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Kenneth Alexander, Claire S. Barnes, Gerald A. Fishman, Joel Pokorny, Vivianne C. Smith; Spatial frequency characteristics of contrast processing deficits in retinitis pigmentosa. Journal of Vision 2003;3(12):56. doi: 10.1167/3.12.56.
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© ARVO (1962-2015); The Authors (2016-present)
Patients with retinitis pigmentosa (RP) were found previously to have a greater contrast processing deficit under conditions that favor the magnocellular (MC) pathway, yet they also show reductions in visual acuity, consistent with functional impairment within the parvocellular (PC) pathway. The purpose of this study was to clarify the relationship between contrast sensitivity deficits and stimulus spatial frequency in patients with RP, using testing paradigms designed to emphasize either the MC or PC pathway. Contrast sensitivity was measured for spatially localized, narrow-band patterns (sixth derivatives of Gaussians) at peak spatial frequencies ranging from 0.25 to 8 c/deg, using steady-pedestal (brief presentation of the test stimulus against a continuously presented adapting field) and pulsed-pedestal (simultaneous brief presentation of the test stimulus and adapting field) paradigms to emphasize the MC and PC pathways, respectively. The contrast sensitivity functions of 12 patients with RP (mean age, 38.9 + 11.3 years), with visual acuities ranging between 20/12.5 and 20/40, were compared to those of 10 visually normal control subjects (mean age, 39.2 + 13.3 years). The contrast sensitivity functions of the control subjects were low-pass for the steady-pedestal paradigm (presumed MC pathway mediation), and band-pass for the pulsed-pedestal paradigm (presumed PC pathway mediation), with the greatest sensitivity difference between the two paradigms at low spatial frequencies, as described previously. The RP patients showed the greatest reduction in contrast sensitivity at the highest spatial frequencies, but the pattern of contrast sensitivity loss differed for the two testing paradigms. At low spatial frequencies, the RP patients showed a greater loss of steady-pedestal than of pulsed-pedestal contrast sensitivity, indicating a greater impairment under conditions favoring the MC pathway. The differential patterns of sensitivity loss for the steady-pedestal and pulsed-pedestal paradigms illustrate their usefulness in evaluating the effect of photoreceptor degeneration on contrast processing within postreceptoral visual pathways.
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