![]() The Qualisys active markers are also available in various sizes.įor speech research, one disadvantage of active-marker systems is that the wires attached to markers on the face may be inconvenient, restrictive, and obtrusive, especially for populations like children, the elderly, and individuals with disabilities. Following similar principles, the Qualisys Oqus systems (Qualisys AB, Gothenburg, Sweden) feature high-speed digital cameras (1, 3+, 4, and 5+ series) with a rate of 180–500 frames per second (fps) at full resolution and 360–1750 fps at reduced resolution. Speech researchers typically use a reference frame that is static relative to the skull (e.g., affixed to a head band or goggles) so that all recorded data can be analyzed relative to the head-centric coordinate system. ![]() The system is pre-calibrated at the factory, and a frame with three or more IREDs serves as the reference. ![]() ![]() For the Optotrak Certus system, the claimed RMSE is 0.1 mm ( Northern Digital, 2012a). The former is a measurement statistic (often expressed as the root-mean-square error, RMSE) with respect to the true position or displacement of the measured object whereas the latter is a statistic (often expressed as the standard deviation) reflecting the spread of repeated measurements of the position or displacement of an object. The markers suitable for orofacial structures are as small as 4 mm in diameter, and their positions can be sampled at a rate up to 4600/( n+1.3) Hz (where n is the number of markers).Īccuracy and precision are two widely-used performance indices for motion tracking systems. The infrared light emission is captured by three lenses. In the Optotrak system, every strober (i.e., unit of IRED control circuitry) controls multiple markers and communicates with the main capture unit via wired or wireless connection. One widely used example of an active-marker three-dimensional (3D) position tracking system is the Optotrak Certus Motion Capture System (NDI Northern Digital Inc., Waterloo, Ontario, Canada). Each IRED is activated by an electrical signal sent from the control circuitry, and the emitted infrared light is detected by the special cameras’ lenses. In an active marker system, infrared light emitting diodes (IREDs) are attached as markers of the flesh points that are to be tracked. When it is sufficient to track movements of externally visible structures such as the lips and jaw or when non-invasive tracking is required, optical tracking systems can be used. For example, the Haskins Optically Corrected Ultrasound System (HOCUS) incorporates an optical system and an ultrasound system for tracking motion of the tongue, lips, and jaw without head constraints ( Whalen et al., 2005). Sometimes, integration of two or more instrumental techniques allows for better observation of the relevant articulatory movements. Movements of both intra- and extra-oral structures can be tracked using the first four techniques whereas only extra-oral structures can be captured using optical techniques. Examples include x-ray technology, ultrasound, magnetic resonance imaging (MRI), electromagnetic articulography (EMA), and optical or camera-based techniques. Over the past few decades, a wide variety of motion tracking instruments have been developed and applied to speech production (for an overview, see Earnest & Max, 2003). Revealing details regarding the nature and organization of movement patterns – and, thus, indirectly, information about the underlying neural control signals – depends on the availability of accurate, reliable, and safe techniques for transducing the movements of interest. Positioning and moving the articulators (i.e., shaping the vocal tract) directly determines the final outcome of the speech process – acoustic signals corresponding to speech sounds. Studies of articulatory movements contribute critical information to our understanding of the sensorimotor mechanisms involved in speech production.
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