In a lab mouse version of The Truman Show, researchers from Harvard Medical School constructed a little world for a new paper. An eight-inch-wide platform raised 20 inches off the ground stood at the center, covered in mouse bedding. All around curved a tall LED screen, blank until a white, disorienting stripe flashed to one side or the other.
The researchers were looking for head direction cells in the mouse, which act as an inner compass in the brains of humans, insects, animals and fish. While not a proper magnetic compass, this neural compass acts as a relative one based on landmarks instead of Earth’s magnetic field. In humans, it spans several different brain areas, including the anterodorsal thalamus, the area targeted in the mouse study.
An Inner Compass
All neural compasses include brain cells with preferred firing directions, meaning they fire continuously and spew neurotransmitters when the head is pointed in a certain direction. But how do they know when that’s happening?
First, there’s visual data from the eyes, according to a paper in the journal Hippocampus. Second, the brain relies on the proprioceptive sense that tracks limb position.
Third, the compass factors in the important vestibular system (the inner ear), which uses fluid-filled loops to estimate head position.
Mice in the Dark
In the mouse study, the researchers identified compass cells covering all 360 degrees, and noticed some quirks with how they functioned.
For one, a rotated visual cue, one that moved around the screen, held special power over the mice. After about two minutes, the mobile cue hijacked their compass and overrode other types of input, including the mice’s own movement.
When subjected to the dark, the mice’s compass wandered about, sometimes aimlessly, and sometimes with a pattern influenced by the previous cues. Deprived of landmarks, the mice held onto traces of old ones, making them easier to revive.
Beyond the basic compass functioning, the researchers detected an extra layer of network activity they dubbed “gain” that helped the system to change quickly and sort through conflicting landmarks.
Finding Our Way
The human brain maintains a map of where the body is located and uses many senses to do so, including vision, sound, the vestibular system and proprioception. This is what allows us to look around a room, close our eyes, and take five steps without colliding with anything.
The inner compass is crucial to these "place" and "grid" cells and provides live information as to where the human is pointing. Since we live in a three dimensional world, the brain must track a vertical axis as well, and this appears to happen in a section of the brain called the retrosplenial cortex, a 2018 study found.
But how do we make these maps? Scientists have long wondered whether observation is enough, whether one simply has to gaze upon a landscape, or whether one must walk the ground. In 2022, a study concluded that rats could do the latter and learn a cage structure simply by watching a friend.
