Cranial Nerves – There are 12 pairs of nerves that originate from the brain itself. These nerves are responsible for very specific activities and are named and numbered as follows:

1. Olfactory: Smell
2. Optic: Visual fields and ability to see
3. Oculomotor: Eye movements; eyelid opening
4. Trochlear: Eye movements
5. Trigeminal: Facial sensation
6. Abducens: Eye movements
7. Facial: Eyelid closing; facial expression; taste sensation
8. Auditory/vestibular: Hearing; sense of balance
9. Glossopharyngeal: Taste sensation; swallowing
10. Vagus: Swallowing; taste sensation
11. Accessory: Control of neck and shoulder muscles
12. Hypoglossal: Tongue movement

TONGUE MOVEMENTS IN FEEDING AND SPEECH Karen M. Hiiemae* Institute for Sensory Research, Department of Bioengineering and Neuroscience, Syracuse University, Syracuse, NY 15244-5290, USA; Jeffrey B. Palmer Department of Physical Medicine and Rehabilitation, Department of Otolaryngology-Head and Neck Surgery, Johns Hopkins University and Good Samaritan Hospital, Baltimore, MD 21239, USA

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The position of the tongue relative to the upper and lower jaws is regulated in part by the position of the hyoid bone, which, with the anterior and posterior suprahyoid muscles, controls the angulation and length of the floor of the mouth on which the tongue body ‘rides’. The instantaneous shape of the tongue is controlled by the ‘extrinsic muscles’ acting in concert with the ‘intrinsic’ muscles. Recent anatomical research in non-human mammals has shown that the intrinsic muscles can best be regarded as a ‘laminated segmental system’ with tightly packed layers of the ‘transverse’, ‘longitudinal’, and ‘vertical’ muscle fibers. Each segment receives separate innervation from branches of the hypoglosssal nerve. These new anatomical findings are contributing to the development of functional models of the tongue, many based on increasingly refined finite element modeling techniques. They also begin to explain the observed behavior of the jaw-hyoid-tongue complex, or the hyomandibular ‘kinetic chain’, in feeding and consecutive speech. Similarly, major efforts, involving many imaging techniques (cinefluorography, ultrasound, electro-palatography, NMRI, and others), have examined the spatial and temporal relationships of the tongue surface in sound production. The feeding literature shows localized tongue-surface change as the process progresses. The speech literature shows extensive change in tongue shape between classes of vowels and consonants. Although there is a fundamental dichotomy between the referential framework and the methodological approach to studies of the orofacial complex in feeding and speech, it is clear that many of the shapes adopted by the tongue in speaking are seen in feeding. It is suggested that the range of shapes used in feeding is the matrix for both behaviors.

Key words. Tongue, regional anatomy, eating, speech, deglutition

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Interest in the oropharyngeal complex has been rising. For a long time largely ignored, it is now receiving serious attention. A fully functional oropharyngeal complex is essential for normal feeding, breathing, and speech sound production. Degradation of any oropharyngeal function is a serious medical management challenge and a major quality-of-life issue for the patient. The tongue is a very difficult organ to examine. However, new technology, e.g., the invention of advanced imaging techniques, supports highly sophisticated analyses of tongue movement and shape. There is also growing interest in the biomechanics of feeding behavior and speech and their neural control. Clinical issues, such as the role of the tongue in obstructive sleep apnea, are receiving new emphasis. The need for improved synthesized speech has also contributed to these developments.

