## 3 Measurement Results of System Evaluation
### System Performance Evaluation  
Figure 13 shows the experimental setup for the computer access test. After receiving approval from the institutional review board (IRB) at Georgia Institute of Technology, two healthy subjects (33-year old male and 30-year old female) participated in the experiment to evaluate the iTDS-2 performance. Subjects attached small disk-shaped magnetic tracers (\(\diameter\)2.8 mm \(\times\)1.0 mm, K&J Magnetics, Jamison, PA, USA) on their tongues using dental adhesive and wore their customized iTDS-2 dental retainers on the lower jaw. The iTDS-2 transmits raw magnetic sensor data, changing with the tongue position, to the TDS-UI where the iPod-touch identifies one out of seven user-defined commands (left, right, up, down, left-click, right-click, and resting) and sends it to a computer via USB 55. Both subjects had already experienced the eTDS headset and the iTDS-1 palatal dental retainer on an occasional basis. However, this was their first time using the iTDS-2 arch-shaped dental retainer. Subjects trained the system with seven tongue commands and completed three trials for the maze navigation and center-out tapping tasks [55]. The graphical user interface (GUI) for the maze navigation task is shown in Figure 13.  
Figure 14 shows the performance comparison among the eTDS, iTDS-1, and iTDS-2. Figure 14a,b shows the deviation and completion time in the maze navigation task, respectively, and Figure 14c shows the throughput in the center-out tapping task. Deviation was calculated as the sum of area between the desired path on the maze and the actual traversed path of the mouse cursor on the screen, and completion time is the time taken by the subject to reach the end point. The throughput, measured in bits/s, is an indicator of the rate of information transfer from users to computers via a computer input device. Details of these tasks and their performance measures can be found in [55]. Subjects on average demonstrated 22% better performance with the iTDS-2 over the iTDS-1. We speculate that this is because the iTDS-2 does not limit the intraoral space, where the tongue moves, and also preserves the tactile feedback through the palate. The subjects' performance with the iTDS-2 is still inferior to that of the eTDS by \(\sim\)8% on average. The \(FOM_{i}\) in Figure 14d indicates the quality of commands classification, which is found to be closely related to the subjects' performance in computer  
Figure 13: An experimental setup for the computer access test including the maze navigation GUI. access [56]. It can be seen that the iTDS-2 is better than the iTDS-1 but worse than the eTDS, which is in agreement with the other performance measures. We speculate that the iTDS-2 might still be too bulky for easy, comfortable, and long term fit in the buccal shelf area, which might have caused the subjects' lower lips to be stretched a bit, degrading their performance. We should point out that the learning effect might have affected the experimental results, because the same subjects participated in the trials with the eTDS, the iTDS-1, and the iTDS-2. Also, there is not enough statistical power with only two subjects participating in the trial. We intend to conduct more experiments with a larger population over extended periods before drawing a solid conclusion regarding the iTDS-2 performance. Table 1 shows the summary of the iTDS-2 specifications and benchmarking against other intraoral tongue-computer interfaces. The table shows that the iTDS-2 has the longest operating time compared to other tongue-operated ATs. The iTDS-2 also has a robust wireless link, thanks to a dual-band Tx that can automatically switch in the presence of interference in each band. Moreover, the iTDS-2 has a unique arch-shaped design to locate in the buccal shelf area, which preserves tactile feedback and more space for the voluntary tongue motion.