The brain is a strange thing, and hypnosis may be one of the weirdest brain phenomena around. Some dude waves a watch at you and tells you there is an elephant in the room. Voila, your brain invents an elephant, acts out the elephant's behavior, and the rest of your brain responds as if this is real. This weirdness is partially why I noticed a paper on how hypnotism changes visual perception. The rest is paranoia.

I've been suffering from a deadly combination of fascination and suspicion when it comes to hypnotists for a long time. When I was in my teens, I remember a hypnotist visiting town—this was before anyone in New Zealand thought that hypnotism was anything but entertainment, I guess. He set up a little stage in a shopping center and invited a couple of teenage boys on stage.

The two dudes strutted up, full of pride. After hypnotizing them, the hypnotist got them to do all sorts of things, right down to performing a strip show. They were down to their tighty-whities when the hypnotists woke them up. Needless to say the two young men were not just shocked, they were highly embarrassed. To make matters worse for them, the crowd roared. No gentle clapping and polite laughter—these two got the full thank-god-that-isn't-me belly laugh. The hypnotist had chosen a particularly cruel joke to play on a pair of highly suggestible young men.

That left an impression. Beyond deciding I don't think I'll ever volunteer to be hypnotized, I came away with a lingering curiosity. Why are some people susceptible to hypnotism, and how does a suggestion make people do things they would rather not?

Those are big questions that are difficult to answer, but we can focus on some smaller questions. In a recent study, researchers wanted to see how hypnotism changed visual perception. If someone under hypnosis is told that their view is obscured, do they really not see or are they unable act on what their brain is yelling at them?

Watch the watch now

The researchers used a game called the oddball game, in which participants are presented with triangles, squares, and circles on a screen. Triangles appear 80 percent of the time, while squares and circles make up the rest. Participants are asked to click when a square is on the screen. The shapes appear sequentially and for only a short time, meaning participants have to make decisions relatively quickly

This simple task exercises a surprising amount of the brain. Most of the time, triangles appear, so the brain has to recognize that and suppress the urge to click. But when the shape is not a triangle, the urge to click is very strong, so the brain has to quickly determine if the shape is a circle and suppress that urge. Only if it is a square should it allow a click to happen.

Now, if you play that game while wearing electrodes attached to your skull, researchers can determine when you recognize a shape, when you suppress action, and when you are driven to take action. By comparing subjects under normal (control) conditions and under hypnosis, researchers hoped to identify at what level hypnosis gets involved.

The researchers got 60 college students, divided into low, middle, and highly suggestible groups. To obtain data on how the brain was processing the visual stimuli, the participants were wired up to an Electroencephalograph (EEG) while they played the oddball game.

Without hypnosis, the three groups played the game with reasonably high accuracy (around 90 percent), though the highly suggestible group performed slightly better than the rest at about 95 percent accuracy. In the next step, the participants were hypnotized and asked to believe that there was a wooden board obscuring their view of the screen.

As you might expect, performance in the oddball game dropped. For the low and middle groups, the difference was small (about 10 percent) but significant. The most suggestible group, however, went from being the best performing group at 95 percent to the worst performing group at about 55 percent. Yes, they went from correctly clicking on squares almost all the time to missing just about half of them.

So far, interesting, but not so surprising.

Unsee that square

The researchers then turned to the EEG to see if they could unravel how the hypnosis was working. EEGs record voltages as a function of time from different regions of the brain. In particular, the researchers focused on the p3b trace, which is associated with the brain's response to rare events.

During oddball, the trace normally shows a clear increase in the potential as the object is recognized. For triangles, this is followed by the electrode potential returning to zero: action has been suppressed. For circles, the suppression is slower and less strong. However, the response to a square shows no return; instead the potential increases to much higher levels, indicating that the brain knows it needs to take action.

For subjects that were hypnotized, the story is much the same. All subjects in all groups clearly see and recognize the shapes. Indeed, for triangles and circles, I don't think anyone could tell the difference between hypnotized and control groups. For squares, the recognition response occurs as normal, but the follow on—the increase that comes before the button is pressed—is significantly muted.

Breaking this out by group, the researchers showed that, as you might expect, the degree of muting increases with suggestibility. Furthermore, at an individual level, the degree of muting was strongly correlated to performance. So the group average doesn't mask a huge degree of within-group variability.

Although the researchers do not report the statistics for this, the EEG graphs also show that the group with the highest suggestibility (which performed best during the control test) also had a slightly stronger average response to squares. However, I suspect that the observed difference is not statistically significant in this study.

When you wake up, you will remember everything

So, what have we learned? First of all, in this task, we know that the brain still sees the objects on the screen, but that hypnosis suppresses a response to the object. Second, from interviews, the researchers know that the more realistically the participants experienced the bit of wood, the worse their performance at oddball. This was backed up by a strong correlation with EEG measurements.

The researchers also point out the intriguing link between suggestibility and performance. The researchers argue that highly suggestible people have a tighter focus on instructions. In the first case, they are instructed to count, so they do so. In the second, they are instructed that the screen is obscured, so fail to count.

Scientific Reports, 2017, DOI: 10.1038/s41598-017-05195-2 (About DOIs).