Temporal and spatial response properties of optic neuritis patients manifesting statokinetic dissociation

Abstract

Statokinetic dissociation (SKD) refers to a greater reduction in sensitivity to stationary visual stimuli relative to similar targets in motion. We evaluated SKD perimetrically in nine optic neuritis (ON) patients and in nine normals by measuring the difference between static and 4 degrees /s kinetic thresholds for a size I target (0.11 degrees ) along the four oblique meridians. To assess temporal response characteristics, we used flicker perimetry to measure the sensitivity to 2-, 8-, and 20-Hz sinusoidal flicker at 5 degrees intervals along the oblique meridians. The ON patients' sensitivity to all three flicker frequencies was reduced at all eccentricities. In a number of cases, flicker sensitivity was unmeasurable to one or more frequencies within the region where kinetic targets were detectable. This was particularly marked at 20-25 degrees , where in half the cases there was no measurable sensitivity to a flickering target although a moving target could be seen. These results indicate that SKD in these patients may not be due to a selective loss of mechanisms sensitive to low frequency temporal modulation with a relative sparing of mechanisms sensitive to high-frequency temporal modulation as has been previously suggested [A. Safran and J. Glaser, "Statokinetic Dissociation in Lesions of Anterior Visual Pathways," Arch. Ophthalmol. 98, 291-295 (1980)]. We hypothesize that SKD may be due in part to differences in spatial summation in these patients, making a moving target, which has a greater spatial extent, more detectable than a stationary target. Thus ON patients exhibiting SKD should show great improvement in static perimetry thresholds as the target size is increased. We compared the results of standard 30-2 Humphrey perimetry for size III (0.43 degrees ) and size V (1.72 degrees ) targets for normals and ON patients and found that the patients showed a larger sensitivity difference between size III and size V targets than normals.