Brief History of The Pulfrich Effect

In 1922, Carl Pulfrich wrote a paper of the now called Pulfrich effect. In his paper, he gave credit to others for their contribution towards the Pulfrich effect. Pulfrich stated the phenomenon was brought to his attention pior to writing his paper by Professor Max Wolfe. Professor Wolf describe " a noteworthy steroeffect which now and then disturbingly confronted him during rapid movement of a pair of plates in the sterocomparator." (Christianson & Hofstetter, 1972)

Pulfrich also gave full credit to a techical assistant, F. Fertsh, for the findings that the "difference in brightness of the view in front of the two eyes elicited the (Pulfrich) effect." (Christianson & Hofsetter, 1972) Fertsh was also credited for his explanation of the phenomenon in terms of perceptual latency.

Pulfrich's son, Hans, a doctor of Philosophy, was also given due credit for coming up with the theoretical analysis of the "path of apparent movement based on an assumed perceptual latency" ( Christianson & Hofstetter, 1972). Pulfrich only credited himself for the notion of the pulfrich phenomenon could be used in heterochromatic photometry.

In 1922, Pulfrich demonstration apparatus did not include a pendulum as it is usually stated in description of the pulfrich effects. Instead, blocks of square apectures were used. Below are the designs of Pulfrich's earlier models.( Figure 1 and 2)

(Christianson & Hoffsetter,1973)

Demonstration of the Pulfrich Effect

The Pendulum Swing

Instructions

The participant is seated comfortably. The participant is given a pair of dark glasses with one of the lens punched out (Figure 3.). The intact lens acts as an nuetral density filter. The participant is told to put the dark glasses on and told to face and focus on the screen. The screen is put at the eye level of the participant. On the screen, a pandulum is shown to be swinging from left to right repeatedly. After a short period of time, the participant will experience the pulfrich effect.

Figure 3. Dark Glasses with one lens punched out.

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).

Other Variations

In the past thirty years, modifications have been made to the Classical Pulfrich effect to 'eludicate neuronal mechanism for processing delay and disparity" ( Read & Cumming, 2005). The two more prominent modifications are the stroboscopic Pulfrich effect and the dynamic visual noise.

The stroboscopic Pulfrich effect

In the stroboscopic version of the Pulfrich effect, "a target is presented in apparent motion, jumping from one point to another point across the screen instead of moving continous, and is viewed with interocular delay" (Read & Cumming, 2005). Figure 5 shows the space/time diagram for the stroboscopic pulfrich effect. The dotted lines represent the trajectory of the moving objects while the "star" represent its brief appearance. It was found that " at any instant in time, the stimulus is visble to only one eye" (Read & Cumming, 2005). Therefore, there is no spatial disparity but there is temporal delay( the difference between the "stars", J) which causes the perception of depth to be present.

Figure 5. Space/Time diagram for the stroboscopic Pulfrich effect (Read & Cumming, 2005)

Dynamic visual noise

Dynamic visual noise "resembles the "snowstorm" on an untuned television" (Read & Cumming, 2005). When viewed with an interocular delay, "the noise appears to swirl in depth, with points in front of the screeen moving towards the delayed eyes, and points behind it in the opposite direction" (Read & Cumming, 2005). The dynamic visual noise is unlike the pulfrich effect, in that the depth perception experience is not merely a "trival consequences of stimulus geometry" (Read & Cumming,2005).

Thus, we see that the classical explanation of the classical pulfrich effect cannot account for the results experience by the Stroboscopic Pulfrich effect and the dynamic visual noise. In fact, in Lee’s 1970 paper, he explicitly said that the “classical explanation doesn’t seem to work for the Stroboscopic Pulfrich effect” (Lee, 1970).As a result, this gave birth to the joint encoding of motion and depth explanation.

Neurobiological Evidence

The Neurobiologcal evidence
In the stroboscopic pulfrich effect, it is a flashing stimulus which causes one eye to be lagging behind the other. In this case, there is temporal delay but no spatial disparity. Since the Classical explanation of the Pulfrich effect was not applicable to the stroboscopic pulfrich effect other explanations were developed.

