Although no marked differences in the asymmetry of facial expressions distinguished Parkinson’s disease patients and healthy individuals in a study, patients most clearly expressed an emotional reaction on the side of the face corresponding to the side of the body less affected by Parkinson’s motor symptoms, a study found.
Its researchers concluded patients’ facial asymmetry when displaying emotions is a consequence of general motor symptom asymmetry, rather than difficulties or problems in processing emotions.
Parkinson’s is characterized by the gradual loss of muscle control and lack of facial expression, sometimes accompanied by cognitive deficits.
Previous studies have suggested that hemispheric dominance — a phenomenon in which one side of the brain is more important than the other for a given function — in emotional processing can lead to an asymmetric facial expression.
Although Parkinson’s motor symptoms tend to be rather asymmetric (i.e., more pronounced on one side of the body), studies had not addressed facial and emotional asymmetry in Parkinson’s patients.
A research team in London and Italy explored the relationship between motor symptom asymmetry and facial expressiveness in Parkinson’s disease.
The study enrolled 20 patients and 20 healthy people serving as controls, who were video-recorded while displaying facial expressions: one that was neutral, and six basic emotions (anger, disgust, fear, happiness, sadness, and surprise).
The most expressive pictures obtained from the video-recordings were then cut down the middle and put through a program to generate ‘chimeric’ faces that showed only the right side of the face (right side manipulated to make full face) and the left side (a left-left combination). Investigators then asked nine healthy people with no prior connection to the study to rate which one of the two chimeric faces looked more expressive. Raters’ choices, reaction times, and confidence levels were recorded.
To evaluate a possible link between facial expressiveness and motor symptom asymmetry, researchers performed correlation analysis between the global facial laterality index (pooling all emotions together) as well as for the indexes of each emotion separately, and the body laterality index (defined by the side of the body most affected by Parkinson’s symptoms).
No substantial differences were found in how the nine raters judged emotional expressiveness on the two chimeric faces (right-right and left-left), whether within the Parkinson’s and the control group, or between the two groups.
In Parkinson’s patients, however, investigators found a correlation between the global facial laterality index and the body laterality index, suggesting that each patient’s most expressive side of the face corresponded to the body side less affected by Parkinson’s symptom’s.
“Despite the lack of significant facial asymmetry in PD [Parkinson’s disease] and healthy subjects, the relationship we found between the intensity of facial expression and motor symptom lateralization supports the hypothesis that there is some facial asymmetry of emotional expression in PD which relates to the general lateralisation of the motor features of the disorder rather than a specific abnormality in emotional processing,” the researchers wrote.