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.