The mammalian tongue has vital functions in feeding: It plays a major role in ingestion, as in licking, lapping, and browsing; and it moves food distally through the oral cavity from the incisors to the post-canines for chewing, and then to the pharynx for bolus formation and swallowing. In dogs, the tongue has a thermoregulatory function in panting. Tongue position relative to the posterior pharyngeal wall is important in respiration. Chemo-receptors and mechanoreceptors in the tongue surface sense the nature and mechanical properties of ingested food, and prevent the digestion of noxious substances. In addition, tongue shape and position in the oral cavity influence the shape and dimensions of the airway between the palate and the tongue surface in mammals with Type I tongues (Doran and Baggett, 1971). Given the dominant role of speech in human interactions, an overwhelming proportion of the research on tongue movement focuses on its role in speech, specifically vowel and consonant production. [Nor is it surprising that speech research uses an internationally recognized alphabet of notations and abbreviations that are difficult for the oral biologist to follow. Where possible, those specialized usages have been avoided here.] Until recently, studies of the patterns of tongue movement in feeding were focused on non-human mammals (see Hiiemae and Crompton, 1985; Hiiemae, 2000). Since 1992 (Palmer et al.), there have been four reports on tongue, hyoid, and jaw movements in human subjects (complete sequences from initial ingestion to terminal swallow) when consuming foods of different initial consistencies (Palmer et al., 1997; Palmer, 1998; Hiiemae and Palmer, 1999; Hiiemae et al., 2002). In the same period, there have been significant developments in the approaches to tongue behavior in speech.

Doran (1975) and Doran and Baggett (1971) identified two types of mammalian tongues: Type I is the spatulate fleshy tongue found in almost all mammals. It can protrude up to 50% over its resting length and is capable of fairly complex movements. Type II tongues are the highly flexible whip-like organs found in anteaters and other myrmecophagous mammals. [For reviews of tetrapod feeding mechanisms, including a chapter on mammals, see Schwenk, 2000.] Although differing in their details, all Type I tongues share common characteristics and general architectures. Non-human mammals—e.g., opossums, rats, mice, rabbits, cats, tenrec, and a range of primates, but particularly the macaque—have been used in studies of feeding. Cats, rabbits, guinea pigs, and rats have also been used in studies investigating the neural control of that rhythmic activity. Clearly, tongue movements in speech can be studied only in humans.

There is almost no common ground between the ‘feeding/physiology’ literature and that focusing on speech, but a bridge may be appearing. MacNeilage (1998) hypothesizes that the movements of the tongue and jaw in speech (which he terms ‘cyclicities’) evolved from their movements in infantile babbling. This idea has its supporters and detractors but is superficially very appealing. No one has yet attempted, as far as we know, to test it experimentally. This idea is particularly relevant if one is interested in the evolution of speech, since, as should become clear within the body of this review, human tongue behavior in feeding builds on the patterns of movement in the hyolingual complex observed in other mammals. It is, therefore, reasonable to hypothesize that the matrix of tongue movements during human speech was derived from the wide variety of tongue movements found in suckling and feeding, although this view is controversial.
 

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The tongues of mammals share certain important characteristics, but there are also important differences. The mammalian tongue cannot be viewed as a ‘freestanding’ organ. Rather, for almost all its functions, it depends on its linkages with the hyoid apparatus and lower jaw (Fig. 1). This is the hyolingual complex. There is a fundamental anatomical difference between the non-human tongue and that of humans. Non-human mammals have flat hard palates, mostly with well-developed rugae, long tooth rows, and a long flat tongue (small vertical dimension). The hyoid is behind rather than below the oropharyngeal surface of the tongue, with the epiglottis extending dorsally and coming into contact with the soft palate on its pharyngeal surface (see Hiiemae and Crompton, 1985). The non-human larynx is linked to the hyoid bone but positioned behind rather than under it. Human neonates have a comparable relationship among tongue, hyoid, larynx, epiglottis, and soft palate (Negus, 1949). Within a few months of birth, however, the neck begins to elongate, culminating in the ‘descent of the larynx’, creating the vertical component of the supra-laryngeal vocal tract (Fig. 2). As a result, the posterior oral seal and the upper esophageal sphincter (UES) become widely separated, and a ‘bend’ develops in the tongue surface, creating both oral and oropharyngeal surfaces. This change in humans has been attributed to the development of speech and is considered a prime cause of the morbidity and mortality associated with disturbances of swallowing (Palmer et al., 1992).
 

View larger version (27K): [in this window] [in a new window]   Figure 1. The functional linkages among jaw, hyoid, and tongue movements in feeding and speech. Movement of one element affects that of most others. For simplicity, the Fig. does not include the cheeks and lips, the former having an important role in food management in feeding, and the latter being important articulators in speech.

