The Pulfrich Effect

The Pulfrich effect is a perceptual phenomenon that occurs when a moving object appears to be displaced laterally in the presence of an interocular delay. It was first described by the German physicist Carl Pulfrich in 1922. The effect relies on the brain's ability to process the difference in timing between the two eyes' perception of a moving object.

Classical Setup

In the classical setup, the viewer wears a pair of standard glasses with one lens completely darkened (e.g., with a neutral density filter). The darkened lens creates a temporal delay in the visual information reaching one eye compared to the other eye. As a result, the brain receives slightly different visual inputs from the two eyes, creating a perceptual discrepancy.

When a moving object is observed under these conditions, the brain interprets the delayed visual information differently from the immediate information. As a consequence, the object appears to be displaced laterally in the direction of motion. The amount of perceived displacement is influenced by factors such as the interocular delay duration, the velocity of the moving object, and the orientation of motion.


The Pulfrich effect can be demonstrated using various visual stimuli, including pendulums, swinging objects, or moving balls. When viewed with the interocular delay, these objects appear to follow an elliptical or curved trajectory instead of a straight line. The perceived displacement is typically larger when the object moves faster or when the delay duration is increased.


The Pulfrich effect has been extensively studied in psychophysics, and its characteristics have been explored in numerous experimental studies. One notable study by Bridgeman and Stark (1991) investigated the parameters influencing the effect, including interocular delay, velocity, and luminance contrast of the moving object. Another study by Rokers et al. (2009) examined the relationship between the Pulfrich effect and the sensitivity to motion-in-depth.

The underlying neural mechanisms of the Pulfrich effect have also been investigated. Neurophysiological studies using single-unit recordings in animals have provided insights into the processing of temporal disparities in visual areas, such as V1 and MT. For example, DeAngelis et al. (1998) examined the responses of neurons in macaque MT to stimuli with interocular delays, shedding light on how the brain encodes and integrates temporal disparities.


The classical Pulfrich effect has practical applications in stereoscopic displays, 3D movies, and virtual reality technologies. By introducing controlled interocular delays, the perception of depth content can be enhanceand to generate a more immersive visual experience.

In summary, the classical Pulfrich effect is a perceptual phenomenon where a moving object appears laterally displaced due to an interocular delay. It is a well-documented phenomenon with extensive empirical evidence from psychophysical studies and neurophysiological investigations.


  • Bridgeman, B., & Stark, L. (1991). Ocular proprioception and efference copies in perception and action. In Vision and action (pp. 166-182). Springer.
  • DeAngelis, G. C., Cumming, B. G., & Newsome, W. T. (1998). Cortical area MT and the perception of stereoscopic depth. Nature, 394(6692), 677-680.
  • Pulfrich, C. (1922). Die Stereoskopie im Dienste der isochromen und heterochromen Photometrie sowie einiger Untersuchungen der Bewegungswahrnehmung. Physikalische Zeitschrift, 23, 304-312.
  • Rokers, B., Cormack, L. K., & Huk, A. C. (2009). Disparity- and velocity-based signals for three-dimensional motion perception in human MT+.