5/24/2006

Reichardt Detectors and Illusory Motion Reversal

If you've ever watched a movie and noticed that the wheels on a vehicle in the movie appear to be moving backwards, you have experienced illusory motion reversal (also known as the wagon wheel illusion; try it here). This illusion arises from the frame rate of the movie, or the refresh rate of a monitor, making it appear as though objects with a periodic spatial frequency (such as wheels, or repeating lines) are moving in the opposite direction of their true motion.

Interestingly, recent reports that illusory motion reversal happens under direct sunlight have revived claims that the human visual system essentially takes perceptual "snapshots" of the visual field, and hence that the human visual system has some sort of framerate or cycle speed. This explanation fits well with theories of neural function that posit an important role for neural oscillations in attention and memory processes.

In contrast, other authors have argued that a particular type of motion detector - a Reichardt Detector - can explain these effects, without invoking some kind of neural frame rate in the visual system. According to this argument, a neural circuit may serve as a Reichardt detector subunit if it receives input from two receptive fields, where one is temporally delayed from the other. Essentially, such a circuit would detect a particular velocity (rate of motion + direction of motion). In a full Reichardt detector, two of these subunits are compared with one another through subtraction. (If you can't visualize this, try this interactive demonstration.)

According to the motion-reversal-via-Reichardt-detectors argument, if the spokes of the wagon wheel are rotating at a specific frequency, some Reichardt detectors will become active for the opposite direction, because they will essentially "mistake" another spoke as the new location of the first spoke. This motion reversal only enters conscious awareness when the Reichardt detectors tuned to the correct direction of motion are fatigued.

This argument has at least two shortcomings which I have pointed out previously. But as it turns out, a recent paper by Rojas et al. has made the same points. Even better, the authors have replied in another published piece. Here's the essence of their reply:

1) Full Reichardt detectors, as opposed to Reichardt subunits, are not velocity-tuned but rather temporal frequency-tuned. As a result, it is possible to explain the increased probability of illusory motion reversal around 10 Hz with full Reichardt detectors.

2) The Reichardt-based explanation does not posit temporal sampling at all, despite first appearances, because they are not temporally discrete. Instead, they are spatially discrete, and thus sensitive to spatial aliasing if not preceeded by the appropriate pre-filters.

3) Although the arguments presented so far do not argue against temporal sampling that is not uniform across the visual field, a new visual illusion (pictured at the top) provides preliminary evidence against this hypothesis. Specifically, when subjects view a stimulus with moving lines superimposed over rotating fan blades, illusory motion reversal tends to happen for one pattern but not the other (i.e., fan blades but not lines, or vice versa.) A spatial perceptual sampling hypothesis would suggest that both items should reverse simultaneously.

However, this final point does not argue against object-based temporal sampling (as the authors themselves note). It is also possible that temporal sampling occurs at multiple levels at once - i.e., both space-based and object-based - just as in other visual attention experiments. In fact, Reichardt detectors would have to be at least partially object sensitive, otherwise we would have the sensation that objects are moving when another object has merely appeared.

Although I think these replies are interesting, I am also not sure that this argument can actually be resolved. Despite several previously mentioned problems (foremost among them being that Reichardt detectors have never actually been found in the mammalian visual system), it seems like these points of view may not be mutually exclusive. For example, it is still possible that the asynchronous activity of Reichardt detectors is temporally sampled, thus giving rise to rivalry between them.

Related Posts:
Attention: The Selection Problem
Illusory Motion Reversal: Rivalry or Perceptual Sampling?
Perceptual Sampling: The Wagon Wheel Illusion

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