View larger version (26K): [in this window] [in a new window]   Figure 2. Diagrammatic sagittal sections of the oropharyngeal complex. (A) The anatomy of the complex as defined in the Process Model of Feeding. The spaces identified as the oral cavity, oropharynx, and hypopharynx are shown by graded stippling. ATM = anterior tongue marker; PTM = posterior tongue marker; UCM = upper canine marker; LCM = lower canine marker; and UMM = upper molar marker. The supralaryngeal vocal tract, with its vertical (SVTv) and horizontal (SVTh) components, is also shown (heavy dashed lines). (B) Illustration of the coordinate framework for the data shown in Figs. 3 and 4. The X axis is drawn between UCM and UMM markers and parallels the occlusal plane of the upper post-canine teeth, itself reflective of the plane of the hard palate behind the upper incisor alveolus. Movement in that axis is antero-posterior. The Y axis is perpendicular to the X axis, subtended from the upper canine marker (‘0’ point in the grid). Mandibular (LCM) or tongue marker (ATM, PTM) movement in the Y axis is primarily supero-inferior, as is, to a lesser extent, hyoid movement. [For further explanation, see Hiiemae et al.(2002).] Reproduced from Hiiemae et al.(2002), with permission from Pergamon Press.

 
    (1) THE FUNCTIONAL ANATOMY OF THE HYOLINGUAL COMPLEX
Unlike the elephant’s trunk (Kier and Smith, 1985; Smith and Kier, 1989), the mammalian tongue is short and is anchored to the mandible, the hyoid, and cranial base by its extrinsic muscles. Although still an hypothesis, the evidence points to the biomechanics of the mammalian (and human) tongue as being consistent with that predicted for a muscular hydrostat: i.e., the tongue is of fixed incompressible volume such that distortion in one direction/axis affects the other two [see below]. Stone and Lundberg (1996) explain tongue shape based on this principle. Takemoto (2001) bases his recent analysis of the tongue musculature on the ‘hydrostatic’ interpretation of the anatomy of the tongue. Even when it acts as a hydrostat, the movements and shape changes of the human tongue occur in a space whose dimensions are dictated by movements of the jaw and hyoid.

Tongue behavior cannot be divorced from hyoid movement, which is directly linked to motion of the mandible. The length and angulation of the floor of the mouth on which the tongue body rides are dictated by that linkage. Movements of the tongue surface can occur independently of hyomandibular movement within a limited range of jaw motion. [This relationship can be described as one in which a ‘tapered sausage’ is attached to a mobile surface (the oral floor formed by the hyomandibular muscles) so that the ‘sausage’ can change its shape as the floor moves.]

In a trenchant review of the ‘muscles of the mandible’, Last (1954) lays out the principles of these functional relationships. Last does not call these linkages a ‘kinetic chain’, but the thrust is just that. This concept can be represented as a series of linked muscle groups acting on two mobile skeletal elements, the hyoid and mandible (Fig. 1). Their relative positions determine the length and orientation of the floor of the mouth, and so the gross vertical and antero-posterior position of the tongue body relative to the hard palate. This concept is important, because many studies on the jaw musculature in the dental and related literature focus only on the adductors (i.e., temporalis, masseter, medial pterygoid). Some refer to the digastric as the primary abductor of the mandible (see Miller, 1991). By and large, the hyoid complex is ignored. The speech literature is quite different, in that the focus is on the relative position and movement of the articulators, emphasizing the tongue surface, palate, and lips (Fig. 2; also see Folkins and Kuehn, 1982). Folkins and Kuehn advance the concept of ‘bidirectionality’, in which they recognize that movement in one part of the system affects all the others. The literature on the anatomy of the tongue (e.g. Lowe, 1981) dismisses the hyoid muscles as ‘belonging to the floor of the mouth’. It is essential to emphasize that global tongue position and so its movements in feeding and speech are directly correlated with the length and orientation (position) of the floor of the mouth (the base of the tongue body), i.e., hyoid position.

