Do not get lost in three pines: an egocentric representation of the environment

Movement is life. This phrase can be interpreted as a motivation to move forward, not to stand still and achieve what is desired, and as a statement of the fact that almost all living beings are in motion for most of their lives. To ensure that our movements and movements in space each time do not end with bumps on our forehead and broken little fingers on our feet, our brain uses 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 use these cards from the outside, so to speak, but by placing a person on this card and collecting data from a first-person perspective. Scientists from Boston University decided to prove this theory by conducting a series of practical experiments with laboratory rats. So how does the brain actually navigate in space, which cells are involved, and what role does this study play for the future of autonomous cars and robots? We learn about this from the report of the research group. Go.

Study basis

So, the fact established many years ago is that the main part of the brain responsible for spatial orientation 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 and the formation of spatial memory. It is the latter that is the source of the very “maps” that our brain invokes at the right time for more effective 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 claiming that between the actual navigation and the hippocampal maps there is an intermediate stage - the conversion of these maps into a first-person view. That is, a person is trying to understand where what is located not at all (as we see on real maps), but where what will be located relative to himself (as a function of "street view" in Google Maps).

The authors of the work under consideration emphasize the following: Cognitive maps of the environment are encoded in the hippocampal formation in the allocentric system, but motor skills (movements themselves) are represented in the egocentric system.


UFO: Enemy Unknown (allocentric system) and DOOM (egocentric system).

The difference between allocentric and egocentric systems resembles the difference between third-person games (or from a side view, top, etc.) and games with a first-person view. In the first case, the environment itself is important for us, in the second - our position regarding this environment. Thus, allocentric navigation plans should be transformed into an egocentric system for actual implementation, i.e. moving in space.

Researchers believe that it is the dorsomedial striatum (DMS) * that plays a critical role in the above process.


The striatum of the human brain.

Striatum * - part of the brain that refers to the basal nuclei; striatum is involved in the regulation of muscle tone, internal organs and behavioral reactions; the striatum is also called the "striatum" due to its structure of alternating bands of gray and white matter.

DMS demonstrates neural responses related to making decisions and taking actions regarding navigation in space, so this area of ​​the brain should be studied in more detail.

Research results

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 parts of the brain) aimed at DMS ( 1a ).


Image No. 1: the reaction of striatum cells to environmental boundaries in an egocentric frame of reference.


44 experiments were conducted when rats collected randomly scattered food in a familiar space (open, not in the maze). As a result, 939 cells were fixed. From the collected data, the presence of 31 head direction cells (HDC) was found, but only a small part of the cells, or rather 19, had allocentric spatial correlates. Moreover, 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 coding scheme for the boundaries of space.

To assess the possibilities of such an egocentric presentation, on the basis of peak cell activity indicators, egocentric border maps ( 1b ) were created that illustrate the orientation and distance of the borders relative to the direction of the rat’s movement, rather than its head position ( 1g comparison).

18% of the fixed cells (171 out of 939) showed a significant response when the boundary of the chamber occupied a certain position and orientation relative to the experimental one ( 1f ). Scientists called them cells of egocentric boundary cells (EBC). The number of such cells in experimental subjects ranged from 15 to 70 with an average of 42.75 ( 1c , 1d ).

Among the cells of egocentric borders, there were those whose activity decreased in response to the boundaries of the chamber. There were a total of 49 and called them reverse EBC (iEBC). The average rate of cell response (their action potential) in EBC and iEBC was quite low - 1.26 ± 0.09 Hz ( 1h ).

The EBC cell population responds to all orientations and positions of the chamber boundary relative to the subject, but the preferred orientation distribution is bimodal with peaks located 180 ° opposite to each other on both sides of the animal (-68 ° and 112 °), being slightly offset from the perpendicular to the long axis of the animal at 22 ° ( 2d ).


Image No. 2: preferred orientation and distance for the response of egocentric cell borders (EBC).


