title: Individual Letter Contrast Thresholds: Effect of Object Frequency and Noise.
abstract: OBJECTIVE
  To compare differences in contrast threshold among individual Sloan letters presented in additive white luminance noise and in the absence of noise.
  
  
  METHODS
  Contrast threshold for letter identification was measured for three visually normal subjects (aged 22, 25, and 34 years) using letters from the Sloan set (C, D, H, K, N, O, R, S, V, and Z). The letter size was equivalent to 1.5 logMAR (logarithm of the minimum angle of resolution), and the letters were either unfiltered or band-pass filtered to limit the object frequency content (cycles per letter) to a one-octave wide band centered at 1.25, 2.5, 5, and 10 cycles per letter. Letters were presented for an unlimited duration against a uniform adapting field or in the presence of additive white luminance noise. Contrast threshold for each letter was determined using a 10-alternative forced-choice interleaved staircase procedure.
  
  
  RESULTS
  For standard unfiltered Sloan letters presented against a uniform field, contrast threshold for individual letters differed by as much as a factor of 1.5, consistent with a previous report. When measured in luminance noise, the individual letters differed by as much as a factor of 1.8. Band-pass filtering the letters to include only low object frequencies increased the differences in contrast threshold among the individual letters (about a factor of 3) compared with unfiltered letters and letters filtered into high object frequency bands.
  
  
  CONCLUSIONS
  The addition of white luminance noise had relatively small effects on interletter contrast threshold differences, whereas band-pass filtering had large effects on interletter threshold differences, greatly increasing variation among the letters that contained only low object frequencies. Letters that contain only high object frequencies may be useful in the design of letter charts because the interletter threshold differences are relatively small for these optotypes and the object frequency information mediating identification is known.
authors.affiliation: *BA †PhD Department of Ophthalmology and Visual Sciences (all authors), Department of Bioengineering (CH, JJM), Department of Electrical and Computer Engineering (SW), and Department of Psychology (JJM), University of Illinois at Chicago, Chicago, Illinois.
authors.forename: Cierra
authors.initials: C
authors.lastname: Hall
authors.forename: Shu
authors.initials: S
authors.lastname: Wang
authors.forename: J Jason
authors.initials: JJ
authors.lastname: McAnany
mesh_terms: D000328:Adult; D002755:Choice Behavior; D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D009622:Noise; D012684:Sensory Thresholds / Q000502:physiology*; D014787:Vision Tests / Q000295:instrumentation*; D055815:Young Adult