    (2) THE KINETIC CHAIN
The muscular linkages among the mandible/lower jaw, the hyoid, the cranial base (see Fig. 1), and sternum have the following properties:

• First, during feeding, the hyoid is in continuous motion, so that the relationship between it and the lower jaw changes constantly. Hyoid movement is linked to that of the opening and closing of the jaws (the masticatory/chewing cycle) and therefore to activity in the mandibular adductors (Figs. 1, 2, 3, 4). Hyoid motion results from change in the relative positions, and distance between, the hyoid and the mandibular symphysis, which, in turn, depends on mandibular position relative to the cranium. Analysis of the experimental data shows that the hyoid can travel upward and forward toward a slowly opening jaw, and that it can be pulled sharply backward from a jaw held in a wide gape (Hiiemae et al., 2002; also see Carlsöö, 1956; Pancherz et al., 1986). It is also clear that the geometry of the relationship between the hyoid and the mandible in man is such that their relative positions are affected by the direction and amplitude of jaw movement (Folkins and Kuehn, 1982). It has been argued (Thexton and McGarrick, 1988, 1989) that if cinefluorographic (CFG) or videofluorographic (VFG) data are examined with the lower (mandibular) occlusal plane as the reference plane, then hyoid and tongue movements confined within the tongue body can be analyzed without the distorting effect of jaw movement. This is simply not the case for man, where the jaw and tongue are relatively much shorter than in other mammals—e.g., opossum, cat, and macaque (see Hiiemae and Crompton, 1985; Hiiemae, 2000)—and the hyoid with the larynx lies below the posterior tongue rather than behind it.

View larger version (66K): [in this window] [in a new window]   Figure 3. Time/position records for mandible and hyoid over 10 sec of normal feeding (A) and for a 10-second extract from the same subject reading the entire ‘Grandfather Passage’ (B). The food (A) was chicken salad spread. The movements of each marker (hyoid, lower canine) are plotted over time relative to the upper occlusal plane (see Fig. 2). Jaw and hyoid movements are clearly rhythmic in A, with synchronization between their movements. The movements of the jaw in speaking are much less rhythmic, with low-amplitude oscillations. In contrast, the antero-posterior movements of the hyoid seem to be slow, while its vertical movements are more rapid. The relatively slow vertical movements of the jaw (from most ‘up’ to next most ‘up’) could represent a single ‘cyclicity’ (see text and MacNeilage, 1998). Reproduced from Hiiemae et al.(2002), with permission from Pergamon Press.

View larger version (41K): [in this window] [in a new window]   Figure 4. Sagittal domain plots for jaw, hyoid, and tongue markers (anterior and posterior) for the same subject: (A) a complete sequence of feeding on cookie (brittle shortbread fingers, 6 g), and (B) reading the Grandfather Passage. Each recording took about 50 sec and included about 1500 to 2000 datapoints. The XY coordinates for each datapoint were then plotted. The sagittal domain for the hyoid (defined by its centroid) is further forward during speaking than during eating. The ranges covered by sagittal domains in feeding are larger than they are for speaking for every structure [see Hiiemae et al.(2002) and this text]. Reproduced from Hiiemae et al.(2002), with permission from Pergamon Press.

• Second, the relative positions of the hyoid and mandible directly affect the length and angle of the floor of the mouth on which the tongue mass ‘rides’: Increasing the distance between the hyoid and the mandibular symphysis lowers and lengthens the floor of the mouth relative to the lower jaw, and so the relative position of the tongue body will be lower relative to the tooth rows. Conversely, raising the hyoid can shorten the floor of the mouth. Such a movement facilitates tongue-palate contact. The effect of such shortening depends on whether the movement is simply vertical or upward and forward as in swallowing (Ishida et al., 2002).

• Third, the complex biomechanics of the hyolingual apparatus have not been thoroughly studied. An issue here is hyoid movement and the correlated position of the oropharyngeal surface of the tongue. It cannot be assumed that the two exactly parallel each other. The tongue can ‘bunch’, ‘heap’, and twist, increasing its vertical dimension while shortening its postero-anterior axis.

