Understanding the brain’s map and compass
If a man has a map, he can know where he is without knowing which way he is facing. If a woman has a compass, she can know which way she’s facing without knowing where she is. Animals from ants to mice to humans use both kinds of information to reorient themselves in familiar places, but how they determine this information from environmental cues is not well understood.
In a new study in mice, Penn researchers have shown that these systems work independently. A cue that provided both types of information allowed the mice to determine their location but not the direction they were facing.
The study was conducted by graduate students Joshua Julian and Alexander Keinath, assistant professor Isabel Muzzio, and professor Russell Epstein, all of the Department of Psychology in the School of Arts & Sciences.
“When you’re lost,” Epstein says, “how do you reestablish your bearings? People have been studying this for more than 25 years, but they have not focused on the fact that place recognition—figuring out where you are—and heading retrieval—figuring out which way you’re facing—could be two separate systems.”
The team’s experiment was an updated version of one of the original studies of reorientation, conducted at Penn in 1986, which showed that rats use the shape of a room, but not other features, to get their bearings.
To show this, the researchers in the 1986 study used a small rectangular room that had cups in each corner in which food could be hidden. From a bird’s-eye view, the food was always hidden in the northwest corner. From the rat’s perspective, markings on the walls should have told them which way they were facing, enabling them to find the northwest corner every time.
After researchers spun the rats around to get them to lose their bearings, however, the rats were equally likely to search in the southeast as they were in the northwest. This told the researchers that the rats’ “compasses” were calibrated to the geometry of the room, but apparently couldn’t use the markings to tell which short wall was which.
“These earlier reorientation studies taught us how the mental compass worked,” Julian says, “but did not tell us about the mental map.”
In the new version of the experiment, mice were trained on the location of food in two different rectangular rooms. These rooms were identical to each other, except their north walls were decorated with vertical stripes in one and horizontal stripes in the other. The food was also hidden in different corners in the two rooms.
The mice made the same kind of geometry error as in the earlier studies, equally searching in the correct corner and the one that was diagonally opposite it. This showed that the wall markings provided the map of which of the two rooms the mouse was in, but only geometric features were used to set the compass that told the animal which direction was which.
Future work will involve running similar experiments while gathering direct brain recordings from the mice to see how their brain “maps” change when they use a room’s geometry versus marking cues to get their bearings.