The Pulfrich phenomenon in a large population of young healthy subjects

To determine whether the Pulfrich phenomenon, an optical illusion occurring in many ophthalmological diseases, is perceived equally in both eyes in a large group of healthy medical students.

A pendulum bob swinging perpendicular to the direction of observation was observed with either the right or the left eye covered with neutral density filters (50, 80 or 90 % absorption) and the apparent elliptical pendulum movement measured in depth. Interocular time delay was calculated from depth. Data from 65 individuals were included based on having: completed all 7 determinations of depth, a visual acuity of >/= 20/20 on both eyes and an intact stereoscopic perception (Titmus stereotest, acuity of >/= 20/25).

All subjects perceived the phenomenon. Depth perception was not significantly different (p > 0.05; MANOVA) between the two eyes (depth in [mm]; mean +/- sem): OD: 8.5 +/- 0.38, 23.7 +/- 0.53, 36.3 +/- 0.81; OS 9.4 +/- 0.50, 24.1 +/- 0.69, 35.6 +/- 0.92; for 50, 80 and 90 % absorption of the filter respectively. At 0 % absorption the pendulum was seen in average at positive values (0.9 +/- 0.23 mm; p > 0.05). Calculated interocular time delay (ms) was: OD: 8.0 +/- 0.18, 5.1 +/- 0.12, 1.8 +/- 0.08; OS: 7.46 +/- 0.15, 5.0 +/- 0.11, 1.9 +/- 0.10. The average depth perceived without filter corresponded to a time delay for the right eye of 0.2 +/- 0.05 ms. Correction for the depth perception perceived without filter did not alter statistical significance.

The Pulfrich phenomenon is perceived equally in both eyes. Depth perception without filters was not significantly different from zero. The illusion has clinical utility, since in normal subjects reliability of measurements is good and size of the illusion (without filters) small.