Abstract
Abstract: :
Purpose: To evaluate suprathreshold contrast processing in patients with retinitis pigmentosa (RP), using the visual evoked potential (VEP), under chromatic and luminance conditions designed to emphasize the parvocellular (PC) and magnocellular (MC) pathways, respectively. Methods: Swept–parameter VEPs, with contrast manipulated parametrically, were obtained in response to modulation (appearance/disappearance mode) of an array of 32 x 32 isolated checks presented against a steady yellow background of 55 cd/m2. The checks were modulated either in chromatic contrast at 6 Hz (isoluminant red checks) to favor the chromatic PC pathway, or in luminance contrast (increment and decrement yellow checks) at both 6 Hz and 12 Hz to favor the MC pathway. The VEP responses of 10 patients with RP (mean age, 44 ± 12 years), whose visual acuities ranged from 20/20 to 20/40, were compared to those of 10 age–equivalent visually normal control observers (mean age, 42 ± 13 years). Response amplitude and phase at the stimulus fundamental frequency were derived from Fourier analysis. Results: The contrast–response functions of the control observers were similar to those reported previously. The function for chromatic contrast increased approximately linearly with increasing contrast and did not saturate, whereas the function for luminance contrast increased sharply and saturated at contrasts above approximately 30%. For the patients with RP, the mean response function for chromatic contrast was approximately linear but had a reduced slope compared to normal, indicating a reduced contrast gain. The patients’ mean response function for luminance contrast at 6 Hz saturated at the same contrast as for the control observers, but amplitudes were reduced overall. A phase advance with increasing contrast was observed in both the patient and control groups, indicating similar nonlinear contrast gain control effects. At 12 Hz, the patients’ mean response amplitudes for luminance contrast were reduced substantially below normal at all contrasts. Conclusions: Contrast processing was impaired within both MC and PC pathways in these patients with RP, but response deficits were greatest for luminance contrast at a temporal frequency of 12 Hz. These VEP results are congruent with recent psychophysical studies of contrast sensitivity deficits in RP patients in showing that visual function within the MC pathway can be more severely affected than PC–pathway function in this group of hereditary retinal degenerations.
Keywords: electrophysiology: clinical • degenerations/dystrophies • contrast sensitivity