Mentally Overestimating Motor Tasks May Cause Cognitive Fatigue in MS

People with multiple sclerosis (MS) tend to mentally overestimate the time required to complete a short walking activity, causing cognitive fatigue that may affect their quality of life, a study reports.

The connection between cognitive fatigue and imagined motor exercises may offer a potential therapeutic approach to improve motor function and quality of life in MS patients, the researchers said. 

The study, “Spatial constraints and cognitive fatigue affect motor imagery of walking in people with multiple sclerosis,” was published in the journal Nature Scientific Reports.

Motor imagery (MI) is the process of mentally simulating or imagining a motor activity like walking, without physically doing the activity. Previous research suggests that the brain functions required to imagine performing a motor activity are similar to actually doing it. In healthy adults, these real and imagined activities are believed to have a temporal coupling — meaning, a similar duration between actual and mental movements.

In MS patients, however, a degree of incongruence is observed between imagined and actual motor tasks, resulting from abnormal brain activity that may indicate extra mental exertion to perform physical tasks.

MI therapy has been shown to improve walking and limit fatigue in people with MS. However, to what degree patients exhibit discrepancies between imagined and physical tasks, and whether such discrepancies affect cognitive fatigue or quality of life has yet to be determined.

“Investigating the relationship between fatigue and MI performances in [patients with] MS could be key to shedding new light on the motor representation of [MS patients] and providing insights into new rehabilitative treatments,” the investigators wrote.

“Thus, to shed light on this aspect, we asked [patients with MS] to actually and mentally walk along three paths with different widths and we compared their performances to those of [healthy subjects],”  the team wrote.

In total, 15 MS patients were recruited from an outpatient rehabilitation center in Genoa, Italy, along with 15 age-matched healthy people (controls). The MS patients had a mean age of 42.67 years, with no relapses in the previous three months and the ability to walk with no more than one aid; the healthy controls had a mean age of 41.60 years.

All of the participants were asked to first imagine walking, then actually walk — or vice versa — down three 5-meter (16.4-foot) paths. The paths had widths of 20 cm (7.9 inches), 35 cm (13.8 inches), or 50 cm (19.7 inches).

Participants performed 12 walking movements for each path width in each experimental condition (actual and mental) for a total of 72 movements distributed over two different sessions separated by a time interval of two days. The experiments were performed with a five-minute break between the physical and mental tasks. Tasks were timed from the “Go” signal to the participant physically crossing the finish line or mentally crossing the finish line and saying “Stop.”

A good MI performance indicates the participants’ ability to accurately estimate the duration of the imagined activity compared with the actual duration of the physical activity. The results showed that both actual and mental task completion were slower in MS patients than in healthy controls.

While controls showed good MI performance, MS patients mentally overestimated the duration of the task, meaning they completed the imagined task significantly slower than the physical task on all path widths tested. Interestingly, actual task completion was significantly slower in MS patients on the narrowest (20 cm) path, suggesting that the spatial restriction affected their ability to walk down the path.

In MS patients, worse MI performance correlated significantly with higher reported cognitive fatigue on all path widths, indicating that cognitive fatigue may affect MI performance. Similar correlations between physical or psychosocial fatigue and performance were not observed.

“Although current results do not have direct therapeutic implications, they provide some critical hints for MI use in the clinical and rehabilitation context,” the researchers wrote, adding that, specifically for MS patients, “it seems that spatial constraints and cognitive fatigue need to be correctly weighted according to the individual rehabilitative intervention.”

The team suggested that “mental practice with MI offers a unique and attractive opportunity to improve locomotor skills through a safe and self-paced training for people with severe disability that, due to their walking difficulties, can be done only in short bursts, especially in the early phase of rehabilitation.”