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P. C. Knox, N. Amatya, Q. Gong; How Common Are "Express Saccade Makers"? Comparisons Between Chinese and Caucasian Subjects. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2541.
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A gap between fixation target offset and saccade target onset encourages the production of express saccades (ES; latency 80-120ms). Some subjects (express saccade makers, ESMs) produce almost exclusively ES in gap trials, and in overlap trials (fixation target remains present when the saccade target appears), a large proportion (>30%) of saccades have latencies in the ES range. The literature suggests that this is rare in the absence of pathology and/or extensive training.
We exposed 33 Chinese (median age: 27y), and 40 Caucasian (median age: 27.5y) naive, healthy, adult subjects to gap (duration 200ms) and overlap trials (2 x 200 trials for each condition, run in separate blocks, targets 10° left/right randomised, randomised fixation time 1s-2s, order of blocks randomised between subjects). While the groups were tested in two different places, we standardised testing conditions (eg viewing distance, subject instructions) carefully. Eye movements were recorded using an infrared reflectance eye tracker. For each subject, the percentage of saccades with latencies of 80ms to 120ms (ES) was calculated for all saccades with latencies between 50ms and 500ms.
Of 33 Chinese subjects, 10 exhibited a high proportion (>30%) of ES in overlap conditions, while this was true of only 1 of 40 Caucasian subjects (proportions of 30% vs 2%). In the Chinese ESMs, ES made up 53±19% (mean±SD) of saccades in gap, falling to 42±9% of saccades in overlap conditions. In non-ESMs, ES made up 20±14% of saccades in gap, falling to 14±10% in overlap conditions. Comparable figures for the non-ESM Caucasian group were 27±19% falling to 6±6%.
These results demonstrate that while "express saccade makers" are indeed relatively rare (2%) in the Caucasian group, they are much more common among Chinese subjects (30%). Thus using a high proportion of express saccades as a biomarker for pathology may be unwise in a population in which 30% of healthy subjects already meet the definition of ESM.
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