For more than ten years, Prof. Yitzhak Farid has been researching brain activity while performing actions related to memory, by recording the activity of nerve cells. Fried - a professor of neurosurgery at the Tel Aviv University School of Medicine and the University of California, Los Angeles, and director of the functional neurosurgery unit at the Tel Aviv Medical Center - treats severe epilepsy patients who are candidates for surgical treatment after their drug treatment has failed. He and his staff insert electrodes into the patients' brains in order to locate the epileptic focus. If the focus is located, it can be excised and thus cure the disease in many cases.
After the electrodes are inserted into the brain, the patients are hospitalized for about two weeks in the hospital where they are connected to a monitoring unit, which records their brain activity. In such a situation - which is usually only possible in animals - it is possible to record, with the patients' permission, the activity of individual nerve cells. "Our patients are awake, which allows us to give them cognitive tasks - for example, play video games - and in the process check the activity of individual nerve cells related to memory," says Fried. "The test is based on the fact that we know that the areas involved in epilepsy are also related to memory."
Two years ago Fried published an article in the journal "Nature" in which he showed that when subjects were asked to imagine different situations and then envisioned the same situations - the same nerve cells were activated in both cases. The next question that preoccupied him is related to spatial memory. What happens when people navigate their immediate environment, do they create a spatial map of the environment in their mind?
This question arose, among other things, after cells called "place cells" were discovered in the brains of rats. When you put a rat in its natural environment, let it move freely and record the activity of its brain cells, you see that when it is in different places in the brain, in the hippocampus area, different nerve cells are activated. A different cell is activated everywhere, and this is how a cognitive map of the environment is built in the rat's brain. According to Fried, this is one of the dramatic examples of the link between the activity of brain cells and complex behavior in mammals. However, he emphasizes that it is not clear to the researchers what causes the place cells to work. These cells operate according to the position in space at that moment, regardless of the visual information.
Until now, it was not known whether the place cells also work in primates and humans, in whom vision has developed impressively. It is known that in humans the hippocampus is associated with memory, but it was not clear if it is associated with spatial memory, and in what way spatial memory is organized in the neural network. "The epileptics gave us an excellent opportunity to test this," says Fried. "Our two main questions were, do humans also have place cells that create a map of the environment, and if so, are there also other cells that play a role when navigating the environment."
Fried, in collaboration with Prof. Michael Kahane, a researcher in the field of spatial navigation from the University of California, Los Angeles, and research student Arna Ekstrom, asked the epileptics to play a video game in which the subject is a taxi driver traveling through a virtual city consisting of several streets where there are shops. The driver does not know the city; He gets to know her when he picks up passengers who appear in random places in a taxi, and drives them to different shops.
The researchers monitored the activity of the nerve cells in the brains of the patients during the game. They found three groups of cells that functioned during navigation. In the first group there were place cells: in different places where the "taxi driver" arrived, different place cells were activated, which created a kind of spatial map of the city in the brain of the subject. This complex system has therefore also been preserved in humans.
But in addition to the place cells, the researchers found in the subjects' brains two additional systems that help with navigation. One system consists of cells that respond to the visual information during navigation. The researchers called these cells "gaze cells". No cells of this type were found in studies in rats. According to Fried, there is no research evidence that the rats use visual information for spatial orientation.
The third group of cells responded to the target of the navigator. As long as the driver was looking for a certain "address", and this was his final goal for that round, activity was recorded in certain cells. As soon as he moved to another address, other cells responded. The researchers called this group "target cells". Spatial orientation therefore creates a response in the brain that relates to where you are, what you see and what you are looking for. Mainly place cells were found in the hippocampus region. In an area close to it, the pre-hippocampus, mainly gaze cells were found. The target cells were not found concentrated in unique areas. Many of them were found in the frontal lobe, which is related among other things to planning.
"These findings allow a first look at the cellular code that people use to create spatial maps of their environment and to navigate from place to place," says Fried. "Damage to these cell groups can cause people to lose the ability to navigate in space and remember new places." The study was published late last week in the journal." "The information revealed in Nature about the functioning of the human brain may open new research directions in the treatment of memory-related degenerative brain diseases such as Alzheimer's disease.
The brain savant
