Subject inefficiency phenomenon of motor imagery brain-computer interface: Influence factors and potential solutions

Motor imagery brain–computer interfaces(MI-BCIs) have great potential value in prosthetics control, neurorehabilitation, and gaming; however, currently, most such systems only operate in controlled laboratory environments. One of the most important obstacles is the MI-BCI inefficiency phenomenon. The accuracy of MI-BCI control varies significantly(from chance level to 100%accuracy) across subjects due to the not easily induced and unstable MI-related EEG features. An MI-BCI inefficient subject is defined as a subject who cannot achieve greater than 70% accuracy after sufficient training time, and multiple survey results indicate that inefficient subjects account for 10%–50% of the experimental population. The widespread use of MI-BCI has been seriously limited due to these large percentages of inefficient subjects. In this review, we summarize recent findings of the cause of MI-BCI inefficiency from resting-state brain function, task-related brain activity, brain structure, and psychological perspectives. These factors help understand the reasons for inter-subject MI-BCI control performance variability, and it can be concluded that the lower resting-state sensorimotor rhythm(SMR) is the key factor in MI-BCI inefficiency, which has been confirmed by multiple independent laboratories. We then propose to divide MI-BCI inefficient subjects into three categories according to the resting-state SMR and offline/online accuracy to apply more accurate approaches to solve the inefficiency problem. The potential solutions include developing transfer learning algorithms, new experimental paradigms, mindfulness meditation practice, novel training strategies, and identifying new motor imagery-related EEG features. To date, few studies have focused on improving the control accuracy of MI-BCI inefficient subjects; thus, we appeal to the BCI community to focus more on this research area. Only by reducing the percentage of inefficient subjects can we create the opportunity to expand the value and influence of MI-BCI.