Comparison of Brain Activation during Motor Imagery and Motor Movement Using fNIRS

Abstract

Motor-activity-related mental tasks are widely adopted for brain-computer interfaces (BCIs) as they are a natural extension of movement intention, requiring no training to evoke brain activity. The ideal BCI aims to eliminate neuromuscular movement, making motor imagery tasks, or imagined actions with no muscle movement, good candidates. This study explores cortical activation differences between motor imagery and motor execution for both upper and lower limbs using functional near-infrared spectroscopy (fNIRS). Four simple finger- or toe-tapping tasks (left hand, right hand, left foot, and right foot) were performed with both motor imagery and motor execution and compared to resting state. Significant activation was found during all four motor imagery tasks, indicating that they can be detected via fNIRS. Motor execution produced higher activation levels, a faster response, and a different spatial distribution compared to motor imagery, which should be taken into account when designing an imagery-based BCI. When comparing left versus right, upper limb tasks are the most clearly distinguishable, particularly during motor execution. Left and right lower limb activation patterns were found to be highly similar during both imagery and execution, indicating that higher resolution imaging, advanced signal processing, or improved subject training may be required to reliably distinguish them.

Figure 1

Layout of the light sources, light detectors, and optodes (numbered 1–24). Adjacent sources and detectors are 3 cm apart.

Figure 2

Trial timing diagram.

Figure 3

Experiment protocol: each day had three repetitions of the motor execution and motor imagery runs.

Figure 4

Overview of the data analysis procedure, performed separately for motor imagery and motor execution tasks.

Figure 5

Average difference in activation between motor execution and motor imagery. Optodes with a significant difference (p < 0.05, FDR adjusted) between motor execution and motor imagery for a given task are circled.

Figure 6

Average HbO activation across all subjects for motor execution (a) and motor imagery (b). Significant optodes (p < 0.05, FDR adjusted) are circled.

Figure 7

Average in HbO activation over time for motor imagery and motor execution during the right hand task.

Figure 8

Average HbO and HbR activation for a single optode for each task. Standard error of the mean is shown as a faded area around the average. White area from 0 to 15 seconds is the task period; the grey areas are the resting state before and after the task.