Balance control is more affected in older than younger adults by repeated visual perturbations during walking

To stay upright while walking, the human body relies on a constant, silent conversation between the senses. The brain combines what the eyes see, what the inner ear feels regarding motion and orientation, and what the muscles and skin report about the body's position. This sensory blend allows us to estimate where our center of mass is and where our feet need to land to keep us from falling. As people age, the sensors themselves often degrade; the inner ear becomes less sensitive, and the body's ability to process these signals slows down. Consequently, older adults often rely more heavily on vision to maintain their balance. However, vision can sometimes be a trickster. When the visual world moves in a way that contradicts the body's actual motion, it can create an illusion of falling or drifting, forcing the brain to make rapid, often unnecessary, corrections that can actually destabilize a person. Understanding how the aging brain handles these visual tricks is crucial, not just for understanding human movement, but for figuring out why falls become more common later in life.

Researchers at the Vrije Universiteit Amsterdam and the University of Antwerp set out to test how older and younger adults react when their visual world is deliberately confused while they walk. They recruited healthy younger adults, averaging 23 years old, and healthy older adults, averaging 75 years old. All participants walked on a treadmill at a steady pace while wearing a safety harness. The researchers projected a visual scene onto a large screen in front of them and introduced three different types of visual disturbances. In one scenario, the background stripes moved to the right while the participants were told to stare at a fixed red dot in the center. In a second scenario, the background stayed still, but the red dot moved to the right, and the participants were asked to track it with their heads. In the third scenario, the dot moved to the right again, but this time the participants had to keep their heads perfectly still and track the dot only with their eyes. These visual tricks were repeated about fourteen times during each walking session to see if the participants could learn to ignore the confusing signals.

The study measured how much the participants' bodies swayed from side to side, known as the center of mass, and how their feet landed. The researchers had expected that older adults would sway more overall and would be slower to adapt to the repeated visual tricks. Surprisingly, when looking at the entire walking session, the older adults did not show greater overall instability than the younger group. However, the story changed when the researchers looked closely at the moments the visual tricks were happening. In the condition where the background moved while the target stayed still, the younger adults consistently swayed to the left, the opposite direction of the moving background, as if their brains were tricked into thinking they were falling that way. The older adults, however, did not show this consistent reaction. Instead, after a few repetitions of the trick, the older adults began to sway in the opposite direction, moving to the right, the same way the background was moving. This suggests that the older adults' brains may have lost focus on the fixed target and started tracking the moving background instead, or they began using a different, faster strategy to correct their balance that involved their standing leg rather than their foot placement.

When the participants had to track a moving target, the differences became even more pronounced. In the condition where they tracked a moving dot with their heads, the older adults began to sway to the right much earlier than the younger adults and swayed much more intensely. Their bodies moved in the direction of the target, but the magnitude of this movement was roughly eight times larger during the initial movement phase and nearly four times larger during the stationary phase compared to the younger group. This indicates that when the head moves, the older adults struggle to separate the motion of their own body from the motion of the visual world, leading to a much stronger, and potentially dangerous, reaction. In the condition where they tracked the target with only their eyes, both groups swayed in the direction of the target, but the older adults started reacting sooner.

Perhaps the most revealing finding concerned how the participants changed their behavior over time. When the visual tricks were repeated, the younger adults generally maintained a consistent reaction. The older adults, however, showed a remarkable ability to learn. In the eye-tracking condition, their swaying decreased significantly as the repetitions continued, suggesting they were successfully learning to ignore the confusing visual signal and rely on other senses. This ability to "downweight" or reduce the influence of unreliable visual information remained intact in older age. Yet, this learning did not happen when they had to move their heads. In that scenario, neither group showed a reduction in their reaction, likely because moving the head also disturbs the inner ear and neck sensors, making it too difficult for the brain to figure out which signals to trust.

The study concludes that while older adults retain the capacity to learn from repeated visual confusion and adjust their balance strategies, their initial reactions are often larger and occur sooner than those of younger adults. This suggests that the primary issue is not a total loss of balance control, but rather a difficulty in accurately estimating self-motion when visual information conflicts with other senses. The older brain seems to have a harder time filtering out the visual noise, especially when the head is moving, leading to exaggerated corrections that could increase the risk of a fall in real-world situations where the visual environment is constantly shifting.