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You may have heard that it's impossible to grow more brain cells. All the more reason to wear your bike helmet or drink less alcohol.

But research in the last few decades has shown that neurogenesis, or the formation of new neurons, can happen in adulthood. In fact, a paper published this week in the journal Nature Medicine found evidence that we make new neurons into our late 80s. The idea that humans don't make brain cells as adults arose because it's kind of true — at least, the neurons that we have at birth are the bulk of the neurons we'll ever have for life.

Plus nerve cells are notoriously slow to repair themselves, which is especially important in conditions like Alzheimer's or Parkinson's. But, back in 1998, research showed that an adult human hippocampus can make new neurons. Still, questions remained, like how many years of neurogenesis we have, and of course, whether we can apply this information medically.

So for this new study, researchers took brain samples from dozens of freshly deceased humans ranging in age from 43 to 93 — 45 of the brains were people with Alzheimer's, while 13 were considered controls. They zeroed in on several regions, especially a part of the hippocampus where previous studies have seen neurogenesis and looked for it by finding cells expressing a protein called doublecortin, or DCX. It's associated with microtubules, one of the structural proteins in cells that helps them move things around.

Previous work confirmed that when the cells that grow into neurons start dividing, they express more DCX, and the stuff is still around as neurons mature. That's what makes it a good marker — the presence immature neurons tells the researchers that new neurons are still popping up. As they predicted, the researchers found a notable difference in the number of DCX-positive cells depending on if the person had Alzheimer's and how severe the condition was.

But they still found thousands of young neurons per cubic millimeter of tissue in all of the hippocampuses they examined. And in the controls, they found tens of thousands, even in an 87-year-old's brain, though there appeared to be a slight decline in neurogenesis with age. This isn't the first time scientists have looked for neurogenesis in adult brains, though, and previous studies found much less — which made people wonder if maybe adult brains don't really make new neurons.

Now, the researchers say, they're sure that they do. They credit their more robust findings to improved methodology. They say detecting these doublecortin-expressing cells can depend on how the brain tissue is prepared and stored.

More time in the brain bank makes these cells harder to find, which might ultimately make it seem like neurogenesis doesn't happen when it does. This team made sure they were using fresher specimens and prepped them under just the right chemical conditions. And now that the methods for finding new neuronal cells are more fine-tuned, hopefully, future studies can confirm these results, so we can make progress towards understanding and someday treating brain conditions like Alzheimer's.

Meanwhile, in other brain news, new research in the journal Cell Reports may have figured out the mechanism behind hallucinations — and it's basically the opposite of what we thought. When you hear the word “hallucination”, it might conjure up images of the psychedelic 60s, like peak Jimi Hendrix, Pink Floyd, or The Beatles, flower clowns. But when scientists talk about hallucinations, they don't necessarily mean the bright lights and rainbow colored trips of “I Am the Walrus”.

Hallucinations are any disruption in how the brain receives sensory input. So that sometimes means seeing or hearing things that aren't actually there, but they vary from person to person. Scientists are really keen to understand the neural basis for them because they're a major symptom of some psychiatric disorders.

Also, the drugs that make people hallucinate seem to help with conditions like depression, but scientists aren't totally sure why. And knowing what's really happening in the brain could help them figure that out. For a long time, people have thought that hallucinations are caused by an increase in activity in relevant brain regions.

Which makes sense, because, well, if you're seeing things that aren't there, that would suggest the part of your brain that handles visual information is going a little overboard. And previous research has connected the effects of hallucinogenic drugs with the activation of serotonin-2A receptors specifically. So in the new study, researchers gave their mice test subjects a drug called DOI, which, like LSD or magic mushrooms, turns on those serotonin-2A receptors.

But unlike LSD, it's not a schedule 1 controlled substance, which means it's easier to experiment with in the US. The team then showed images to the mice while measuring and imaging the activity in their brains to see which regions were affected. Sure enough, they could see activity in the visual cortex as the mice were processing the images.

And the researchers saw the same general pattern in brain activity in both the mice that did get the drug and the mice that didn't get the drug. But for the ones that did get the drug, the timing was off. And instead of more signaling, there was actually less.

Which actually could explain why people see things when they take hallucinogens. According to the researchers, the brain likely compensates for the decrease in activity by over-analyzing the information it does have. Now, there have been some studies that look at the whole human brain under the influence of hallucinogens, but the imaging methods used in these mice went way beyond your typical brain scan.

The mice were genetically modified so their cells would create a fluorescent molecule that reacts with calcium ions. And since these ions move about when a cell reacts to things, neuronal activity can be measured at a much smaller scale, like, individual neuron scale. Though the mice's skulls had to be open during the experiment so the microscopes could zero in on the right cells, so not necessarily a technique you wanna use on...people.

But that doesn't mean the study is perfect or anything. Like we always say, mice aren't people, so it's possible their brains don't work quite the same way as ours do. Also, the research team couldn't exactly ask their subjects what they saw — they had to infer hallucinations from things like paw or head twitches on top of their other data.

So it's not 100% clear the other drug actually induced the same kind of hallucinogens that drugs like LSD would. But, this study provides some intriguing hypotheses that can be tested further. If the same general mechanism for hallucinations does occur in people, then understanding it in depth could lead to novel ways of treating conditions like schizophrenia where hallucinations are a big part of the symptoms.

And it could help scientists understand why some psychoactive substances seem to help with disorders like depression, potentially allowing them to develop medications that mimic the good effects without so much of the tripping. Thanks so much for watching this episode of SciShow news! If the two studies mentioned today seemed kind of mind-blowing to you, we have a podcast you might wanna check out.

It's called SciShow Tangents, we produced it in collaboration with WNYC Studios and it's really fun. There are four of us in the podcast — including me! — and every week, we share the most mind-blowing facts we can find about a topic. Along the way, we also tend to go on lots of tangents, which is why we call the show “SciShow Tangents.” There are different segments, like one where we answer listener questions, and another where someone presents one true fact and two fake ones and the rest of us have to guess which one is true one.

Which, for the record, is a lot harder than you'd think. And we try to end every episode with a fact about butts. Because you can never learn too much about butts.

And there are always more butt facts. You never run out. You can check it out wherever you get your podcasts! [ ♪OUTRO ].