Neurons doing tango
Every animal navigates in space and we humans have perfected navigation and dedicated a considerable part of our technology at its service. We can navigate in real, virtual, and information spaces. Although we often lose that certain “thread,” thousands of neurons in our brains ensure that we still reach our destination and then find our way back to our starting point.
Researchers have known for some time that the space around us is encoded by nerve cells in our brain, and from the signals, the medial temporal lobe (the hippocampus and entorhinal cortex together) creates a neural model of the environment. In this model,
the cells signal our current coordinates as that certain blue dot on the Google map.
But how this takes place is still the subject of lively debate. An international research team has recently managed to get closer to answering the question.
A large number of experiments in rodents have proven that our spatial position is indicated by the spikes of so-called place cells, special neurons in the hippocampus. These cells represent the place that the person in motion is just passing through. From a series of spikes, our brain is able to reconstruct the path we take. For this, it uses a coordinate system, which was discovered by researchers in 2005 in the entorhinal cortex adjacent to the hippocampus. A group of scientists started to explore this part of the brain.
Dozens of electrodes were implanted in the temporal lobe of patients suffering from epilepsy, including microelectrodes that made it possible to follow the activity of individual neurons. The subjects were asked to play a computer game on a tablet they held on their lap, thus performing orientation tasks in a virtual environment – with electrodes in the brain throughout the activity. This was necessary because it is difficult to give bedridden people a task that requires movement but they are just as capable of doing it mentally. With the procedure, the researchers could locate the places of epileptic seizures and test the patients’ spatial navigation and memory without stopping the recording of clinical data for a moment.
The targets of the experiments were the so-called grid cells, specific neurons in the entorhinal cortex. Grid cells are known to fire spikes at periodically repeated grid points in space. Watching the patients play, the researchers could also see that these
cells fire not only at specific points in space but also at given times.
How is it possible? How can a cell obey both rules at the same time? Researchers found themselves faced with the old question again: what creates harmony among neurons?
When the avatar in the computer game moved systematically along a line, the researchers expected that the location and timing of the firing would be synchronised to form a periodic pattern in both space and time. However, when the movement of the avatar is randomised, temporal and spatial periodicity comes into conflict unless both are synchronised by a common brain rhythm, and that rhythm is fast enough to prevent the spatial position from changing much in one of its periods. There is only one such continuous and rapid rhythm in the brain, the gamma oscillation.
Zoltán Nádasdy, the first author of an article reporting on the research, has previously suggested that
the exchange of information between nerve cells may not take place at random times
as it was previously believed. The firing pattern of cells in the brain – that is, the time and location of action potentials – can be regulated by a rhythm, the gamma oscillation. He set up an intriguing theory of phase coding that gives the accurate prediction of the speed of gamma wave propagation after recording gamma frequency. It predicts the delay of the gamma phase in the cerebral cortex or between distant parts of the brain and also indicates network properties caused by oscillation propagation. When these oscillations are unnatural, the behaviour changes, and the same happens during epileptic seizures, which is why researchers started to monitor patients suffering from epilepsy.
During the examination, the senior research fellow of the ELTE Faculty of Education and Psychology as well as his colleagues from Austin could also observe that the phase of the spikes is constant near some spatial points compared to the gamma rhythm, then, getting further away, it changes systematically and becomes constant again at a certain distance. This rhythm creates an environment-specific map over time.
tüzelési ráta = firing rate; tüzelés fázisa = firing phase; tüzelési ráta térkép = firing rate map; fázis térkép = phase map; aktuális tüzelés fázisa = actual firing phase; lehetséges tüzelés fázisa = potential firing phase; gamma mező potenciál = gamma field potential
In the image above, the colour rectangles show the phases and their spatial relationships. The points in space where the cells fire in a certain phase (marked with the same colour on the black plane in the image) show a similar pattern to the firing of the grid cells. However, while the latter captures the change in the frequency of firing, the former captures the firing phase. These cells, therefore, coordinate their firing with gamma oscillations in both time and space as if they were performing strictly choreographed dance steps. The phenomenon can be best compared to tango, where
although the dance steps are set, the leader is free to lead his partner through the dance floor.
If the neurons encode space in this way, the position of the avatar can be predicted from the phase of the spike, the researchers assumed. This is exactly what happened. The spatial position could be predicted from the phase with a precision of plus or minus one virtual metre – which is more accurate than decoding positional information from place cells and grid cells. If we can read this code so well, the neurons are also highly likely to read it.
“Such fast and precise phase coordination as we could currently demonstrate has not yet been observed in a spatial task,” says Zoltán Nádasdy. “We succeeded in doing so because we worked out how to calculate and represent the firing phase in two dimensions from the movement of the avatar.”
This scientific breakthrough not only supports the principle of phase coding but also offers an answer to the question as to what the time base for reading the code is. This is nothing but the gamma rhythm. If this is truly so, researchers have discovered another function of gamma rhythm in neural coding. The discovery not only explains how our brain encodes space but can also take us closer to curing brain disorders.
Gamma rhythm is present everywhere from arthropods to vertebrates, from insects to primates, including humans, too. The rhythm that appears in all brain types throughout evolution certainly plays a key role in coding neural information, and this will hopefully be demonstrated by further research.
Source of the image: Lightspring/Shutterstock