Movement is life. This phrase can be interpreted both as a motivation to move forward, not to stand still and achieve what you want, and as a statement of the fact that almost all living beings are in motion most of their lives. In order to ensure that our movements and movements in space do not end with bumps on our foreheads and broken little fingers on our legs each time, our brain uses the saved “maps” of the environment that unconsciously pop up at the moment of our movement. However, there is an opinion that the brain does not apply these cards from the outside, so to speak, but by placing a person on this card and collecting data when viewed from the first person. Scientists from Boston University decided to prove this theory by conducting a series of practical experiments with laboratory rats. How does the brain actually navigate in space, which cells are involved, and what role does this research play for the future of autonomous cars and robots? We learn about this from the report of the research group. Go.
Research basis
So, the fact established many years ago is that the main part of the brain responsible for orientation in space is the hippocampus.
The hippocampus is involved in a variety of processes: the formation of emotions, the transformation of short-term memory into long-term memory, and the formation of spatial memory. It is the latter that is the source of the very “maps” that our brain calls at the right moment for more efficient orientation in space. In other words, the hippocampus stores three-dimensional neural models of the space inside which the owner of the brain is located.
Hippocampus
There is a theory stating that there is an intermediate step between actual navigation and maps from the hippocampus - the transformation of these maps into a first-person view. That is, a person is trying to understand where something is located not at all (as we see on real maps), but where something will be located relative to him (like the “street view” function in Google Maps).
The authors of the work we are considering emphasize the following: Cognitive maps of the environment are encoded in the hippocampal formation in the allocentric system, but motor skills (the movements themselves) are represented in the egocentric system.
UFO: Enemy Unknown (allocentric system) and DOOM (egocentric system).
The difference between allocentric and egocentric systems is like the difference between third person games (or side view, top view, etc.) and first person games. In the first case, the environment itself is important for us, in the second, our position in relation to this environment. Thus, allocentric navigational plans must be converted into an egocentric system for actual implementation, i.e. movement in space.
Researchers believe that it is the dorsomedial striatum (DMS)* plays an important role in the above process.
The striatum of the human brain.
Striatum* - part of the brain that belongs to the basal ganglia; the striatum is involved in the regulation of muscle tone, internal organs and behavioral responses; The striatum is also called the "striatum" because of its structure of alternating bands of gray and white matter.
The DMS demonstrates the neural responses associated with decision making and action in relation to spatial navigation, so this region of the brain should be studied in more detail.
Results of the study
In order to determine the presence/absence of egocentric spatial information in the striatum (DMS), 4 male rats were implanted with up to 16 tetrodes (special electrodes connected to the desired areas of the brain) targeting DMS (1a).
Image #1: Striatal cell response to environmental boundaries in an egocentric frame of reference.
Explanations for image #1:а - points of location of tetrodes;
b - egocentric map of boundaries;
с — allocentric spatial maps (4 squares on the left), color-coded trajectory plots of the locations of cell response peaks relative to body position, and egocentric maps (4 squares on the right) based on the response of EBC cells at various orientations and distances between the rat and the wall;
d - as on 1s, but for EBC with preferred distances away from the animal;
e - as on 1s, but for two inverse EBCs;
f — distribution of the average resulting length for the observed cells;
g - the distribution of the average resulting length for EBC using the direction of movement and the direction of the head;
h — distribution of the average response of cells (total and EBC).
Forty-four experiments were conducted when rats collected randomly scattered food in a space familiar to them (open, not in a maze). As a result, 44 cells were recorded. From the collected data, the presence of 939 head direction cells (HDCs) was established, however, only a small part of the cells, and more precisely 31, had allocentric spatial correlates. At the same time, the activity of these cells, limited by the perimeter of the environment, was observed only during the movement of the rat along the walls of the test chamber, which suggests an egocentric scheme for encoding the boundaries of space.
To assess the possibilities of such an egocentric representation, based on peak cell activity indicators, egocentric boundary maps were created (1b), which illustrate the orientation and distance of the borders relative to the direction of the rat's movement, and not the position of its head (comparison to 1g).
18% of the captured cells (171 out of 939) showed a significant response when the chamber boundary occupied a certain position and orientation relative to the subject (1f). Scientists called them egocentric boundary cells (EBCs). egocentric boundary cells). The number of such cells in the experimental subjects ranged from 15 to 70 with an average of 42.75 (1c, 1d).
Among the cells of the egocentric boundaries, there were those whose activity decreased in response to the boundaries of the chamber. There were 49 in total and they were called inverse EBCs (iEBCs). The average index of cell response (their action potential) in EBC and iEBC was quite low - 1,26 ± 0,09 Hz (1h).
The EBC population responds to all orientations and positions of the chamber boundary relative to the subject, but the distribution of preferred orientation is bimodal with peaks located 180° opposite each other on either side of the animal (-68° and 112°), being slightly offset from perpendicular to the long axis of the animal by 22° (2d).
Image #2: Preferred orientation and spacing for egocentric boundary cell (EBC) response.
