Is your mind playing tricks on you — or is it something more?

What do Mary Todd Lincoln, Jerry Garcia and I have in common? We’ve all had hallucinations.

Lincoln’s were schizophrenia related and landed her in an insane asylum called Bellevue Place. Garcia’s were drug related. In 1989 he told Jeremy Alderson “I always keep some psychedelics around. It’s like a coffee break almost. It’s something that I fear and that I love at the same time. I never take any psychedelic without having that feeling of, ‘I don’t know what’s going to happen.'”

Mine, research says, are just associated with normal brain function. They are auditory and happen almost exclusively when I’m very sleepy.

If you’ve ever been scared you were going crazy — thought you saw or heard something that really wasn’t there — don’t worry too much. You’re probably one of the 5 percent of people who, like me, experience hallucination at some point in life.

A study in Proceedings of the National Academy of Sciences of the United States of America (PNAS) says that hallucinations, which were once seen only as symptoms of psychosis and serious mental illness, may just be the result of your brain doing the best it can to make sense of a hectic and unclear world. “It’s not a broken brain that generates hallucinations, it’s just part of how the brain works,” neuroscientist Christoph Teufel, author of the study, said.

Most people think of their eyes and ears working like a video camera recording sounds and images and sending them directly to the brain to process.

We can sure do some of that, but this isn’t exactly what’s happening.

It turns out, the information sent to the brain isn’t very detailed at all. The brain has to fill in the information gaps based on previous knowledge and context clues. Most of the time this  process goes off without a hitch and the brain fills in correct information. But when it doesn’t… welcome to a hallucination.

Study scientists think that hallucinations are results of the brain’s over reliance on old information and that sometimes the brain fills in a gap with details that aren’t really there. The brains of people with signs of serious psychosis are merely better at using prior knowledge, albeit incorrect, to fill in the gaps delivered from their visual systems.

So what’s the differences between a “normal” hallucination and the ones Mary Todd Lincoln had that got her committed?

The PNAS study is important because it supports the idea that psychosis is a spectrum disorder, not a condition that you either have or don’t have. Hallucinations seem to be normal, as long as they don’t take over a person’s life. “There’s a continuum between mental illness and mental health,” Teufel says, and we’re all on it.”

Non-disease associated hallucinations are known to manifest more often as a result of bereavement, stress or sleep deprivation. High caffeine consumption has also been linked to increased rates of hallucination. As few as five cups of coffee per day can trigger the phenomenon.

A higher than average number of Alzheimer’s and Parkinson’s disease patients report having hallucinations. If you have macular degeneration or other vision problems there is a condition known as Charles Bonnet syndrome wherein vision loss is accompanied by hallucinations.

Barring any of the aforementioned medical conditions and psychedelics, if you experience a relatively small number of mild hallucinations — like hearing someone call your name — you can relax. It’s  likely just your normal brain trying to sort things out.