Cognitive Daily

Flash-lag demos galore!

The flash-lag effect is difficult to explain, but amazingly cool to see. Over at Mixing Memory, Chris has a great post where he links to two examples of the phenomenon and discusses what might be causing it. Cool, isn’t it?

I can’t resist linking to one more example, created by none other than CogDaily’s own Greta Munger. Here are her instructions for seeing the illusion: “Take a look at the movie below, and decide whether or not the blue flashed object is exactly aligned with the end of the gray rod. To start the movie, click on the rod.” Greta discusses the illusion further in this post.

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You can see for yourself that the blue object is indeed actually aligned with the rod by using the slider to stop the movie. So why do we see the illusion of the flashing object trailing the moving object? Chris has a couple possible explanations:

Whitney and Murakami argue that the illusion is due to differences in the amount of time it takes to process a flash vs. motion (the latency difference hypothesis). Motion, they argue, gets processed faster, and therefore the perception of the motion biases the later processing of the flash, even though the flash actually occurred first (in other words, the visual system ain’t psychic, it’s just slow).

[Another] theory argues that the illusion is the result of postdiction (as opposed to prediction). Under this view, the visual system uses both pre- and post-flash motion cues from the object (in the demonstrations above, the rings) to estimate the position of the object, and this results in the object appearing to be ahead of the flash. Evidence for the latency difference hypothesis (which involves postdiction as well) is also consistent with the postdiction hypothesis.

I can’t offer any more light on the phenomenon than this, but since it’s one of Greta’s particular areas of expertise, I’ll see if I can get her to respond in the comments (though since she’s just returned from Psychonomics, she may not have time to get to it today).

In other news:

Comments

  1. #1 RSG
    November 20, 2006

    I initially saw the flash as aligned on the counterclockwise turn, but not on the clockwise. It did vary somewhat, though. I have no idea why that would be.

  2. #2 david1947
    November 20, 2006

    I also only saw the lag on the clockwise rotation, and that only about half the time.

    Looks like the time difference between predictive image reconstruction and reactive. The flashed dot takes a frame time or so to become perceived(?) while the bar’s motion is predictable and is therefore perceived to be at the advanced position more by neurological computation than by direct perception. We are talking about maybe 30 msecs here.

    I haven’t tried watching for a long time, but I suspect the flash can also become part of the expected reconstruction and that then it will be perceived more reliably to be aligned with the bar.

  3. #3 Dave Munger
    November 20, 2006

    I haven’t found that the effect diminishes with time. Even though I know it’s actually aligned with the rod, I still see the blue object lagging, every time.

  4. #4 Peter
    November 20, 2006

    I saw the illusion of the flashing object trailing the moving object. I think this might be an attention effect. I saw the moving rod and waited for the flash. Then the blue object flashed. This led to an orienting reaction, and, in consequence, to an attentional shift to the blue object. But my task was to decide whether or not the object was exactly aligned with the end of the rod. So, after I recovered from the orienting reaction, I remembered what I had to do and attended to the rod, “Where is that rod?” I saw the rod, but too late, some time was gone since the blue object had flashed. Thus, the rod was always ahead of the blue object.
    The idea that the flash-lag effects are generally due to the differential speeds of flash vs. motion perception is compelling, but I wondered whether the effects might be a consequence of the tendency of our brains to make sense of what we see rather than to some failure of the system. This is especially true for example “two”, in which the visual system might filling in steps between the two states of the disc (yellow and blue) which are not presented (like when watching a movie that consists of discrete pictures but you see motion) when visual information is poor.

  5. #5 david1947
    November 25, 2006

    “The idea that the flash-lag effects are generally due to the differential speeds of flash vs. motion perception is compelling, but I wondered whether the effects might be a consequence of the tendency of our brains to make sense of what we see rather than to some failure of the system.”

    I think that is what I meant by “neurological computation” and “expectation”. This kind of processing is what makes it possible to play rythms with millisecond precision even though reactive times are about 2 orders of magnitude slower. The rotating illusion is more predictable, by the low level mechanisms, than is the flash, if for no other reason than the time intervals involved. (hypothesis alarm – I have only thought experiments for this one, but a heavily computational background).

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