Contrast discrimination deficits in retinitis pigmentosa are greater for stimuli that favor the magnocellular pathway

Luminance contrast discrimination was measured in 14 patients with retiniti s pigmentosa (RP) and 14 control observers with normal vision, using steady -pedestal and pulsed-pedestal paradigms [Pokorny, J., & Smith, V. C. (1997) . Psychophysical signatures associated with magnocellular and parvocellular pathway contrast gain. Journal of the Optical Society of America A, 14, 24 77-2486] to bias performance toward the magnocellular (MC) or parvocellular (PC) pathway, respectively. The aim was to determine the relative effects of retinal degeneration on MC- and PC-pathway function in RP. For five of t he RP patients, contrast discrimination thresholds were within normal limit s for both the steady-pedestal and pulsed-pedestal paradigms. The other nin e RP patients showed threshold elevations for the steady-pedestal paradigm (presumed magnocellular mediation), whereas their thresholds for the pursed -pedestal paradigm (presumed parvocellular mediation) were within normal li mits for all but the two patients who had the most extreme threshold elevat ions using the steady-pedestal paradigm. A control experiment on four of th e RP patients, using a greater number of pedestal contrasts, verified that the patients' thresholds for the pulsed-pedestal paradigm showed the patter n expected for contrast discrimination mediated by the PC pathway. The high er threshold elevations for the steady-pedestal paradigm than for the pulse d-pedestal paradigm indicate that the retinal degeneration that occurs in R P predominantly disrupts contrast discrimination under stimulus conditions that favor the MC pathway. (C) 2001 Elsevier Science Ltd. All rights reserv ed.