Potential participants had to complete a short survey outlining their medical history. If an individual had any prior symptoms of swallowing difficulties, or had a history of stroke or other neurological conditions, head or neck cancer, neck or spinal injury or a tracheostomy, he/she was excluded from the study. In total, fifty consenting healthy adults (24 males) participated in this study ranging from 18 to 65 years of age (19 participants were 18-34 years old; 9 participants were 34-44 years old; 13 participants were 45-54 years old; and 9 participants were 55-65 years old). The research ethics board of Bloorview Kids Rehab (Toronto, Ontario, Canada) approved the study protocol.
Upon the completion of the short survey, participants were seated comfortably in a chair for the rest of the experiment. To record cervical accelerometry signals (i.e., accelerometry signals without swallowing), a dual-axis accelerometer (ADXL322, Analog Devices) was placed on the neck of each participant anterior to the cricoid cartilage and secured with double-sided tape. The accelerometer has a measurement range of ± 5 g, a bandwidth of 2.5 kHz, a resonant frequency of 5.5 kHz, and sensitivity of 174 mV/g. A voltage supply (Model 1504, BK Precision) set at 5 V was used to power the acceleremoter. Furthermore, the two axes were positioned in the anterior-posterior (A-P) and superior-inferior (S-I) anatomical directions as shown in Figure1 . All participants were advised to refrain from swallowing during each task, but were permitted to swallow accumulated saliva between successive steps. No data were recorded during those swallows. Three additional sensors confirmed that the participants followed the data collection protocol properly. We collected signals from a triple-axis accelerometer (MMA7260Q, SparkFun Electronics) attached to a headband and centered on the participant's forehead to monitor head motions; a respiratory belt (1370G, Grass Technologies) secured around the participant's diaphragm to monitor breathing patterns; and a microphone placed 30 cm away from the participant's mouth to capture any vocalizations. The dual-axis accelerometer signals were filtered (passband 0.1-3000 Hz) and amplified in hardware (Grass P55 pre-amplifier). The subsequent signals were acquired at 10 kHz using a data acquisition card (NI USB-6210, National Instruments) and custom Labview software. The data were stored on a hard drive for subsequent analyses.
The data collection procedure included five primary tasks. Participants remained silent and were asked to:
1. tilt their head to the left side 10 times.
2. tilt their head to the right side 10 times.
3. tilt their head down 10 times.
4. tilt their head back 10 times.
5. rotate their head from right to left 5 times, and from left to right 5 times.
Participants performed each task only once, and the data collection did not generally last longer than 15 minutes per participant.
The participants also engaged in other tasks as part of the data collection protocol for a different study, hence, the additional sensors. For the current study, the additional sensors (head accelerometer, respiratory belt and microphone) simply served to confirm participant compliance with the experimental protocol. For example, head accelerometers were used to confirm that head motion was generated only when cued. Similarly, a microphone was used to ensure that participants did not vocalize during these tasks, while the respiratory belt was used to ensure that participants maintained normal breathing patterns during these tasks. Nevertheless, we did not engage in an extensive analysis of data collected using these sensors beyond a qualitative inspection to confirm participant compliance to the experimental protocol.
Upon the completion of the short survey, participants were seated comfortably in a chair for the rest of the experiment. To record cervical accelerometry signals (i.e., accelerometry signals without swallowing), a dual-axis accelerometer (ADXL322, Analog Devices) was placed on the neck of each participant anterior to the cricoid cartilage and secured with double-sided tape. The accelerometer has a measurement range of ± 5 g, a bandwidth of 2.5 kHz, a resonant frequency of 5.5 kHz, and sensitivity of 174 mV/g. A voltage supply (Model 1504, BK Precision) set at 5 V was used to power the acceleremoter. Furthermore, the two axes were positioned in the anterior-posterior (A-P) and superior-inferior (S-I) anatomical directions as shown in Figure
The data collection procedure included five primary tasks. Participants remained silent and were asked to:
1. tilt their head to the left side 10 times.
2. tilt their head to the right side 10 times.
3. tilt their head down 10 times.
4. tilt their head back 10 times.
5. rotate their head from right to left 5 times, and from left to right 5 times.
Participants performed each task only once, and the data collection did not generally last longer than 15 minutes per participant.
The participants also engaged in other tasks as part of the data collection protocol for a different study, hence, the additional sensors. For the current study, the additional sensors (head accelerometer, respiratory belt and microphone) simply served to confirm participant compliance with the experimental protocol. For example, head accelerometers were used to confirm that head motion was generated only when cued. Similarly, a microphone was used to ensure that participants did not vocalize during these tasks, while the respiratory belt was used to ensure that participants maintained normal breathing patterns during these tasks. Nevertheless, we did not engage in an extensive analysis of data collected using these sensors beyond a qualitative inspection to confirm participant compliance to the experimental protocol.
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