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The tongue moves rapidly in both speech and feeding. Abd-el-Malek (1939, 1955) provided the first description of its movements (1955) after studying its anatomy (1939). Using naked eye observation and still camera exposures to record what he determined to be the core tongue shapes involved in feeding, he demonstrated, in a human subject, that the tongue could protrude, retrude, twist, and produce a variety of ‘intrinsic’ shape changes (see Fig. 5). He could not provide data on the rate of tongue movement or how it changed from one posture to another. Beyond such straightforward descriptions, the issues in studying the tongue hinge on the question(s) to be addressed and the length of the behavioral sample needed to examine the problem at issue, e.g., whether: (a) actual motion is to be examined during the course of complete behaviors, such as a feeding sequence or reading a test paragraph with almost all the vowels and consonants in American English (e.g., the ‘Grandfather Passage’, Darley et al., 1975; also see Hiiemae et al., 2002); or (b) whether the changes in tongue body and surface shape produced during the production of a vowel sound or a consonant-vowel (C-V) phoneme are the subject of enquiry (e.g., Perkell, 1969; Kent, 1972; Stone and Lundberg, 1996). If the objective is to examine global measures such as rigid body motion, range of motion, and repetitive patterns, then long recordings such as those needed for (a) would be the choice. If local tongue shape is to be examined in depth, then approach (b) would be optimal. Another approach is to model the tongue, but that can only follow the acquisition of data on basic anatomy (Takemoto, 2001) and tongue function, such as electromyography (EMG) of tongue and hyoid musculature or its shape derived from images and tissue-point tracking (see Stone, 1990; Wilhelms-Tricarico, 1995; Akgul et al., 1999). Whatever the approach, some reference system is essential if shape changes are to be plotted, analyzed, and compared.

View larger version (101K): [in this window] [in a new window]   Figure 5. Tongue shapes in feeding as presented in Abd El Malek (1955). These drawings were based on still photographic images taken from a single subject. The upper pair of drawings shows the shape of the tongue as food is about to enter the mouth: The anterior tongue surface is hollowed to receive the ‘bite’, and the back is heaped. This shape is seen in VFG records as Stage I Transport is initiated. The bite is cradled in the depression on the tongue surface during ‘pull back’ (see text) until ‘tipped’ onto the occlusal surface of the post-canines by tongue movements analogous to those shown in the lower drawings. The lower pair of drawings illustrates the movements of the tongue as it rotates about its postero-anterior long axis and ‘tips’ the food onto the active side post-canine cheek teeth during processing. This behavior occurs in most chewing cycles, but when the bite is moved to the other side, the rotated tongue ‘collects’ the food and, by twisting in the other direction, ‘tips’ it onto the other post-canine occlusal table. Reproduced from Abd-el-Malek (1995), with permission from Blackwell Publishing.

 \\The history of tongue movement studies is correlated with the focus of the pioneers in the field who used what was then ‘state-of-the-art’ technology to address their questions. The classic swallowing paper (Ardran and Kemp, 1955) addressed the movements of the tongue using the earliest cinefluorographic (CFG) systems. The earliest CFG studies in speech focused on the supralaryngeal vocal tract, and particularly tongue-palate-lip interactions in distinct isolated phonemes or syllables (Perkell, 1969; Kent, 1972). These early and quite different disciplinary foci led to a situation where there were essentially no studies comparing movements in the two behaviors using the same subjects. The only comparative study (Hiiemae et al., 2002) shows, with subjects as their own controls, how mandibular, hyoid, and gross tongue movements differ between eating and speaking (Figs, 3, 4).

There are two continuing problems with investigations of tongue motion: first, the 2D representation of 3D events when standard imaging techniques are used (see Stone, 1990); and second, the speed with which these events can be recorded relative to their actual time course. These issues are discussed below in the context of each of the major data acquisition methods currently in use.

