Retinal and cortical evoked responses to chromatic contrast stimuli. Specific losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis

It is known that colour vision may be altered in optic neuritis. Our aim was to establish whether chromatic and achromatic vision are differentially impaired using stimuli designed to favour the activity of either the magnocellular or the parvocellular stream of the visual pathway. Fourteen patients with a past history of unilateral optic neuritis in the course of multiple sclerosis and 10 age-matched control subjects were included in the study. Patients had relatively good visual acuity in the affected eyes and no gross colour deficits. Stimuli were alternating gratings of low spatial frequency and of different chromaticity along the red-green axis. The psychophysical contrast sensitivity (CS) was measured at 5 Hz as a function of colour ratio [red/(red+green)] to evaluate both the equiluminant point (the colour ratio corresponding to the lowest CS) and the CS for isochromatic, luminance gratings (red-black and green-black). Steady-state (2-24 Hz) and transient pattern electroretinograms (PERGs) and visually evoked potentials (VEPs) were recorded in response to high contrast (90%) stimuli of low spatial frequency (0.3 cycles deg^-1) modulated in either pure chromatic contrast (equiluminant red-green) or pure luminance contrast (yellow-black). On average, CSs were reduced (10 dB) in optic neuritis eyes compared with controls for both luminance and chromatic gratings. In the VEPs (both transient and steady-state) amplitude losses and latency delays were far larger for the chromatic VEPs than for the luminance VEPs. Chromatic VEP latency delays were remarkable also in the fellow, clinically normal, eyes. Significant losses were apparent in both the luminance and chromatic PERG. However the chromatic PERG was comparatively more altered. In agreement with previous reports, selective losses were not apparent at threshold. By contrast suprathreshold electrophysiological responses displayed a clear dissociation between luminance and colour; suggesting that the parvocellular stream, compared with the magnocellular stream is more impaired in optic neuritis.