Coupling Eye Movements and Postural Sway Significantly Enhances Stability in Parkinson Disease

Fabio A. Barbieri, PhD

(Credit: MOVI-LAB UNESP)

A recent study conducted by researchers at São Paulo State University (UNESP) in Brazil and the University of Lille in France showed that coupling eye movements and postural sway when performing horizontal saccadic eye movements improved body sway in patients with Parkinson disease (PD) on dopaminergic medication compared with performing vertical saccadic eye movements. Published in Biomechanics, these findings suggest that incorporating saccadic eye movements into postural training could be an effective intervention strategy for patients with PD.¹ 

In the study, results revealed that horizontal saccadic eye movements significantly reduced body sway for both healthy participants (n = 11) and those with PD (n = 10) rather than gaze fixation, regardless of side-by-side or tandem position of the feet. Notably, vertical saccadic eye movements increased body sway in contrast to horizontal ones, and not to fixation, for patients with PD. Additionally, patients with PD showed a higher number of fixations in all experimental conditions, without changes in the mean duration of fixations in both body positions.

“It’s not an automatic strategy to avoid losing balance and falling, since routinely moving the eyes rapidly from side to side is difficult. You can train horizontal saccadic eye movement, but this study was aimed at basic research rather than practical recommendations,” lead author Fabio A. Barbieri, PhD, associate professor and head of the Human Movement Research Laboratory (MOVI-LAB) at São Paulo State University (UNESP), said in a statement.² “We didn’t expect people with Parkinson’s to be capable of combining these two movements. Our findings furnish new knowledge of the disease, and of its motor and cognitive consequences.”

The study involved participants over 60 years old who could stand independently during postural tasks. Each participant completed 12 trials, which consisted of 2 postural tasks—standing in a side-by-side stance and a tandem stance—under 3 randomized visual conditions: fixation on a central target, horizontal saccadic eye movements (tracking side-to-side), and vertical saccadic eye movements (tracking up and down). Each trial lasted 70 seconds, with 1-minute rest periods after every 3 trials to prevent fatigue. This structured approach ensured consistency and reliability in assessing the effects of eye movement conditions on postural stability.

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“Individuals with this disease have a postural deficit that impairs stability and sway control. They also have difficulty controlling their eye movements. Their blinking tends to be slow, and it’s hard for them to pick information from the environment,” Barbieri said in a statement.²

The results of the study revealed several significant effects related to postural sway and eye movement tasks. First, a significant group effect was observed for anterior-posterior (AP) displacement, mean velocity of sway, root mean square (RMS), and area of sway, all of which were higher in the PD group compared with the control group. Specifically, the PD group exhibited greater AP displacement, mean velocity of sway, RMS, and area of sway (P <.001, η² values ranging from 0.35 to 0.58). Additionally, there was a significant effect of the visual task on AP displacement and RMS (P <.005, η² = 0.24-0.25), with both groups showing reduced AP displacement and RMS during horizontal saccadic eye movements compared with the fixation condition.

Another notable finding was the effect of base of support on postural sway. A significant effect was found for medial-lateral (ML) displacement, AP and ML mean velocity of sway, RMS, and area of sway, with the tandem stance resulting in greater levels of sway compared to the side-by-side stance for both groups (P < 0.001, η² values ranging from 0.48-0.78). Furthermore, the PD group demonstrated a greater AP mean velocity of sway during the tandem stance compared to the side-by-side stance (P < 0.007, η² = 0.33), a difference that was not observed in the control group.

“For young adults, vertical saccadic eye movements are beneficial, but for older adults they don’t reduce body sway. Although Parkinson’s patients can make vertical saccadic eye movements, they can’t adapt [by reducing sway] as well as they can to horizontal saccadic eye movements because up-and-down movements are more difficult in general and involve a lower degree of eye rotation, hindering integration between the sensory system [i.e. the eyes] and the postural system [i.e. the body],” senior author Sérgio Tosi Rodrigues, PhD, head of the Laboratory of Information, Vision and Action (LIVIA) at UNESP’s Department of Physical Education in Bauru, where he also serves as a full professor, said in a statement.²

Interactions between group and visual task were also identified, particularly for the area of sway. For the PD group, the area of sway was significantly higher when performing vertical saccadic eye movements compared to horizontal saccadic movements (P <.005), though there was no significant difference between the fixation and vertical saccadic conditions (P = 0.124). Regarding eye movements, the PD group showed a higher number of fixations and shorter fixation durations compared to controls (P <.001, η² = 0.59 for number of fixations; P < 0.03, η² = 0.23 for duration). Both groups exhibited fewer fixations during vertical saccadic eye movements compared to horizontal saccadic movements (P <.05, η² = 0.21). Additionally, the tandem stance resulted in a higher number of fixations compared to the side-by-side stance for both groups (P <.02, η² = 0.26).

“Besides limitations due to a disease, such as Parkinson, control of gaze and posture seems to vary with age. Generally speaking, the natural aging process leads to a deterioration of motor control and visual perception, among other alterations. Visual functions perform less well in older than younger people, for example, potentially making older people more susceptible to falls,” Tosi Rodrigues said in a statement.²

“Combining static balance with vertical saccadic eye movements may be difficult for Parkinson’s patients, increasing body sway as a result,” Barbieri said in a statement.² “This happens because postural control is backgrounded. To try to simplify what’s complex, there’s a change in attention. The subject shifts attention from posture to focus on eye movement, requiring more caution from the brain in order to control posture and stability so as to avoid a fall.”

REFERENCES
1. Barbieri FA, Polastri PF, Barela JA, Bonnet CT, Brito MB, Rodrigues ST. People with Parkinson’s Disease Are Able to Couple Eye Movements and Postural Sway to Improve Stability. Biomechanics. 2024; 4(3):460-472. https://doi.org/10.3390/biomechanics4030032
2. Rapid horizontal eye movement can improve stability in people with Parkinson’s. News Release. Agência FAPESP. Published October 30, 2024. Accessed November 21, 2024. https://agencia.fapesp.br/rapid-horizontal-eye-movement-can-improve-stability-in-people-with-parkinsons/53155