The human magno and parvo systems and selective impairments of their functions.

The magno and parvo systems, consisting of large and small cells respectively and showing the greatest differences in temporospatial characteristics, are two of the most interesting channels in the organization of retinotopic input pathways of the human visual system. We tested the hypothesis that selective lesions of fibers of a specified diameter occurring in pathology of the conducting pathways, i.e., multiple sclerosis, could be used as a "key" for studies of the different functional properties of the magno and parvo systems. Measurements were made of spatial-frequency contrast sensitivity in healthy subjects and patients in the early stages of multiple sclerosis. Decreased sensitivity at low and intermediate or at high spatial frequencies provided indirect reflections of changes in the activities of the magno and parvo systems. These psychophysical data are compared with electrophysiological results. The amplitudes of the early components of visual evoked potentials reflect activation of the two systems in healthy people and patients with multiple sclerosis. We identified two groups of patients, with statistically significantly dominant lesions of the magno and parvo systems respectively.