Though neither of these likely comes as a surprise, their cause is cool: when you first look at or experience a stimulus, the neurons that recognize it get excited. They spring into action, processing the new information and forcing it into your consciousness: wow, look at all that falling water! Then the neurons get bored. The stimulus gets blasé and drifts into the background. And your neurons adjust their expected baseline——falling water starts to look stationary——and you start to interpret things in relation to this baseline. And if falling is stationary, then stationary is up, so when you look from a waterfall to a rock, your baseline is off and the rock appears to levitate until your neurons readjust.
But what's really cool is this: looking at something moving (a waterfall) can make you feel like you're moving (the walkway). In other words, the phantom sensations cross. Maybe you've had the jolting sensation of moving backwards as a large truck slowly passes your car on the freeway.
Researchers demonstrated this phenomenon by showing participants images of black and white bars moving up or down through a computer screen, and then touching participants' fingertips with a small electric device. Participants felt the device moving even if it wasn't.
But what's really, really cool is this: the effect crossed the other way, too. A moving touch stimulus made participants feel as if a screen of black and white bars was moving, even if it wasn't.
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