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Lei Liu, Hillary Gauthier; Perceptual instability of low contrast letters. Journal of Vision 2004;4(8):252. doi: https://doi.org/10.1167/4.8.252.
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The role of a new perceptual instability in low contrast letter recognition was investigated. Fourteen characters, which were morphs of a block-E letter, were created. They all had the 3 horizontal bars of the E, and differed in the way the 4 end gaps were sealed. For example, if the upper left and the lower right end gaps were sealed, the resulting character was a square “5”. If all 4 end gaps were sealed the character was a square “8”. The stroke width was 10′. Each character was 50′×50′ in size. These characters were displayed in 30-second periods. Periods were either varying, or constant. In a varying period, a character was displayed for 3 ± 0.5 seconds, and then was quickly morphed into another randomly selected character without changing character contrast. In a constant period, one character was shown continuously for 30 seconds. Constant and varying periods were randomly interlaced to make a 3-minute display of characters of the same contrast. The subjects' task was to verbally report the characters they saw. A voice recognition utility was used to record stimulus and response letters. Subjects with normal visual acuity participated in the study. They were trained to recognize and report the 14 characters. Their contrast thresholds for detecting the characters were also measured. When character contrast was greater than 5 times the detection threshold, stimulus characters were reported correctly. When contrast was 3 to 4 times the detection threshold, subjects began to make reporting errors, including reporting character changes during a constant period (typically, reporting a different character during the 30 second display, and then quickly reporting the correct character). At 2 to 3 times the detection threshold, more response errors were made, including 4 or 5 letter changes during a constant period. This new observation suggests that a perceptual instability that is analogous to monocular rivalry may impose a contrasts limit on letter recognition.
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