When we move, we have a sense of the distance we have already covered. This also works if the movement is only virtual, i.e. we only have visual cues. But what happens in our brain? A study now shows that certain slow brain waves indicate when we reach a given distance. These have previously been associated with spatial perception. The research team was also able to confirm that brain activity is lower in self-controlled movements than in observed movements.
It doesn’t matter whether we move under our own power or just watch the surroundings pass us by: We have an intuitive feeling of the approximate distance we have covered. Behavioral experiments have shown that it is enough if we see on a screen how a poorly designed floor panel is moving. In corresponding experiments, the test subjects were able to actively reproduce the observed distance by controlling the movement with a joystick. However, what is going on in her brain was not clear until now.
On a virtual journey
A team led by Constanze Schmitt from the Philipps University of Marburg has now dealt with this. To do this, the researchers first showed 15 subjects on a screen how a floor panel moves under a point fixed in the center of the screen. They then asked the test persons to control the movement themselves with the help of a joystick and to cover exactly twice the previously seen distance. The people could set the pace themselves by tilting the joystick. During all experiments, Schmitt and her team measured the subjects’ brain activity using an electroencephalogram (EEG).
“The aim of our study was to find out whether the point in time at which the test subjects completed the first half of their self-controlled journey, i.e. the distance originally shown, is accompanied by specific brain activity,” explains Schmitt. The fact that the subjects did not have to stop exactly after the originally shown distance, as in other experiments, but were supposed to estimate twice the distance was a trick used by the researchers: “In this way, we were able to observe the neuronal signature at the single distance without the subjects having to go to the move their hand at the same time to end the virtual forward drive.” Motor activity would also have shown up in the EEG and the signal would have been falsified.
Brain waves as a “stop signal”
Each subject went through several experiments in which the researchers varied the length of the route and the speed of movement shown. In line with previous studies, it was found that the subjects tended to overestimate the distance shown, i.e. to drive a little too far in the reproduction task – especially the shortest distance. The researchers also observed that almost all test subjects adjusted their speed to that shown previously, thus including a time component in their estimation.
The EEG showed that certain signals, the so-called event-related potentials, decreased as soon as the subjects began the self-determined virtual movement. “This finding was expected,” says Schmitt. “It corresponds to the idea that stimuli that you generate yourself and can therefore predict are processed less intensively in the brain than unexpected stimuli.” In addition, the researchers actually found a brief increase in the brain activity measured in the EEG when the test subjects reached the simple distance had. This activation primarily affected the theta band, i.e. low-frequency brain waves. These occur, among other things, during sleep and deep relaxation, but have also been linked to spatial perception in earlier studies.
Subjective Estimate
The current study suggests that theta waves do play a role in estimating distances and represent a neural correlate of reaching the target distance. “Another question was whether this is an objective or subjective measure,” explains Schmitt’s colleague Frank Bremmer. In other words, does activation occur when subjects have traveled the given one-way distance, or when they are halfway through their estimated distance?
The results show that theta-wave activity is related to the subjectively estimated distance, not the objectively given one. “The activation found was very precise in terms of timing,” says Bremmer. “However, we have not yet been able to say which areas of the brain are responsible for them.” The research team wants to pursue this question in future studies.
Source: Constanze Schmitt (Philipps University of Marburg) et al., eNeuro, doi: 10.1523/ENEURO.0137-21.2022