Contrast response properties of magnocellular and parvocellular pathways in retinitis pigmentosa assessed by the visual evoked potential

Abstract

Purpose: To evaluate the contrast response of the visual system in retinitis pigmentosa (RP) under conditions designed to emphasize the parvocellular (PC) and magnocellular (MC) pathways.

Method: Visual evoked potentials (VEPs) were measured in 10 patients with RP and in 10 age-equivalent control subjects with normal visual acuity and color vision, by using an array of isolated checks that were presented against a steady yellow background. The checks were modulated sinusoidally, either in isoluminant chromatic contrast (5.6 Hz), to favor the chromatic PC pathway, or in luminance contrast (5.6 and 11.2 Hz), to favor the MC pathway. Response amplitude and phase at the stimulus (fundamental) frequency were derived from Fourier analysis, and contrast response functions were fit with a Michaelis-Menten equation to derive R(max), the maximum response amplitude, and sigma, the contrast necessary to produce R(max)/2.

Results: In the control subjects, the mean amplitude function for chromatic modulation increased approximately linearly with increasing contrast, whereas the function for luminance modulation increased sharply at low contrasts and saturated at contrasts above approximately 30% for both temporal frequencies, as expected. The patients with RP showed primarily a reduction in R(max) with little change in sigma in all testing conditions. The reduction in R(max) was equivalent for chromatic modulation and luminance modulation at 5.6 Hz, but was substantially lower for luminance modulation at 11.2 Hz.

Conclusion: Contrast processing was impaired within both the MC and PC pathways in these patients with RP, but the degree of impairment within the MC pathway depended on temporal frequency. These VEP results are in general agreement with recent psychophysical studies of contrast sensitivity losses in patients with RP, and further they characterize contrast processing deficits in these patients at suprathreshold levels.

Fig. 1

Mean VEP fundamental amplitude as a function of the angular width of the isolated checks, on semi-log coordinates, for the patients with RP (diamonds) and control subjects (circles), for increment (open symbols) and decrement (filled symbols) checks. Error bars represent ±1 standard error of the mean (sem).

Fig. 2

Mean VEP contrast response functions for chromatic modulation, on semi-log coordinates, for the patients with RP (filled circles) and control subjects (open circles). Error bars represent ±1 sem. Curves represent the least-squares best-fits of Eq. 2.

Fig. 3

Mean VEP contrast response functions for luminance modulation at 5.6 Hz (squares) and 11.2 Hz (triangles), on semi-log coordinates, for the patients with RP (filled symbols) and control subjects (open symbols). Error bars represent ±1 sem. The curves represent the least-squares best-fits of Eq. 2.

Fig. 4

Mean VEP fundamental phase as a function of luminance modulation at 5.6 Hz (squares) and 11.2 Hz (triangles), on semi-log coordinates, for the patients with RP (filled symbols) and control subjects (open symbols). Error bars represent ±1 sem. Solid lines represent least-squares regression lines fit to the mean phases for each group.