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Ignacio Serrano-Pedraza, Vina Manjunath, Olaoluwakitan Osunkunle, Michael P. Clarke, Jenny C. A. Read; Suppression in Intermittent Exotropia during fixation. Journal of Vision 2010;10(7):361. doi: 10.1167/10.7.361.
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© ARVO (1962-2015); The Authors (2016-present)
Intermittent exotropia (X(T)) is a common oculomotor anomaly where one eye intermittently deviates outwards. Patients with this type of strabismus are often not aware of the exodeviation and do not experience diplopia (Jampolsky 1954, American Orthoptic Journal, 4). The absence of diplopia during the divergent phase has been explained by suppression of the deviated eye. Since X(T) patients have stereopsis, it is widely believed that suppression occurs only during deviation. Here, we show that dichoptic images trigger suppression even during correct fixation. We studied 12 X(T) patients aged between 5 and 22 years. All had functional stereo vision with stereoacuity similar to that of 20 age-matched controls (0.2-3.7 arcmin).We measured suppression during fixation at 120cm. Each eye viewed an identical cartoon face (6x6 deg) dissociated by polarizing filters and presented for 400 msec. In one eye, the face was presented at the fovea; in the other, at different retinal positions along the horizontal axis. We also included catch-trials where two faces were presented in both eyes. The task was to indicate whether one or two faces were present. To ensure correct fixation, in between stimuli, subjects viewed a nonius image composed of a dissociated butterfly and a net on a binocularly-viewed forest background. All X(T) patients showed normal diplopia when the non-foveal face was presented in the nasal area of the retina. However 83% of X(T)s reported perceiving only one face when the non-foveal face was presented to temporal retina, indicating suppression during fixation. In a follow-up experiment, we examined which eye was suppressed. Some subjects suppressed the temporal stimulus regardless of which eye viewed it, so they always perceived the central stimulus; others always suppressed the same eye even when it viewed the central stimulus, so they then perceived the peripheral stimulus; others showed a mixture of both strategies.
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