Scienists put a tiny camera inside a mouse's hippocampus. Here's what they saw.
A few weeks ago, Japanese scientists released video footage of the neurons firing in a baby zebra fish brain while it hunted for food. It was pretty cool. This week, researchers at Stanford University announced they have used a similar technique to capture real-time neuronal images from a mouse brain. It’s pretty cool, too.
As in the fish study, Stanford scientists used gene therapy to elicit a bright, fluorescent green flashes from mouse neurons whenever they fired. From there, they implanted an extremely small microscope in the mouse’s brain, just above the hippocampus. According to a university press release describing the experiment, the camera was able to capture the activity of about 700 hippocampal neurons. A microchip wired to the camera transmitted the images to a monitor for the scientists to view.
Which is what you see in the video above. Released just a few days ago to accompany a new study in the journal, Nature Neuroscience, the video shows in split screen what’s happening in the mouse hippocampus while it negotiates the boundaries of an enclosure.
Part of what you're seeing here is learning. Watch closely and you’ll observe that the neurons that fire when the mouse is in a particular location fire the same way when the mouse returns to that same spot. “The hippocampus is very sensitive to where the animal is in its environment, and different cells respond to different parts of the arena,” explained associate professor of biology and of applied physics, Mark Schnitzer, in the press release. “Imagine walking around your office. Some of the neurons in your hippocampus light up when you're near your desk, and others fire when you're near your chair. This is how your brain makes a representative map of a space.”
In that sense, the footage to the right serves as a direct transposition of real-world space into gray matter topography. As Schnitzer put it, “We can literally figure out where the mouse is in the arena by looking at these lights.” What’s more, the mouse’s neurons fire the same way in relation to its location when the mouse is tested a month later.
The hippocampus plays a complex role in human cognition, which makes this newest batch of video imagery very exciting. As with mice, the human hippocampus is crucial to our spatial intelligence. A famous study published in the Proceedings of the National Academy of Sciences in 2000 found that the posterior hippocampi of London cab drivers were actually larger than in control subjects. When a person’s job requires memorizing the byways of a city as large as London (the greater metropolitan area, according to the Encyclopedia Britannica, covers 610 square miles), the hippocampus effectively grows to make room.
But the hippocampus is also a crucial instrument in forming and retrieving what are called “declarative” or “explicit” memories. These are memories like facts and places that can be actively recalled and expressed. (“Implicit” or “procedural” memory, by contrast, involves learned but unconscious processes like the complex muscle and sensory coordination it takes to ride a bike.) For declarative memories, it’s believed the hippocampus acts as a sort of processing hub, assigning the different sensory components of an experience to different places in the neocortex, where they eventually reside as “pieces” of a whole memory. Scientists are still figuring out exactly how it works, but declarative memories seem to live at least partially in the hippocampus for a while before eventually moving entirely to the brain's various cortices (visual, auditory, etc.) for long-term storage.
When we recall memories, the hippocampus acts “something like an orchestra conductor in directing the symphony of our conscious memory,” to borrow a different metaphor from science writer, Nicholas Carr. Based on studies of rats, monkeys and humans from the last few decades, scientists think the hippocampus retrieves and synthesizes whole memories—how a memory looks, feels and smells, for example—from different parts of the brain. Neuroscientists also think, as Carr notes, that the hippocampus helps link new and old memories together.
Which means that part of what we’re seeing in the mouse video could be the beginnings of long-term memory formation. The practical applications of such research are huge. The Stanford scientists believe this and subsequent studies could, for example, contribute greatly to our understanding of neurodegenerative diseases, like Alzheimer’s.
Video from the Japanese zebra fish experiment.
In the Japanese fish video (above), the neuronal firings we observe act like something of map, too, albeit a map of a different kind. In that case, scientists were looking at the fish’s optic tectum. The flashing lights mapped what the fish saw, not where it was. Taken together, studies like these are starting to give us a clearer picture of how different areas of the brain work, and work together, in ways we can actually see.