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H. Cheng, B. Zhang, K. Wicks, Y. Chino; Binocular Interactions in Human Strabismus: A Multifocal VEP Study. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1982.
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© ARVO (1962-2015); The Authors (2016-present)
To study the characteristics of binocular interactions in human strabismus
MfVEPs were recorded in 17 normal and 8 strabismic subjects with a 60-sector pattern-reversal dartboard stimulus (VERIS). Normal subjects were tested under 3 viewing conditions: 1) monocular 2) bi-foveal and 3) diplopic. Strabismic subjects were tested under conditions 1) and 2), and their natural viewing condition. A haploscope coupled with an alternating cover test was used to achieve bi-foveal or diplopic conditions. Response amplitude (amp) was the root-mean-square amplitude in the signal window. Binocular interaction was assessed by a binocular-monocular ratio (BMR) averaged over 60 sectors, and BMR for each sector was the ratio of a sector’s amp under two-eye viewing condition to the better monocular amp at that sector, and expressed in dB.
Normal subjects showed binocular summation (BMR>0) under bi-foveal condition and binocular inhibition (BMR<0) under diplopic condition for all eccentricities with a mean BMR±SD of 0.47±0.46 dB and -1.47±0.74 dB, respectively. All strabismic subjects tested under natural viewing condition and 7 under bi-foveal condition reported single vision (presumably due to clinical suppression), and no binocular inhibition was observed for bi-foveal (mean BMR: 0.82±0.60 dB) or natural viewing (mean BMR: 1.07±0.61 dB) conditions. One subject couldn’t achieve bi-foveal viewing due to horror fusionis. He noted diplopia when targets were brought as close as possible to his foveas and showed inhibition under this condition (BMR = -1.21 dB). Another strabismic subject could see single or double under natural viewing condition. Her BMR was 1.03 dB and -0.64 dB under single and diplopic conditions.
At the onset of strabismus (hence diplopia), V1 cortical physiology is likely to be dominated by inhibitory binocular interactions. However, since binocular inhibition could be demonstrated in our strabismic (and normal) subjects only under diplopic conditions and mfVEP signals are known to largely reflect the activity of V1 neurons, the neural mechanisms underlying clinical suppression in strabismic humans appear to reside beyond V1.
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