Qian & Anderson (1997) explained that in the stroboscopic Pulfrich effect, the moving stimulus “activates cells which are sensitive both to direction of motion and interocular disparity” (Qian & Anderson, 1997). Therefore, the cells activated would “jointly encode motion and disparity” (Qian & Anderson, 1997). This implies that receptive fields are space/time inseparable which explains that the receptive fields are tilted in respect to the axes of space and time (Figure 6).

Figure 6. Direction-selective cells’ receptive fields which are tilted in space and time. (Read & Cumming, N.D.)
As see in Figure 6, the pink blobs represent the receptive fields extending through space and time. The receptive fields are tilted in space-time and are “activated more strongly when the stimulus crosses in one direction than the other” (Read & Cumming, N.D.)

Figure 7 & 8. Direction-selective cells in space and time. (Read & Cumming, N.D.)

In the research of Read and Cumming, they claimed that if we have receptive field is tuned to a non-zero disparity and direction of motion, than the left and right eye receptive field must be separated in space. Thus, this would allow response to a disparate object moving across a visual field (Figure 7) and allow response to a moving object with temporal delay and zero disparity (Figure 8).

In 2001, Anzai et al., published a paper on the joint-encoding of motion and depth by the visual cortical neurons using the neural basis of the Pulfrich effect. The research was based on looking into the striate cortex of cats. The researchers observed responses to the stimuli with both spatial disparity and intercocular time delay.

They established that the tilted space-time profile was “the neuronal basis of the Pulfrich effect” (Anzai et al., 2001).Furthermore, since the tilted space-time profile was direction-selective than “ the Pulfrich effect is mediated by direction-selective cells” (Anzai et al.,2001).

In the cat’s striate cortex, most cells were found to be direction-selective. Thus, Anzai et al., 2001’s research supported the direction-selective hypothesis on the Pulfrich effect.

However in 2005, Read & Cumming wrote a paper to refute Anzai, et al.’s claims. Read and Cumming’s research paper was based on the striate cortex of the primates. It was found that the primates’ V1 only showed approximately 25% of the cells to be direction-selective unlike the large percentage of direction-selective cells in the cats in Anzai, et al.’s study. In Read & Cumming’s study, the established that joint encoding may “not be required to explain depth perception in Pulfrich-like stimuli” (Read & Cumming, 2005). In fact, they claim that a brain which “encodes motion and disparity in entirely separate neuronal pathways could experience such illusions” (Read & Cumming, 2005) too.

In conclusion, the neurological evidence of the Pulfrich effect is still up for debate. Researches on the Pulfrich effect has been going on for the past 80 years (since Pulfrich’s 1922 paper) and it has not shown any signs of slowing down. I believe that further research should be done on the Neurobiological evidence of the Pulfrich effect before one theory can emerge as a winner.

References

Anzai, A., Ohzawa, I. and Freeman, R.D. (2001). Joint-encoding of motion and depth by visual cortical neurons: neuronal basis of the Pulfrich effect. Nature Neuroscience. 4(5). 513-518.

Christianson,S. & Hofsetter, H.W. (1973). Some Historical notes on Carl Pulfrich. American Journal of Optometry and Archives of the American Academy of Optometry. 49. 944-947.

Lages, M., Mamassian, P. and Graf. E.W. (2003). Spatial and Temporal tuning of motion in depth. Visual Research. 43. 2861-2873.

Lee, D. N. (1970). A stroboscopic sterophenomenon. Vision Research. 10.587-593.

Qian, N. & Anderson, R.A. (1997). A psychological model of motion-stero intergration and a unified explanation of the Pulfrich-like phenomena. Vision Research. 37.1683-1698.

Read, J. C. & Cumming, B.G. (2005). All Pulfrich-like illusion can be explained without joint encoding of motion and disparity. Journal of Vision. 5. 901-927.

Read, J. & Cumming, B. (N.D.). The Neuronal basis of the Pulfrich effect in Primate. Retrieved on December 18, 2009 from http://www.staff.ncl.ac.uk/j.c.a.read/publications/presentations/SFN03.ppt

Relaxing Swinging Pendulum. Retrieved on January 3, 2010 from http://www.youtube.com/watch?v=lZTaX4u-Z_k&feature=PlayList&p=6E437569B3F8ECDE&index=0&playnext=1