Suprathreshold contrast perception of resolvable high spatial frequencies remain intact in keratoconus

Contrast detection thresholds are elevated with optical quality loss in keratoconus. This study hypothesized that suprathreshold contrast perception is also impaired in keratoconus, with the impairment being predictable from the pattern of loss in threshold-level performance. Contrast detection thresholds were determined across a range of spatial frequencies in 12 cases with mild to severe keratoconus and 12 age-similar controls. These values were used to predict the contrast needed to achieve perceptual matches between reference and test spatial frequency pairs (peak of CSF Vs. 0.3x, 0.5x, 2x or 3x spatial frequency from the peak) for stimuli at 10% and 50% suprathreshold contrast. Contrast thresholds predicted a 1.5 to 6.7-fold increase in the test pattern's contrast to obtain a perceptual match with the reference pattern in keratoconus, relative to controls. Contrary to predictions, the empirical data of contrast matches between test and reference patterns were similar for higher than peak spatial frequencies at both contrast levels. However, as predicted, test patterns required higher contrast than the reference pattern for a perceptual match for lower than peak spatial frequencies. These results were similar to controls and invariant of disease severity, interocular asymmetry and short-term changes in optical quality. Unlike thresholds, suprathreshold contrast perception of resolvable high spatial frequencies appears immune to optical quality losses in keratoconus. These results are discussed in the context of the prevailing models of contrast constancy in healthy humans. Breakdown of contrast constancy at lower than peak spatial frequencies may reflect the properties of the testing paradigm employed here.