The last two posts have focused on the navigation methods of non-mammals, such as birds and insects, which may be a little bit difficult to fully relate to. But today, we will be discussing mammals, including humans, as the navigation technique is one that humans actually have. Path integration, the subject of today’s blog post, is the ability to navigate and be able to return to a starting point using cues from self-motion (also known as proprioceptive and vestibular senses), such as turning right or left. This ability, also called “dead reckoning”, does not require any sort of landmarks to be used, although landmarks can still be helpful in navigation. Today we’ll take a look at how many different mammals, including humans, use path integration.
The term “dead reckoning” is short for “deduced reckoning”, and was originally coined by sailors who used path integration as a way to navigate the seas, where everything looks the same. Since there are no real identifiable features to the open seas, early sailors had to keep careful track of all the turns they made in order to be able to return home safely. Similarly with other mammals, when no fixed landmark is available to navigate by, they are able to internally measure the amount of turns they make, and use this information to navigate back to where they came from. This “homing” behavior is often shown when the animals are foraging, and can be seen when an animal explores a new environment collecting food, and when it is finished, immediately returns to its “home base”, usually a nest or burrow. Proving an animal uses path integration can be difficult, as the animal has to be shown using nothing but its own motion to navigate. All external cues for navigation must be eliminated from the testing area. One interesting way developed to test for path integration involved placing the animals (in this case, gerbils) on a slightly rotating platform. The platform rotated so slowly that the gerbils weren’t able to detect it, but by the time the gerbils had completed their objective (picking up a pup from a location and moving it back to their nest), any landmarks in the room would have moved significantly, making it difficult to navigate back to the nest using landmarks alone. The gerbils, however, successfully made it back to their nests.
The neural mechanisms of path integration, unlike most other things I have discussed on this blog, are very well studied and understood. Path integration appears to be controlled by the hippocampus and surrounding areas of the brain, and in exploring rats, the firing neurons are localized to specific parts of the hippocampus, generating “place fields” representing different locations of the animal in its space. Single cells in a place field are called place cells, which get directional information from a network near the hippocampus called the head direction system. Neurons firing in the head direction system strongly correlate with the direction the animal is moving in. All together, the place fields and head direction system are able to form somewhat of an internal map with a compass, with the place field being the map, and the head direction system being the compass.
Path integration is not flawless, and errors do tend to occur when using path integration alone as a primary method of navigation. This is where landmarks come in to play. Using both landmark based navigation and path integration gives the animal much more accuracy than using either method alone. For example, landmark navigation is useful up until the landmarks are no longer visible, and if a landmark is moved, the animal has nothing else to go off of. Path integration by itself carries the risk of the animal not remembering the correct number of turns or approximate distance. However, both at the same time minimize the risks of getting lost, and are quite effective at getting mammals where they need to go. The next time you’re out in a busy, unfamiliar area, use the gift of path integration that your brain has given you!