Watch out for that parked car!

Have you ever noticed that when you drive by a car whose hazard lights are blinking, something doesn't look quite right? You know those blinking hazard lights are really on the car, but they seem off, somehow. Part of what might be going on is something called the flash-lag effect. 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.


In general, it looks as if the blue object is a bit behind the rod; or that the flashed object is lagging relative to the moving rod. This is called the flash-lag effect (FLE) and has been studied in lots of labs, but today I'm going to tell you about an experiment by Romi Nijhawan of the University of Sussex that explored the role of eye movements and FLE (The flash-lag phenomenon: object motion and eye movements, Perception, 2001). Nijhawan asked participants to focus on the small cross at the center, and showed participants a ring, rotating about a fixed point on screen (Panel A). At some point, a flash perfectly filled the rotating ring (Panel B), but participants actually saw a small gap (Panel C) as the flashed object lagged behind the moving one in their perception.


You can play with a cool interactive version of this display here.

So, when your eye is fixed on the cross at the center, you misperceive the flash behind the moving ring. What does this tell us about the car parked on the side of the road? In an additional experiment, Nijhawan used a ring that was stationary and asked participants to follow a small pursuit point around the path of motion (Panel A).


Now, when your eye is moving up, any image on your retina is moving down (think about how the road moves opposite the direction you are going when you look out the passenger window). Just when the pursuit point was in the center of the stationary ring, Nijhawan again perfectly filled the center with a flash (Panel B). And again, participants saw a flash-lag effect (Panel C), but this time it wasn't the ring that was moving, it was their own eyes! No matter the source of the movement (the object moving, or your eyes), we misperceive the relative locations of briefly flashed objects. When you are passing that parked car with its hazard lights on, the blinking lights might look a little off because your motion is creating a flash-lag effect, leading you to see the lights a bit disconnected from the car.

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I would like to point out another effect that I noticed while looking at your interactive display linked above. While I stared at the middle X I noticed that the spinning annulus was creating the visual of a blue circle in the path where the annulus follows. The effect seems to work better from a distance, the intensity of the circle being zero at a very close proximity, then becoming stronger as I pull away. I also noticed that the circle was more noticeable as the annulus moved faster, but even at a slow speed it is viewable. I tried it on two monitors just to make sure it wasn't my monitor. This interests me, do you happen to know if there is a name for this effect?

By Robbie Ferrero (not verified) on 22 Mar 2005 #permalink

I don't know a specific name for this, but it sounds like a motion blur for the trailing edge. I can't seem to see it, though I do get a hint of some kind of trail at faster speeds (mine is just black, no blue). It's good that checked it out on two monitors, do they have the same refresh rates?

One monitor is at 75 hertz on an LCD monitor, the other was at 60 hertz on a CRT monitor. I tried different refresh rates and monitors to see if it was something hardware based, and it may be. But... What I did notice is that if I don't focus directly on the center X then I don't see the halo. Is there anything you've heard about that suggested that movement in peripheral vision could cause an effect like this? Could it just be me?

By Robbie Ferrero (not verified) on 23 Mar 2005 #permalink

I commonly experiment with this "flash-lag" effect as I drive down the road at night. [!:~)] E.g. when I see a radio tower with a light flashing on it I deliberate "wiggle" my eyes [something I am capable of doing] while looking at it and or will look past it at the moment that it flashes. If it is a strobe and not a continuous light, I will see a series of dots. [I once thought that this was do to saccadic movement in my eyes but ruled that out by videotaping it.] The rapid movement of my eyes allows the stroboscopic effect to be detected. [It does not flicker during daylight hours because it is then one big flash rather than a series of dimmer ones.] The fact that I detect the strobe is NOT due to any flickering that I can see—it is due to the persistence of the light in my perception; I see a trail of briefly persisting dots. The dots are superimposed on the background away from the tower. The challenge is to count them. What is really peculiar is that the dots frequently appear to be about 10 feet in front of me!

By Tom Wagner (not verified) on 24 Mar 2005 #permalink