title: The Effect of Stimulus Size on the Reliable Stimulus Range of Perimetry.
abstract: OBJECTIVE
  Automated perimetry uses a 3.5 log unit (35dB) range of stimulus contrasts to assess function within the visual field. Using 'Size III' stimuli (0.43°), presenting stimuli within the highest 15dB of available contrast may not increase the response probability at locations damaged by glaucoma, due to retinal ganglion cell response saturation. This experiment examines the effect of instead using 'Size V' (1.72°) stimuli.
  
  
  METHODS
  Luminance increment thresholds for circular spot stimuli of each stimulus size were measured in 35 participants (mean deviation -20.9 to -3.4 dB, ages 52-87) using the method of constant stimuli, at four locations per participant. Frequency-of-seeing curves were fit at each size and location, with three free parameters: mean, standard deviation, and asymptotic maximum response probability. These were used to estimate the contrasts to which each participant would respond on 25% of presentations (c25).
  
  
  RESULTS
  Using segmented orthogonal regression, the maximum observed response probabilities for size III stimuli began to decline atc25= 25.2 dB (95% confidence interval 23.3-29.0 dB from bootstrap resampling). This decline started at similar contrast for the size V stimulus:c25= 25.0dB (22.0-26.8 dB). Among locations at which the sensitivity was above these split-points for both stimulus sizes,c25averaged 5.6 dB higher for size V than size III stimuli.
  
  
  CONCLUSIONS
  The lower limit of the reliable stimulus range did not differ significantly between stimulus sizes. However, more locations remained within the reliable stimulus range when using the size V stimulus.
  
  
  CONCLUSIONS
  Size V stimuli enable reliable clinical testing later into the glaucomatous disease process.
authors.affiliation: Devers Eye Institute, Legacy Research Institute, Portland, OR, USA.
authors.forename: Stuart K
authors.initials: SK
authors.lastname: Gardiner
authors.affiliation: Devers Eye Institute, Legacy Research Institute, Portland, OR, USA.
authors.forename: Shaban
authors.initials: S
authors.lastname: Demirel
authors.affiliation: Devers Eye Institute, Legacy Research Institute, Portland, OR, USA.
authors.forename: Deborah
authors.initials: D
authors.lastname: Goren
authors.affiliation: Devers Eye Institute, Legacy Research Institute, Portland, OR, USA.
authors.forename: Steven L
authors.initials: SL
authors.lastname: Mansberger
authors.affiliation: Optometry and Vision Science, Indiana University, Bloomington, IN, USA.
authors.forename: William H
authors.initials: WH
authors.lastname: Swanson
keywords: glaucoma; perimetry; psychophysics; retinal ganglion cells