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Lucie Sawides, Adrian Gambin, Alberto de Castro, Benjamin Lochocki, Pablo Artal; High-speed visual stimuli generator reveals minimum time required for letter identification. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5629.
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© ARVO (1962-2015); The Authors (2016-present)
How brief can be a visual stimulus, such as a letter, to be seen and identified? This important question is elucidated here by measuring time identification thresholds of briefly presented tumbling E letters.
We developed a new experimental apparatus for controlled presentation of extremely brief visual stimuli. The system is also capable to project high quality images into the retina of subjects with a large range of uncorrected refractive errors. The system is based in a Digital Micromirror Device (DMD) operating at high speed (22000 Hz). The DMD was illuminated with homogeneous white light (luminance at subject’s pupil =825 cd/mm2) and placed in a Maxwellian view configuration with an effective pupil diameter of 1 mm. High contrast Snellen E letters subtending 1.2°, 2.35° and 7° on the retina respectively were presented. For each letter size, four sequences of images were generated consisting of white Snellen E letters – either pointing right, left, up or down – on black background temporarily sandwiched between empty white background images to avoid the effect of “flashes” and minimize visible persistence. Time identification thresholds were measured in 5 normal young subjects by using a QUEST algorithm to vary the presentation duration of the letters. A test consisted of at least 25 sequence presentations, in each of them the letter orientation was randomly chosen and subjects had to report its orientation. Additionally, 3 subjects repeated the experiment for one letter size with induced myopic defocus of 3D, 6D and 9D for 1mm and 0.3mm pupil size.
Subjects were able to identify letters presented as briefly as 5 ms. Time identification threshold decreases with increasing letter size, on average across subjects: 7.9±3.5ms (E letter subtending 1.2°), 6.5±2.3ms (for 2.35°) and 5.3±1.6ms (for 7°). With induced myopic defocus, time identification thresholds increase with a 0.39ms/D slope (R2=0.99) for 1mm subject’s pupil from 4.45±0.57ms (no defocus) to 7.94±1.09ms (9D myopic defocus) whereas almost no increase was found for 0.3mm pupil: threshold = 4.39±0.32ms (0D) to 5.09±0.68ms (9D).
The high speed Maxwellian visual stimuli generator allowed us to present high quality retinal images during brief intervals in subjects with a large range of uncorrected refractive errors. We found that the minimum duration of a letter to be seen and identified was as small as 5 ms.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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