Comparaci´ on de los sistemas

Several techniques have been used to measure muscle activities, jaw, head and tongue movements during oral processing.

2.6.1. Functional MRI (magnetic resonance imaging)

Functional MRI (fMRI) is a type of specialized MRI scan and one of the most recently developed forms of neuroimaging instruments. It measures the haemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. In mastication research, fMRI has been used to investigate the variation in the blood- oxygenation-level-dependent (BOLD) signals in both hemispheres before chewing and after chewing, and variations in BOLD during tongue protrusion, leftward and rightward tongue movement (Shinagawa et al., 2004). Echo-planar imaging (EPI) is one of the most efficient forms of MRI. It has the fastest acquisition method (100ms/slice), but limited spatial resolution. It has the capacity to image lingual soft tissue in 3 spatial dimensions during physiological movement (Gilbert, et al., 1998).

MRI has low invasiveness and radiation exposure, and relatively wide availability. But the limitation of this technique for feeding research is that the subject has to be lying down, which can affect bolus handling in the oral cavity before swallowing initiation and the time of pharyngeal transit. The other limitation is it cannot clearly show intramural anatomy and mechanics, particularly the relationship between the intrinsic and the extrinsic musculature of the tongue (Gilbert, et al., 1998). It is also relatively costly.

2.6.2. EMG activity

Electromyography (EMG) is a technique used to measure and record the electrical activity in muscles (Jonas, 2005). When muscles are active, they produce an electrical current which is usually proportional to the level of muscle activity (Shiel, 2009). EMG is often used in case of muscular dystrophy, inflammation of muscles, pinched nerves,

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and peripheral nerve damage, to detect abnormal electrical activity of muscles or isolate the level of nerve irritation or injury (Jonas, 2005). EMG has also been used in mastication and swallowing research (Casas, Kenny, & Macmillan, 2003; Foster, et al., 2006; Gonzalez, Sifre, Benedito, & Nogues, 2002; Kohyama, et al., 1998; Neyraud, et al., 2005; Pereira, et al., 2006; Taniguchi, Tsukada, Ootaki, Yamada, & Inoue, 2008). Two kinds of electrodes are often used: surface or needle electrodes. Needle electrodes have better muscle selectivity, but are invasive (Huang, Chen, & Chung, 2004).

Studies of mastication which employed EMG have found that the masseter and styloglossus are active in the jaw-closing phase, which perform tongue protrusion. The digastric and genioglossus are active in the jaw-opening phase, which perform tongue retraction (Kakizaki, et al., 2002). In the jaw-closing phase, digastric short burst (DSB) and genioglossus short burst (GgSB) have often been observed in EMG, and often accompanied by an inhibitory period of relevant antagonistic muscles in rabbits. These are thought to be induced by a peripheral sensor input, reflex response (Kakizaki, et al., 2002).

EMG is an effective way to detect the electrical activity of muscles, but does not show the movement of the muscle directly. Surface electrodes which are commonly used are also subject to disturbance and variability due to placement on the skin.

2.6.3. Electromagnetic Articulography (EMA)

The earliest procedure of articulography was carried out by Sonoda in 1974. The technology was developed further until in 1982, the first 2-dimensional Electromagnetic Articulography (EMA) appeared. In 1988 Carstens Medizinelektronik developed the first commercial EMA, AG100. This evolved through several stages into the AG500, which captured 5-dimensional images (three Cartesian coordinates: x, y, and z; and two angular coordinates: azimuth and elevation). This instrument was invented to investigate the movement of articulators inside the oral cavity during speech, but it has also been used to study feeding and swallowing.

In speech research, EMA is used to observe tongue action, especially the action of the tongue tip, tongue body and tongue dorsum during speaking (Cheng, Murdoch, Goozee, & Scott, 2007; Kuruvilla, Murdoch, & Goozee, 2007; Marino et al., 2002). Linguists have used several methods to position sensors on the surface of the tongue to trace

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tongue movement in a sealed mouth. Similarly, in feeding studies, EMA is used to investigate lingual movements during oral processing. The receiver coils (sensors) have usually been fixed to the tongue blade, tongue body and tongue dorsum, but the distance from the anatomic tongue tip could be different in different research (Goozee, et al., 2000; Simonsen, Moen, & Cowen, 2008; Tabain & Perrier, 2007). In 2004, Steel and Van Lieshout used an articulograph AG100, to study swallowing (Steele & Van Lieshout, 2004b). In a later study, they observed the dynamics of lingual-mandibular coordination during liquid swallowing, but they fixed receiver coils to various locations on the tongue surface (Steele & Van Lieshout, 2008). Schwestkapolly et al. (1995b) observed tongue movements while biting and swallowing using EMA.

