Blue and green cone mechanisms in retinitis pigmentosa

Pronounced abnormalities in the increment threshold curves for the blue and green cone mechanisms only 10 degrees above the fovea, and in some cases in the fovea as well, have been found with a 2.5 degrees stimulus in different genetic types of retinitis pigmentosa. Whenever cone thresholds were abnormal, thresholds determined by the blue cone mechanism (pi1 and pi3) were more elevated than those determined by the green cone mechanism (pi4). Thresholds determined by the green cone mechanism were consistently more elevated at low background intensities than at intermediate and high background intensities. Threshold elevations for the blue and green cone mechanisms relative to each other in the perifovea in a given patient tested with the 2.5 degrees stimulus could be simulated in normal observers tested with a single small stimulus. These findings support the idea that the summation pools for blue and green cone mechanisms are proportionally reduced below normal in retinitis pigmentosa, at least in the perifovea, and are compatible with the idea that both mechanisms are comparably involved at the photoreceptor level.