What is the Pulfrich effect?

The Pulfrich effect is a psychophysical perception where an "apparent spatial disparity is due to the unilateral increase in visual latency associated with the decrease retinal image intensity in the filtered eye. " (Nakamizo, Nickalls and Nawae, 2004) As a result of the "illusionary delay in one eye" ( Read & Cumming, N.D.) , the pendulum ,which is actually swinging back and forth in a frontoparallel plane, will be perceive to be swinging in an ellipse, moving back and forth in depth.

Reality Vs Perceived Reality : Introducing the Classical Explanation

Figure 4. Illustration of the Classical Pulfrich effect. (Lages,Mamassian and Graf, 2003)

In the Classical explanation, a light attenuating filter (eg. the neutral density filter) is placed in front of one eye "causing a time delay for the neural signal in the eye." (Anzai, Ohzawa and Freeman, 2001) Therefore, the perceived position of the swinging pendulum in the filtered eye falls " slightly behind that in the other eye for a given moment, creating spatial offset between what is perceive in the two eyes" (Anzai, Ohzawa and Freeman, 2001). This in turn causes the object to be perceive in depth.

When the pendulem swings from left to right, when the right eye is covered by the filter, the "spatial offset corresponds to a binocular disparity that shifts the perceived depth of the pendulum towards the observer (hatched dotted circle)" (Anzai, Ohzawa and Freeman, 2001) However, when the pendulem swings from right to left, when the right eye is covered, the "spatial offset corresponds to a binocular disparity that shifts the perceive depth of the pendulum away from the observer( hatched dotted circle)."(Anzai, Ohzawa and Freeman, 2001)

The amount of binocular disparity depends on "the pendulem position along the fronto-parallel plane" (Anzai, Ohzawa and Freeman, 2001) as the speed of the pendulem motion varies with the position. Hence, even when the pendulem is swinging back and forth on the fronto-parallel plane (the solid arrows), it would appear to move in depth along an ellipitical trajectory ( The dotted arrows).