Dropping a tab of acid is a surefire way to experience a hallucination, but for a less chemical-heavy trip, set aside two minutes and watch this video. Then immediately look away. Come back when you're done.
Freaky, right? If your experience is anything like mine, after you watched the video, whatever you looked at should appear to be pulsing and swirling, like a slightly more subtle version of what is happening in this GIF--itself widely used on the Internet to demonstrate what the world looks like to someone on acid.
In reality, of course, what you just experienced is not a hallucination in the traditional sense. Instead, it's just a very well-designed video from The Illusion Lab that takes advantage of neurological phenomena we've known about since the days of Aristotle. Here's how it works.
When we look at rapidly moving objects for long periods of time, the neurons in our brains adapt to the stimulus by increasing their sensitivity to objects moving in the opposite direction. For example, if you stare at a waterfall for a few minutes, then look at a rock, the rock will appear to swim upwards for a short period of time.
This is called motion aftereffect. Presumably, the reason our brains do this is so that we don't become so absorbed watching something move, like a rabbit running across a field, that we don't notice another thing running in the opposite direction, like a lion coming towards us.
But whatever the reason, motion aftereffect can be leveraged to make some trippy optical illusions, as the Illusion Lab has done here. By blasting our eyes with rapidly swirling patterns, our neurons get so keyed up to see something, anything, that moves in the opposite directions that when we look away, we start seeing imaginary vortexes everywhere until they calm down.
This isn't the only kind of "hallucination" the motion aftereffect can make us see: another popular version of this same optical illusion can make the world appear to grow around you, instead of pulsing and swirling. And it's certainly a more work-safe way of tripping out than a mushroom cap of Psilocybin.
[H/T Sploid]