The distribution of the preferred distance to the border contained three peaks: 6.4, 13.5, and 25.6 cm, indicating the presence of three different preferred distances between EBC ( 2f - 2h ), which may be important for the hierarchical navigation search strategy. The size of the receptive fields of the EBC increased depending on the preferred distance ( 2i ), which indicates an increase in the accuracy of the egocentric representation of the boundaries with a decrease in the distance between the wall and the subject.

Both in the preferred orientation and in the distance, there was no clear topography, since active experimental EBCs with different orientations and distances relative to the wall appeared on the same tetrode ( 2a , 2b , 2e, and 2f ).

It was also revealed that EBCs stably respond to the boundaries of space (chamber walls) in any version of test chambers. To confirm that the EBCs respond to the local boundaries of the camera, and not to its distal features, the scientists “turned” the camera position by 45 ° and made several walls black, making it different from that used in previous tests.

Data was collected both in a conventional test camera and in a rotated one. Despite the change in the test chamber, all the preferred orientations and distances relative to the walls of the EBC 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 reaction near the corners and the reaction near the middle of the wall, a subset of EBC cells (n = 16; 9.4%), which exhibit an increased reaction to the corners, were isolated.

Thus, we can make an intermediate conclusion that it is EBC cells that perfectly respond to the perimeter of the camera, that is, to the walls of the test chamber and to its corners.

Then the scientists checked whether the reaction of the EBC cells to the open space (the test arena without a maze, i.e. just 4 walls) is the same with different options for the area of ​​the test room. There were 3 visits, in each of which the length of the walls differed from the previous ones by 50 cm.

Regardless of the size of the test chamber, EBC reacted to its borders at the same distance and orientation relative to the subject. This indicates a lack of reaction scaling depending on the size of the environment.


Image 3: Stable response of EBC cells to the boundaries of space.


Since the striatum receives environmental information from several areas of the visual cortex, scientists also checked whether the appearance of the walls ( 3c ) of the chamber affects the response of EBC cells.

The change in the appearance of the boundaries of space did not affect the reaction of EBC cells and the distance and orientation necessary for the reaction relative to the experimental one.


Image No. 4: EBC cell response stability regardless of environment.


It was also found that the reaction of EBC cells, as well as the necessary orientation and distance relative to the experimental one, do not change over time.

However, this “temporary” test was carried out in the same test chamber. It was also necessary to check the difference between the reaction of EBC to known conditions and new ones. For this, several visits were made when the rats studied the camera, which they already know from previous tests, and then new cameras with open space.

As you may have guessed, the reaction of the EBC cells + the 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 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 subject in the chamber.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue

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 between the allocentric spatial representation and the actual action there is an intermediate process in which certain cells of the striatum, called the cells of egocentric borders (EBC), are involved. It was also found that EBC is more associated with controlling the movement of the whole body, and not just the heads 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. Researchers are extremely pleased with the results of their work, which give reason to continue to study the relationship between certain parts of the brain and how navigation is carried out in space.

Thank you for your attention, remain curious and have a good working week, guys! :)

Thank you for staying with us. Do you like our articles? Want to see more interesting materials? Support us by placing an order or recommending it to your friends, a 30% discount for Habr users on a unique analogue of entry-level servers that was invented by us for you: The whole truth about VPS (KVM) E5-2650 v4 (6 Cores) 10GB DDR4 240GB SSD 1Gbps from $ 20 or how to divide the server? (options are available with RAID1 and RAID10, up to 24 cores and up to 40GB DDR4).

Dell R730xd 2 times cheaper? Only we have 2 x Intel TetraDeca-Core Xeon 2x E5-2697v3 2.6GHz 14C 64GB DDR4 4x960GB SSD 1Gbps 100 TV from $ 199 in the Netherlands! Dell R420 - 2x E5-2430 2.2Ghz 6C 128GB DDR3 2x960GB SSD 1Gbps 100TB - from $ 99! Read about How to Build Infrastructure Bldg. class c using Dell R730xd E5-2650 v4 servers costing 9,000 euros for a penny?