The Pulfrich Phenomenon

The classical Pulfrich phenomen refers to a peculiar distortion of binocular vision that involves dynamic dysmetropia in depth. The phenomenon is most commonly elicited by means of a pendulum that is made to swing in the observer's frontal plane. When a light-attenuating filter is put in front of one eye, the pendulum will appear to swing in and out of the plane, following a more or less ellipsoidal trajectory.

The Pulfrich phenomenon is attributable to a relatively slower excitation of the neural channels in the eye with the dimmer image. This leads to asynchronous cortical stimulation and sets the scene for dynamic stereo disparity [1, 2]. The schematic figure to the right summarizes key elements, as seen from above. The pendulum path is represented by the blue line and the pendulum bob swings in the direction of the arrow. At the depicted moment in time, the normal right eye sees the bob in the veridical direction (green dot) whereas the bob appears to be lagging (red dot) to the "slow" left eye. The binocular percept (yellow dot) is located to a point outside the actual path. Move the cursor over the figure to change the swing direction. Now, the binocular percept will be located inside the pendulum path, that is, closer to the viewer. The sum of all projection points approximates an ellipse. The magnitude of the deviation in depth depends on both the density of the filter and the velocity of the movement.

A pendulum is easily improvised with a bob attached to a string or a ruler but a computer graphics display may be more advantageous, not the least because of the possibility of quantitative nulling of any perceived disparity. The anaglyphic display presented below depends on red and cyan (or green) filters put in front of the left and right eyes, respectively. Click the Start button to initiate a simple harmonic (sinusoidal) oscillation of the test target. The sliders allow control over rates of swing and disparities. Note that the indicated rate may require a few swings to settle at a stable value. The target can be made to follow a pendular path by clicking the Arc button but there is much to speak for the perceptually simpler alternative of a linear path.

Using red-cyan glasses, normal subjects are expected to see the test target swing back and forth in the plane of the computer screen. The addition of a neutral filter in front of either eye is expected to cause the target to move closer to the subject when swinging in the one direction and further away when swinging in the other direction. The induced Pulfrich effect can be counteracted by introducing a disparity between the red and cyan images. The disparity can be changed by clicking the left or right mouse buttons inside the display area, or adjusting the disparity slider.

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Dynamic Stereo Display © L. Frisén 2016

Path     Arc Line

The Pulfrich phenomenon is not limited to simple harmonic oscillations in the frontal plane, of course. All kinds of target trajectories will be affected in similar fashions.The phenomenon may also appear in unexpected situations, like viewing the "war-of-ants" pattern on an old-fashioned analogue television screen lacking an input signal. Hold a light-attenuating filter in front of one eye (a sunglass lens usually does very well) to see the randomly flickering pattern transformed into a slowly rotating cylinder. Don't close the other eye! A related type of simulation can be viewed on Michael Bach's Optical Illusions Site.

Acquired Pulfrich phenomena

An apparent dynamic stereo disparity may also arise from non-optical causes, namely, disorders that cause an asymmetry of neural conduction rates between the eyes. Optic neuropathies with bilaterally asymmetric demyelination are probably the most common causes of acquired dynamic stereo dysmetropia. It is not difficult to imagine how acquired Pulfrich phenomena may produce many kinds of problems in daily life, for example, in sports and in traffic situations. The obvious remedy, namely, to put a carefully titrated neutral filter in front of the quicker eye, actually has been reported to work [3]. Real-life utility is presumably limited because of wide variations in luminances and rates of movement.

Monocular counterparts to Pulfrich phenomena

The classical Pulfrich phenomenon has attracted much attention during the years but little if any notice has been given to the possible existence of monocular counterparts. Yet, there are good reasons to assume that such animals do exist. This topic is treated elsewhere on this site.