal using a STFT analysis. This analysis likely extracts epochs in which the retinal image motion is minimal allowing for better visual sampling. This finding indicates a loss of visual cortical signals to motion relative to transient stimuli, likely due to the retinal image motion caused by INS. Reduction in VEP to motion stimuli is associated with loss of VEP coherency or an absence of cortical phase-locking.
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