When walking, we not only have to put one foot in front of the other, but we also have to keep an eye on our path. A study now shows how our visual perception adapts to our steps. Using VR glasses, researchers had their test subjects walk through a virtual environment in which short visual stimuli flashed repeatedly. Depending on which phase of the movement the participants were in, they perceived it better or worse. This suggests that vision is subordinated to motor control in the initial phase of each step.
Movement influences our perception and our attention. Several studies have already proven this in different contexts. The result has usually been that we are more alert during or immediately after physical activity. However, how our visual perception and our motor control interact at the finest level was previously unclear.
Walking in virtual reality
A team led by Matthew Davidson from the University of Sydney has now addressed this question using virtual reality. “Our work reveals a previously unknown relationship between perception and movement,” says Davidson. “We found that walking brings about rhythmic changes in perceptual performance within each step cycle, even though this everyday behavior seems continuous and effortless to us.” The team had already shown in previous studies that our brain does not perceive our visual environment continuously, but rather captures individual images several times per second, which it combines into a seamless experience like a film. Studies on the interaction between image and sound came to rates of around eight images per second.
For the current experiment, the researchers equipped 45 test subjects with VR glasses and had them walk along a virtual path. The participants carried out the steps in reality at a self-selected, natural pace. Meanwhile, visual stimuli repeatedly flashed on the VR display for 20 milliseconds. Whenever the test subjects perceived such a stimulus, they were asked to press a hand-held button. Davidson and his colleagues tracked the participants’ head and eye movements and evaluated in which movement phases particularly many or particularly few stimuli were recognized.
Perception in the rhythm of the steps
The result: “Thanks to VR technology, we discovered that our vision goes through a good and a bad phase with every step,” says Davidson. During the start phase of each step, the test subjects often overlooked the flashing stimuli, whereas in the swing phase of their steps they noticed most of the stimuli and reported them by pressing a button.
“The key new finding of this study is that these oscillations in the brain’s sampling of the world slow down during walking to match the step cycle,” says Davidson’s colleague David Alais. “Humans take about two steps per second when walking and generally maintain a steady rhythm. The reported oscillations in visual sensitivity also occur at approximately two cycles per second and are tied to the step cycle. For some participants, these rhythmic oscillations occurred at four cycles per second, but these too were tied to the step cycle.”
Switch between seeing and moving
But why do we perceive our environment more slowly when walking, which requires constant monitoring of the terrain? “One possible explanation is that vision becomes subordinate to motor control while the foot is on the ground and the next step is being planned,” explains Alais. “As soon as you are in the swing phase between steps, the brain switches back to the primary perception of the world and creates a continuous perceptual rhythm that harmonizes with the step frequency.”
In further studies, the team would now like to clarify whether our perception of other stimuli, including sounds and touch, also changes when walking. “An obvious question is whether these perceptual oscillations are more pronounced in older people, since we have difficulty with balance and coordination as we age,” says Davidson. Tests with VR glasses could possibly even help to identify neuro-muscular disorders at an early stage.
Source: Matthew Davidson (University of Sydney, Australia) et al., Nature Communications, doi:10.1038/s41467-024-45780-4