Orientation-specific losses of contrast sensitivity in multiple sclerosis

The authors measured grating contrast sensitivity over a range of 1 to 25 cg/deg for vertical, horizontal, and both oblique orientations in eight 'definite' multiple sclerosis patients and 11 controls, using a two-alternative forced-choice technique. Only two of eight patients did not show orientation-specific losses of contrast sensitivity. Six of eight patients had significant orientation-specific losses in one or other single eye (p <0.05), and two of eight had significant orientation-specific differences between the left and right eyes. Some patients' orientation-specific losses were restricted to lower spatial frequencies, sparing high spatial frequencies and leaving Snellen acuity unaffected. The disorder is different from the oblique effect and from meridional amblyopia, since these affect high spatial frequencies more than lower spatial frequencies. Since there is no separate neural mechanism for oblique eye movements, these orientation-specific losses cannot be due to disordered eye movements, at least in the patients whose contrast sensitivity was depressed much more for an oblique than for the horizontal or vertical orientation. It is proposed that the responsible pathology is chiefly at, or central to, visual cortex. Nevertheless the disorder is puzzling because the existence of orientation-specific defects in multiple sclerosis (a disease characterized by focal demyelination) would be most easily explained in terms of some gross physical correlate of orientation-specificity in cortical neurons, and no physiological evidence for this has yet been described.