Evaluation of a wireless wearable tongue-computer interface by individuals with high-level spinal cord injuries

Abstract

The tongue drive system (TDS) is an unobtrusive, minimally invasive, wearable and wireless tongue-computer interface (TCI), which can infer its users' intentions, represented in their volitional tongue movements, by detecting the position of a small permanent magnetic tracer attached to the users' tongues. Any specific tongue movements can be translated into user-defined commands and used to access and control various devices in the users' environments. The latest external TDS (eTDS) prototype is built on a wireless headphone and interfaced to a laptop PC and a powered wheelchair. Using customized sensor signal processing algorithms and graphical user interface, the eTDS performance was evaluated by 13 naive subjects with high-level spinal cord injuries (C2-C5) at the Shepherd Center in Atlanta, GA. Results of the human trial show that an average information transfer rate of 95 bits/min was achieved for computer access with 82% accuracy. This information transfer rate is about two times higher than the EEG-based BCIs that are tested on human subjects. It was also demonstrated that the subjects had immediate and full control over the powered wheelchair to the extent that they were able to perform complex wheelchair navigation tasks, such as driving through an obstacle course.

Figures

Figure 1

The latest external tongue drive system (eTDS) prototype and its main components, built using only commercial off-the-shelf components, including a wireless headphone.

Figure 2

TDS-wheelchair interface: (a) the GUI provides users with visual feedback on the commands that have been selected. (b) Adapter circuitry connecting a laptop to the powered wheelchair controller via USB and standard DB-9 connectors, respectively. This circuit uses the TDS commands to change the values of the wheelchair analog state vectors.

Figure 3

Pre-training session GUI showing the 3D representation of the current (trace of green stars) and previous (other markers) tongue positions for different TDS commands. The normalized minimum distance between the current tongue position and all previous positions is shown by a dial on the right. The subjects were instructed to define their tongue positions for TDS commands in a way that markers of different commands are separated from each other and the dial stays in the bright zone, where the normalized distance is greater than 1.5.

Figure 4

The TDS training data for four commands, individually marked and projected on to the 3D PCA space,. (b) The table in the training GUI, in which each button is associated with one training point in the PCA space with a command name (columns) and an index number (rows). This table allows the operator to identify and remove the outcast training points. (c) TDS command clusters in the 3D PCA space after the outcast training point in (a) for the ‘down’ command is removed.

Figure 5

A subject with SCI at level C4 wearing the eTDS prototype and navigating a powered wheelchair through an obstacle course. As a safety measure, the operator walked with the subject while holding an emergency stop button [38].

Figure 6

Plan of the powered wheelchair navigation track in the obstacle course showing dimensions, location of obstacles and approximate powered wheelchair trajectory.

Figure 7

Average completion time along with 95% confidence interval across 13 subjects for three trials in the CA-2 session.

Figure 8

Response time measurement results. (a) Mean probability of correct choices versus response time. (b) Information transfer rate, calculated using the definition in [6], versus response time.

Figure 9

Average navigation speed and number of collisions for discrete and continuous control strategies, with and without visual feedback (VF).

Figure 10

Comparing the effects of different factors on the subjects' performance in computer access and powered wheelchair control tasks through the ITR from the CA-3 session and wheelchair navigation speed and number of collision from the PWCN session by (a, b) younger (age ≤ 50) versus older (age > 50) subjects, (c, d) subjects with shorter (≤6 months) versus longer (>6 months) duration of injury and (e, f) female versus male subjects.