Earlier EMA studies used the more limited AG100 or AG200 device (Cheng, Goozee, & Murdoch, 2005; Cheng, Murdoch, Goozee, & Scott, 2007; Fagel & Clemens, 2004; Murdoch, Goozee, Theodoros, & Stokes, 2000; Steele & Van Lieshout, 2004; Tabain & Perrier, 2007). In these earlier models, the receiver coils had to be fixed on the midsagittal line of the tongue surface, and their axes had to be perpendicular to the midsagittal plane, parallel with the axes of the transmitter coils. Therefore, only the data in the midsagittal plane could be collected during articulators’ movement. These problems were resolved with the AG500 which can collect more dynamic data, not only from the midsagittal plane in the oral cavity, but also from other locations (Blissett, Prinz, Wulfert, Taylor, & Hort, 2007; Wang, Nash, Pullan, Kieser, & Rohrle, 2013). Consequently, it is possible to collect further data of tongue movements during oral processing and observe more tongue functions during feeding.

AG500 collects data of distance from the centre of cube, and the distance between the coils must be greater than 0.8cm (Carstens, 2006). One important limitation is the coils and wires may restrict chewing behaviour.

2.6.4. Videofluorography (VFG)

VFG is a new generation of cineradiography, which requires much lower radiation levels and has become a standard radiological diagnostic tool (Hiiemae & Palmer, 2003). It has been widely adopted in studies of mastication and swallowing in recent years (Heath, 2001; Hiiemae, 2004; Hiiemae & Palmer, 1999; Hiiemae et al., 2002; Mioche, et al., 2002; Okada, et al., 2007; Palmer, et al., 1997; Palmer, et al., 2006). For

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mastication behaviour, it does not permit fine-scale analysis of tongue fleshpoint movement (Beck & Gayler, 1990). VFG has been used to measure tongue and jaw movement (Palmer, et al., 1997), and investigate patterns of food movement and breakdown in the coronal plane (Mioche, et al., 2002). Using this method, the observer can easily see dynamic images from the video, and directly determine the contour of movement pattern. But this technique still has some defects: specific food position cannot be identified because of the shadow between food and the teeth/tongue (Mioche, et al., 2002); two dimensional data collection means that it is impossible to monitor lateral movement during feeding; and only limited data can be collected from subjects in order to minimise exposure to radiation.

2.6.5. Ultrasonograph

Ultrasonic echo-sonography appeared in the 1950’s and began to be widely applied in the medical field and research. Recently, more and more studies have been conducted using ultrasonography during feeding, especially for the study of tongue movement (Blissett, et al., 2007; de Wijk, et al., 2006; Imai, et al., 1995). The ultrasonograph has two imaging types: B-mode images, which show a 2 dimensional recording of the slice through the oral cavity; and M-mode images, which depict the movements of the tongue during chewing, as a function of time. In M-mode, a single column of pixel is abstracted from each frame of the B-mode image and plotted sequentially. The resulting plot allows movement to be seen (de Wijk, et al., 2006). Normally, the probe is positioned laterally underneath the chin to record the movement of a coronal section through the tongue (Blissett, et al., 2007). To quantify the tongue movement, the ‘marker’ pellet technique can be used in ultrasonographic research as well.

It can image soft tissue in real time and the imaging has a rate of data capture of up to 30 frames / s. It is also non-invasive and cheaper than MRI. Ultrasonography has no physical interference, so does not restrict tongue movements at all. The major disadvantage of ultrasonography is that it cannot record the information of specific lingual fleshpoints, such as the data between the tongue surface and vocal tract, and its resolution is limited. Therefore, most researchers do not use it as the only way to study the whole oral process, but combine with other techniques. However, latest and more complex ultrasound techniques can elicit data and produce 3D models of the tongue surface (Hiiemae & Palmer, 2003).

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2.6.6. Tongue pressure measurement

During mastication, swallowing and speech, the pressure in the oral cavity changes dynamically.

Kieser et al. (2008) applied a new method to measure the pressure in the mouth during swallowing. They used 8 miniature pressure transducers fixed on the chrome-cobalt palatal appliance to measure pressure between tongue and lip, the lateral tongue margin and cheeks, and tongue pressure on the midline of the palate. They found some unexpected pressure changes in the mouth, recording negative pressure in the mouth during swallowing, not only at rest. The tongue pressure on the midline of the hard palate increased anteromedially, and then fell rapidly toward the back of the mouth before a wave of increased pressure propelled the bolus toward the back of the palate (Kieser et al., 2008). In an earlier study, it had been found that tongue pressure on the hard palate was generated anteromedially, then followed by a wave of pressure anterolaterally, midmedially, posterolaterally, and posteromedially (Hori, Ono, &

Nokubi, 2006; Ono, Hori, & Nokubi, 2004). Casas et al. (2003) combined three

techniques (EMG, dynamic ultrasound imaging and the miniature pressure transducer) to study the oral processing during chewing of solid food. They found that the intervals between peak pressure recorded at each pressure transducer and peak jaw-closing activity for each masticatory cycle were not significantly different and displayed large statistical variation.