Functional Loss in the Magnocellular and Parvocellular Pathways in Patients with Optic Neuritis

Purpose: To evaluate contrast threshold and contrast gain in patients with optic neuritis under conditions designed to favor mediation by either the inferred Magnocellular (MC-) or Parvocellular (PC-) pathway. Methods: Achromatic and chromatic contrast discrimination was measured in 11 patients with unilateral or bilateral optic neuritis and 18 age-matched controls with normal vision, using achromatic steady-pedestal and pulsed-pedestal paradigms to bias performance toward the MC- or PC- pathway respectively. Additionally, L-M chromatic discrimination at equiluminance was evaluated using the steady-pedestal paradigm. A physiologically plausible model could describe the data with parameters accounting for contrast gain and contrast sensitivity in the inferred MC- or PC- pathway. The fitted parameters from the affected eye by optic neuritis were compared with those from the normal eyes using Generalized Estimation Equation (GEE) models that can account for within-subject correlations. Results: Compared with normal eyes, the affected eyes had significantly higher saturation parameters when measured with both the achromatic pulsed-pedestal paradigm [GEE: β(se) = 0.35(0.06), p < 0.001] and the chromatic discrimination paradigm [β(se) = 0.18(0.08), p = 0.015], suggesting contrast gain in the inferred PC- pathway is reduced; the affected eyes also had reduced absolute sensitivity in the inferred MC-pathway measured with the achromatic steady-pedestal paradigm [β(se) = 0.12 (0.04), p = 0.005]. Conclusion: Optic neuritis produced large sensitivity loss mediated by the MC- pathway and contrast gain losses in the inferred PC- pathway. A clinical framework is presented for interpreting contrast sensitivity and gain loss to chromatic and achromatic stimuli in terms of retinal and post-retinogeniculate loci contributions to detection and discrimination.