title: Effect of luminance noise on the object frequencies mediating letter identification.
abstract: OBJECTIVE
  To determine if the same object frequency information mediates letter contrast threshold in the presence and absence of additive luminance noise (i.e., "noise-invariant processing") for letters of different size.
  
  
  METHODS
  Contrast thresholds for Sloan letters ranging in size from 0.9 to 1.8 log MAR were obtained from three visually normal observers under three paradigms: (1) high- and low-pass Gaussian filtered letters were presented against a uniform adapting field; (2) high- and low-pass Gaussian filtered letters were presented in additive white luminance noise; and (3) unfiltered letters were presented in high- and low-pass Gaussian filtered luminance noise. A range of high- and low-pass filter cutoffs were used to limit selectively the object frequency content of the letters (paradigms 1 and 2) or noise (paradigm 3). The object frequencies mediating letter identification under each paradigm were derived from plots of log contrast threshold vs. log filter cutoff frequency.
  
  
  RESULTS
  The object frequency band mediating letter identification systematically shifted to higher frequencies with increasing log MAR letter size under all three paradigms. However, the relationship between object frequency and letter size depended on the paradigm under which the measurements were obtained. The largest difference in object frequency among the paradigms was observed at 1.8 log MAR, where the addition of white noise nearly doubled the center frequency of the band of object frequencies mediating letter identification, compared to measurements made in the absence of noise.
  
  
  CONCLUSIONS
  Noise can affect the object frequency band mediating letter contrast threshold, particularly for large letters, an effect that is likely due to strong masking of the low frequency letter components by low frequency noise checks. This finding indicates that noise-invariant processing cannot necessarily be assumed for large letters presented in white noise.
authors: Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago Chicago, IL, USA ; Department of Bioengineering, University of Illinois at Chicago, Chicago IL, USA.
authors: Cierra
authors: C
authors: Hall
authors: Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago Chicago, IL, USA ; Department of Electrical and Computer Engineering, University of Illinois at Chicago Chicago, IL, USA.
authors: Shu
authors: S
authors: Wang
authors: Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago Chicago, IL, USA.
authors: Reema
authors: R
authors: Bhagat
authors: Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago Chicago, IL, USA ; Department of Bioengineering, University of Illinois at Chicago, Chicago IL, USA ; Department of Psychology, University of Illinois at Chicago Chicago, IL, USA.
authors: J Jason
authors: JJ
authors: McAnany
keywords: contrast sensitivity; letter identification; object spatial frequency; optotype; retinal spatial frequency; visual noise