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.