Explanations for image #2:a - egocentric boundary maps for four simultaneously studied EBCs with different preferred orientations indicated above each graph;
b - the position of the tetrodes in accordance with the cells from 2a (the numbers indicate the tetrode number);
с — probability distribution of preferred orientations for all EBCs of one rat;
d — probability distribution of preferred orientations for EBC of all rats;
е - the position of the tetrodes for the cells shown in 2f;
f — egocentric boundary maps for six simultaneously recorded EBCs with different preferred distances indicated above each graph;
g is the probability distribution of the preferred distance for all EBCs of one rat;
h is the probability distribution of the preferred distance for EBC of all rats;
i - polar plot of preferred distance and preferred orientation for all EBCs with space size represented by color and dot diameter.
The distribution of the preferred distance to the boundary contained three peaks: 6.4, 13.5 and 25.6 cm, indicating the presence of three different preferred distances between EBCs (2f—2h) that may be important for a hierarchical navigational search strategy. The size of the EBC receptive fields increased with the preferred distance (2i), indicating an increase in the accuracy of the egocentric representation of boundaries as the distance between the wall and the subject decreases.
There was no clear topography in both the preferred orientation and the distance, as the subject's active EBCs with different orientations and distances from the wall appeared on the same tetrode (2a, 2b, 2e и 2f).
It was also found that EBC stably respond to the boundaries of space (chamber walls) in any test chambers. To confirm that the EBCs are responding to the local boundaries of the chamber rather than its distal features, the scientists “rotated” the camera position by 45° and made several walls black, making it different from those used in previous tests.
Data were collected both in a conventional test chamber and in a rotated one. Despite the change in the test chamber, all preferred orientations and distances relative to the walls of the EBC test subjects remained the same.
Given the importance of angles, the possibility that EBCs uniquely encode these local environmental attributes was also considered. By isolating the difference between the response near the corners and the response near the middle of the wall, a subset of EBC cells (n = 16; 9,4%) were identified that show an increased response to the corners.
Thus, we can make an intermediate conclusion that it is the EBC cells that respond perfectly to the perimeter of the chamber, that is, to the walls of the test chamber and to its corners.
Next, the scientists tested whether the response of EBC cells to an open space (a test arena without a maze, i.e. just 4 walls) is the same for different test room sizes. Three visits were made, in each of which the length of the walls differed from the previous ones by 3 cm.
Regardless of the size of the test chamber, EBC responded to its boundaries at the same distance and orientation relative to the test subject. This indicates that the response does not scale with the size of the environment.
Image #3: stable response of EBC cells to space boundaries.
Explanations for image #3:а — egocentric EBC maps under normal conditions (left) and when the test chamber was rotated by 45° (right);
b — egocentric EBC maps for a chamber measuring 1.25 x 1.25 m (left) and for an enlarged chamber 1.75 x 1.75 m (right);
с — egocentric EBC maps with ordinary black chamber walls (left) and with patterned walls (right);
d—f - graphs of the preferred distance (top) and changes in the preferred orientation relative to the baseline (bottom).
Since the striatum receives information about the environment from several areas of the visual cortex, the scientists also tested whether the appearance of the walls is affected (3s) chambers for the reaction of EBC cells.
Changing the appearance of the boundaries of space had no effect on the reaction of the EBC cells and the distance and orientation required for the reaction relative to the subject.
Image #4: Stability of EBC cell response regardless of the environment.
Explanations for image #4:а — egocentric maps for EBC in familiar (left) and new (right) environments;
b - egocentric maps for EBC obtained in the same environment, but with a time interval;
с - Graphs of preferred distance (top) and change of preferred orientation relative to the baseline (bottom) for new (unfamiliar) environments;
d - graphs of the preferred distance (top) and the change in preferred orientation relative to the baseline (bottom) for previously studied (familiar) environments.
It was also found that the response of EBC cells, as well as the required orientation and distance relative to the subject, do not change over time.
However, this "temporary" test was conducted in the same test chamber. It was also necessary to check what is the difference between the reaction of the EBC to known conditions and to new ones. To do this, several visits were carried out, when the rats studied the chamber, which they already know from previous tests, and then new chambers with open space.
As you may have guessed, the response of the EBC cells + desired orientation/distance remained unchanged in the new chambers (4a, 4c).
Thus, the EBC reaction provides a stable representation of the boundaries of the environment relative to the test subject in all types of this environment, regardless of the appearance of the walls, the area of the test chamber, its movement, and the time spent by the test subject in the chamber.
For a more detailed acquaintance with the nuances of the study, I recommend looking at и to him.
Finale
In this work, scientists managed to confirm in practice the theory of the egocentric representation of the environment, which is extremely important for orientation in space. They proved that there is an intermediate process between allocentric spatial representation and actual action, in which certain cells of the striatum, called egocentric boundary cells (EBCs), participate. It was also found that EBCs were more related to the control of movement of the entire body, and not just the head of the subjects.
This study was aimed at determining the complete mechanism of orientation in space, all its components and variables. This work, according to scientists, will further help improve navigation technologies for autonomous cars and for robots that can understand the space around them, as we do. The researchers are extremely excited about the results of their work, which give reason to continue to study the relationship between certain areas of the brain and how space is navigated.
Thank you for your attention, stay curious and have a great week everyone! 🙂
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Source: habr.com