High speed visual stimuli generator to estimate the minimum presentation time required for an orientation discrimination task

Abstract

How brief can a visual stimulus be and still be seen? To answer this question, we developed a digital micromirror device (DMD) based system operating at high speed (22.7 kHz) to control the rapid presentation of visual stimuli and estimated the minimum time required to identify the orientation of tumbling Snellen E letters. Time thresholds were measured in five subjects using a QUEST algorithm to vary the presentation time of the letters subtending either 0.75°, 1.5° and 4.5° on the retina, for two different effective pupil sizes (0.3 and 1 mm). Additionally, to evaluate the effect of defocus on time thresholds, the experiment was repeated with 1.5° letters and induced myopic defocus with 3, 6 and 9 D trial lenses placed in a conjugated pupil plane. We found that subjects were able to identify the orientation of the letters presented as briefly as 5 ms.

Keywords: (230.0230) Optical devices; (230.3990) Micro-optical devices; (230.6120) Spatial light modulators; (330.0330) Vision, color, and visual optics; (330.5510) Psychophysics; (330.6790) Temporal discrimination.

Fig. 1

Optical system and image sequence used to produce a fast visual stimuli presentation. Upper row: Maxwellian view system with the DMD homogenously illuminated with a broadband Xenon lamp and a variable diaphragm, AP1, placed in a conjugated pupil plane to control the effective pupil size. Bottom row: Sequence of images uploaded on the DMD to control the presentation time (t) of the letter E. The sequence of images uploaded on the internal memory of the device contained 100 images, among them 10 white letters on black background. The frequency could be varied between a few images per second and 22.7 kHz which means that, in this configuration, at the highest switching rate of the DMD, the letter E is displayed for 0.44 ms.

Fig. 2

Time threshold as a function letter size. Upper row: Time threshold measurements for the 5 participants and on average (green diamonds) across subjects for a 1 mm (A) and 0.3 mm (B) effective pupil size. Error bars stands for standard deviation from at least 3 repeated measurements in each condition. Lower row: Difference in the minimum time required to identify a 4.5° versus a 0.75° letter size as a function of subject’s refraction (absolute values). Data are for the 5 participants (black diamonds) and on average (green diamonds) across subjects for the 1mm (C) and 0.3 mm (D) effective pupil size.

Fig. 3

Time thresholds as a function of induced myopic defocus with trial lenses of 3, 6 and 9D. Data are for the 5 subjects (S1 to S5) and on average (AV, green diamonds) across subjects for a 1mm (left panels) and 0.3mm (right panels) effective pupil size. Error bars stands for standard deviation from at least 3 repeated measurements in each condition.