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High-speed visual stimuli generator reveals minimum time required for letter identification


Purpose : 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.

Methods : 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.

Results : 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).

Conclusions : 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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