09/11/2012 06:15 pm ET Updated Dec 06, 2017

How Your Sense of Rhythm Makes You Hallucinate

A new research project has people seeing ghosts. By flashing an image on a screen in synch with signal rhythms in certain brain areas, scientists are making people hallucinate the image even after it's disappeared.

This happens for a simple reason: Our brains are pros at picking up and predicting patterns, and if we expect to see (or hear, or feel) something strongly enough, we may actually experience it. Ever felt your cellphone buzzing when it really wasn't? Then you've experienced a very similar phenomenon.

Triggering hallucinations with technology is nothing new; studies have found that a little electrical stimulation at just the right point in the visual cortex can cause people to hallucinate dancing lights and colors. In fact, some researchers have suggested techniques like these may someday give sight to the blind. Even weirder, one recent study found that stimulating a visual area known as MT+ can cause people to hallucinate movement of motionless objects.

Meanwhile, in a 2010 study, a team of scientists made a different kind of discovery: When they flashed an image at people a few times, then flashed a very faint followup image, people only reported seeing the faint image if it had appeared in synch with the rhythm of the previous images. In other words, we're much more likely to see a visual stimulus if we can instinctively predict when it'll appear.

2012-09-09-300pxKanizsa_triangle.svg.png So this year, those scientists decided to try going a step further. They wondered if it'd be possible, using this kind of rhythmic flashing, to not only make people more aware of faint images but to trick their brains into seeing an image that wasn't there at all. So they hooked up some volunteers to an electroencephalograph (EEG), which detects electrical oscillations passing across a person's scalp, and recorded the frequencies of the oscillations of volunteers' visual brain areas as they noticed an image. When the scientists flashed images to the volunteers at this same frequency, the volunteers' brains apparently came to expect the image, even when it was no longer onscreen.

As researcher Kyle Mathewson explains it, "We showed that peoples' visibility of the target fluctuated depending on the timing with respect to their [brainwave] rhythm. So we locked in the timing of their brainwaves and that locked in their ability to see the world at a certain time."

This is actually pretty similar to the way video technology works. Although we might perceive reality as a continuous flow, our brains actually process it in what you might call "slices" or "chunks." Electrical signals in your visual cortex oscillate around 60 to 90 times a second -- about the same refresh rate as your computer monitor -- while other areas of your brain communicate at a range of higher and lower frequencies.

In fact, the latest research suggests that groups of neurons communicate by literally "tuning in" to specific signal frequencies and "tuning out" of others. In other words, our brains don't resemble computers so much as networks of specialized radio operators.

Unlike radios, though, our brains are adept at filling in gaps with "best guesses" about what information they'd expect to see (or hear, or feel) in them. When survival comes down to split-second decision making, those leaps can save lives (after all, the universe is filled with predictable rhythms), but our brains' gap-filling talents also make us maddeningly easy to trick.

So next time you find yourself wondering what's real and what"s an illusion, remember that illusions are central components in your perception of reality. The real question isn't whether you're being tricked, but whether you're aware of the trickery.