first postulated an inertially-based navigation system in animals in 1873. Studies beginning in the middle of the 20th century confirmed that animals could return directly to a starting point, such as a nest, in the absence of vision and having taken a circuitous outwards journey. This shows that they can use cues to track distance and direction in order to estimate their position, and hence how to get home. This process was named path integration to capture the concept of continuous integration of movement cues over the journey. Manipulation of inertial cues confirmed that at least one of these movement cues is information from the vestibular organs, which detect movement in the three dimensions. Other cues probably include proprioception, motor efference, and optic flow. Together, these sources of information can tell the animal which direction it is moving, at what speed, and for how long. In addition, sensitivity to the earth's magnetic field for underground animals can give path integration.
Mechanism
Studies in arthropods, most notably in the Sahara desert ant, reveal the existence of highly effective path integration mechanisms that depend on determination of directional heading and distance computations. In mammals, three important discoveries shed light on this issue. The first, in the early 1970s, is that neurons in the hippocampal formation, called place cells, respond to the position of the animal. The second, in the early 1990s, is that neurons in neighboring regions, called head direction cells, respond to the head direction of the animal. This enables a much more fine-grained study of path integration, since it is possible to manipulate movement information and see how place and head direction cells respond. The third finding was that neurons in the dorso-medial entorhinal cortex, which feeds information to the place cells in the hippocampus, fire in a metrically regular way across the whole surface of a given environment. The activity patterns of these grid cellslooks very much like a hexagonally organized sheet of graph paper, and suggest a possible metric system that place cells can use to compute distances. Whether place and grid cells actually compute a path integration signal remains to be seen, but computational models exist suggesting this is plausible. Certainly, brain damage to these regions seems to impair the ability of animals to path integrate. David Redish states that "The carefully controlled experiments of Mittelstaedt and Mittelstaedt and Etienne have demonstrated conclusively that this ability is a consequence of integrating internal cues from vestibular signals and motor efferent copy". Mice use place cells and grid cells in the brain's hippocampus region to perform path integration.
