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C. Timms, P. West; Suppressed Synkinetic Blinks : An Eye Movement Recording Curiosity . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2271.
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To determine the origin of brief saccadic overshoots in normal subjects when eye movements are recorded with optical eye tracking techniques, such as Video–Oculography (VOG). These overshoots, often attributed to artefact, are characterised by an initially hypermetric saccade followed by a small, apparently, corrective saccade in the opposite direction. The corrective saccade occurs within less than 20ms making it unlikely that it is visually guided.
We recorded eye movements in a simple visually cued saccade paradigm using a high speed precision video eye tracker. Subjects made horizontal saccades to targets presented on a CRT monitor. Digital video of the eye was recorded synchronously with the eye movement recording. The data were analysed by simultaneously inspecting the oculographic record with the associated video frames.
All subjects have these apparently hypermetric saccades. In one subject nearly all their saccades were of this type. In every instance of hypermetria with an associated short latency corrective, a simultaneous slight depression of the upper lid was observed.
Rottach et al have previously reported similar hypermetric saccades in synkinetic blinking and report that when voluntary saccades and blinks are simultaneous hypermetria occurs with short latency correctives. They suggest that this is due to co–contraction of the rectus muscles and invoke the commonality in the neural path for saccades. In our study subjects were instructed to keep their eyes open and to try not to blink, as is common practice. In these circumstances a motor command to blink is generated, as seen as in the twitch of the lid, and only the gross movement is suppressed. If such a suppressed blink occurs simultaneously with the termination of the saccade the innervation to the ocular muscles is largely unsuppressed resulting in co–contraction of the recti and a brief excursion of the eye that adds to the saccade. This finding has importance for clinical eye movement studies where suppressed blinks may be mistakenly diagnosed as saccadic hypermetria.
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