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Jérome Fleuriet, Mark Walton, Seiji Ono, Michael Mustari; Electrical microstimulation of the deep superior colliculus (SC) in strabismic monkeys. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1930.
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© ARVO (1962-2015); The Authors (2016-present)
Although the neural basis for saccadic eye movements is relatively well understood in normal monkeys, very few neurophysiological studies have investigated the integrity of this system in strabismic subjects. The SC is a laminated structure at the interface between sensory systems and saccade premotor centers. The superficial layers contain visual neurons organized in a retinotopic map and deep layers contain a topographic map related to saccadic eye movements. Stimulation of the SC in normal monkeys evokes conjugate saccades. Because the motor layers of SC have never been investigated in strabismic monkeys, it is unclear how their topographic organization develops. For example separate motor maps might exist for the two eyes, with identical topography. In this case, stimulation of a single site might evoke conjugate saccades even in a monkey with strabismus. In contrast, if the topography develops differently for the two eyes, then stimulation would evoke dysconjugate saccades. The present study was designed to assess the organization of the deep layers of SC using microstimulation.
The deep layers of SC were stimulated in esotropic (Monkey-Q, 15° ET) and exotropic (Monkey-P; 30° XT) monkeys during fixation of a 1° laser spot. Stimulation was delivered at 4 sites in each monkey unilaterally or bilaterally (M-Q). Train durations of 50, 100, and 200 ms were used. Current levels (10µA - 50µA) were adjusted to elicit the site-specific maximum saccade amplitude. Frequency was held at 400 Hz. Eye movements were measured with search coils (CNC) and video-oculography (SMI) methods.
Evoked saccades were consistently dysconjugate. Significant amplitude differences (p<0.05) were found for at least one (100% of sites) or both components (50% of sites). For the horizontal component, the mean dysconjugacy was 24% (M-Q) and 27% (M-P). For the vertical component, the means were 27% (M-Q) and 43% (M-P). The most striking effect was found at a site in monkey-P where stimulation evoked rightward saccades of 15° for the right eye and 7° for the left. For two sites, the direction of the vertical component was opposite for the two eyes.
These results suggest at least partly separate control of the two eyes in strabismic monkeys. Single unit recordings will be necessary to determine if dysconjugate saccades result from abnormalities at the level of SC, downstream structures, or both.
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