Path integration
Encyclopedia
Path integration is the name given to the method thought to be used by animals for dead reckoning
.
Charles Darwin
and J.J. Murphy 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 (or idiothetic
) cues is information from the vestibular organs, which detect movement in the three dimension
s. Other cues probably include proprioception
(information from muscles and joints about limb position), motor efference (information from the motor system telling the rest of the brain what movements were commanded and executed), and optic flow (information from the visual system signaling how fast the visual world is moving past the eyes). 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 (e.g., mole rat) can give path integration.
The question of how the animal's brain integrates this dynamic information, in order to draw conclusions about position and homing information, remains unanswered. Extensive studies in arthropods, most notably in the Sahara desert ant
(Cataglyphis bicolor), reveal the existence of highly effective path integration mechanisms that depend on determination of directional heading (by polarizing cues such as the sun) and distance computations (by, essentially, “counting footsteps”). How these sources of information are integrated is not yet known.
In mammals, three important discoveries promise to shed light on this issue. The first,
in the early 1970s, is that neurons in the hippocampal formation
, called place cell
s, respond to the position of the animal. The second, in the early 1990s, is that neuron
s in neighboring regions (including anterior thalamus
and post-subiculum
), 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 (a much simpler procedure than training an animal, which is very slow). 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 cells
looks 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.
Dead reckoning
In navigation, dead reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course...
.
Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
and J.J. Murphy first postulated an inertially-based navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
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
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or 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 (or idiothetic
Idiothetic
Idiothetic literally means "self-proposition" , and is used in navigation models as in the phrase "idiothetic cues" to indicate that path integration was used to determine the present location instead of allothetic, or external, cues...
) cues is information from the vestibular organs, which detect movement in the three dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
s. Other cues probably include proprioception
Proprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
(information from muscles and joints about limb position), motor efference (information from the motor system telling the rest of the brain what movements were commanded and executed), and optic flow (information from the visual system signaling how fast the visual world is moving past the eyes). 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 (e.g., mole rat) can give path integration.
The question of how the animal's brain integrates this dynamic information, in order to draw conclusions about position and homing information, remains unanswered. Extensive studies in arthropods, most notably in the Sahara desert ant
Sahara desert ant
The Sahara Desert ant is a desert-dwelling ant of the genus Cataglyphis. It inhabits the Sahara Desert and is one of the most heat tolerant animals known to date. However, there are at least four other species of Cataglyphis living in the Sahara desert, for example C. bombycina, C. savigny, C....
(Cataglyphis bicolor), reveal the existence of highly effective path integration mechanisms that depend on determination of directional heading (by polarizing cues such as the sun) and distance computations (by, essentially, “counting footsteps”). How these sources of information are integrated is not yet known.
In mammals, three important discoveries promise to shed light on this issue. The first,
in the early 1970s, is that neurons in the hippocampal formation
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
, called place cell
Place cell
Place cells are neurons in the hippocampus that exhibit a high rate of firing whenever an animal is in a specific location in an environment corresponding to the cell's "place field". These neurons are distinct from other neurons with spatial firing properties, such as grid cells, border cells,...
s, respond to the position of the animal. The second, in the early 1990s, is that neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s in neighboring regions (including anterior thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
and post-subiculum
Subiculum
The subiculum is the most inferior component of the hippocampal formation. It lies between the entorhinal cortex and the CA1 subfield of the hippocampus proper.-Paths:...
), called head direction cells
Head direction cells
Many mammals possess neurons called head direction cells, which are active only when the animal's head points in a specific direction within an environment. These neurons fire at a steady rate Many mammals possess neurons called head direction (HD) cells, which are active only when the animal's...
, 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 (a much simpler procedure than training an animal, which is very slow). The third finding was that neurons in the dorso-medial entorhinal cortex
Entorhinal cortex
The entorhinal cortex is located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation. The EC is the main interface between the hippocampus and neocortex...
, 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 cells
Grid cells
A grid cell is a type of neuron that has been found in the brains of rats and mice; and it is likely to exist in other animals including humans...
looks very much like a hexagonally organized sheet of graph paper
Graph paper
Graph paper, graphing paper, grid paper or millimeter paper is writing paper that is printed with fine lines making up a regular grid. The lines are often used as guides for plotting mathematical functions or experimental data and drawing diagrams. It is commonly found in mathematics and...
, 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.