Description and clinical application of the Pulfrich effect

Background: In 1922, German physicist Carl Pulfrich described an illusory binocular perceptual disturbance in which an object moving across an observer's field of vision is perceived as traveling along a curved trajectory.

Objective: To review the discovery of the Pulfrich effect, and subsequent clinical applications.

Methods: We translated Pulfrich's description and searched for subsequent publications using electronic databases and review of reference lists in identified publications.

Results: In 1901, Pulfrich developed an optical device to accurately compare stereoscopic photographs, but brightness difference between plates caused distance misperceptions that interfered with precise measurements. Pulfrich proposed that this Stereo-Effekt resulted from interocular differences in perceptual latency. He induced the effect by placing a smoked glass in front of one eye. The resulting perceptual disparity creates an apparently curved trajectory of an object moving sideways across the field of vision. Pulfrich also recognized that visual pathway disorders can produce a pathologic Stereo-Effekt. In 1925, Grimsdale demonstrated this in a man with unilateral retrobulbar optic neuritis and suggested treatment with a neutral density filter (NDF) over the good eye. Not until the 1970s, however, was the Pulfrich effect evaluated as a diagnostic test for retrobulbar optic neuritis and the therapeutic efficacy of an NDF confirmed.

Conclusion: Although the clinical importance of the Pulfrich effect was suggested by Pulfrich and quickly confirmed, it took decades before its diagnostic utility and the efficacy of an NDF were assessed. Recognition remains clinically important to minimize safety risks and mislabeling, and because resulting misperceptions can be easily treated.