Pulfrich pendulum phenomenon in patients with a history of acute optic neuritis.

The Pulfrich phenomenon is a stereoillusion in which a pendulum swinging at right angles to the line of gaze appears to be describing an elliptical path when absorbing filters are placed in front of one eye. We used two sets of polaroid glasses as adjustable filters. A spot on a modified oscilloscope served at a pendulum bob. Twenty-nine former patients with a history of optic neuritis and visual acuities of greater than or equal to 6/6 in both eyes and twenty-two normal subjects underwent examinations. The patients showed pathological recordings which separated them from the control subjects. The test seems to expose minor residual dysfunction of affected optic nerves where the visual acuity is normalized. This abnormal response when viewing the moving Pulfrich pendulum is probably caused by disturbed neural conduction. The degree of acute visual loss and the time elapsed since the attack did not seem to influence the Pulfrich response. The results may explain why some patients who have recovered from optic neuritis complain of difficulties when viewing moving objects. In addition to the use of Pulfrich illusion test for diagnostic work; i.e. clinical or subclinical attacks of optic neuritis, it can serve as a valuable supplement to the more sophisticated method of visual evoked response.