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    (1) METHODS OF DATA ACQUISITION
Major advances in the methods available for studying the tongue ‘in action’ have occurred in the last 15 or so years with the development of digital technology coupled with sophisticated computer software for data acquisition and reduction. However, there are important differences in the range of methodologies available to the speech language community as compared with those available to oral biologists interested in feeding mechanisms when intra-oral events (such as tongue movement) are to be examined. It is impractical to use intra-oral sensors to study feeding on solid foods.

    (2) ELECTROPALATOGRAPHY (EPG)
Electropalatography (EPG) uses intra-oral sensors. Subjects wear an individualized thin (5 mm thick) plastic ‘base-plate’ over the hard palate anchored to the maxillary teeth. Variable numbers of sensors which respond to tongue contact are embedded in the device (typically 32, 64, or 128 [Folkins and Kuehn, 1982]) or in the EPG3 device, which had 62 sensors (Hardcastle et al., 1991). [The commercially available EPG instrument (Kay Elemetrics Palatometer, 6300 Lincoln Park, NJ, USA) has 96 sensors.] The device has limitations, since the actual movement of the tongue is not measured, only the points of contact between its surface and the hard palate.

Although attempts to use EPG to measure tongue-palate contacts in feeding on solid foods failed (Heath, personal communication; Heath et al., 1980), Jack and Gibbon (1995) successfully measured tongue-palate contacts during the consumption of milk (liquid), yogurt (thick and creamy, but semi-liquid), and ‘jelly’. Chi-Fishman and Stone (1996) argue that EPG can be successfully used to study swallowing. However, the greater value of this method in speech research, when used in conjunction with other methods, is clear from Stone and Lundberg (1996), who compared the data obtained with the results of ultrasound in a study of tongue shape relative to palate. Similarly, the ‘glossometer’ used by Flege (1988) had intra-oral sensors (2 x 3 x 6 mm) embedded in a thin (3 mm) plastic ‘pseudopalate’ (comparable with the EPG device). Each sensor assembly has an LED and paired phototransistor. During data acquisition, the LEDs are pulsed in rapid succession, sending a beam of infrared light downward in a plane perpendicular to the occlusal plane of the teeth, so that the light is reflected from the tongue surface. The method cannot be used if anything is between the tongue and the palate, thus making it unacceptable for studies of feeding.

    (3) ELECTROMAGNETIC ARTICULOMETER (EMA)
Another intra-oral technique, the electromagnetic articulometer (EMA), designed for use in the transduction of articulatory movements during speech production, relies on the attachment of tiny transmitter coils (4 x 4 mm base with a thickness of 2.5 mm) to the tongue surface, lips, and velum (see Fig. 1 in Perkell et al., 1992). Coils of that size would rapidly detach if used during feeding on foods other than liquids, since the tongue surface twists toward the post-canine teeth in every chewing cycle (see Fig. 5). This device requires scrupulous calibration and much manipulation of the data obtained. Recently, Kaburagi and Honda (2001) have used an EMA system to obtain articulatory data to test their dynamic model of the tongue. An equivalent electromagnetic system (the Sirognathograph; see Hiiemae et al., 1996; Kazazoglu et al., 1994) accurately records jaw movement in 3D but cannot be used for the tongue, given the problem of intra-oral transducers when feeding. Other devices, such as strain gauges (Muller and Abbs, 1979), used to measure force or displacement of the lips and mandible, are viable tools for some speech research but, again, are unsuitable for feeding studies, because such methods cannot be applied to the tongue (Folkins and Kuehn, 1982).

    (4) APPLIED DIAGNOSTIC CINERADIOGRAPHY (CFG) AND VIDEOFLUOROGRAPHY (VFG)
This technique (CFG) became available in the 1950s and was used for the earliest studies of human swallowing (e.g., Ardran and Kemp, 1955). Perkell (1969) performed an exhaustive analysis of tongue movements in a single male subject while recording 13 ‘nonsense’ utterances (each with an unstressed followed by a stressed syllable in combinations of 7 vowels and 6 consonants as well as a single short sentence). The first clinical cameras were slow (25–30 frames per sec), and they also used 35-mm film, which had to be laboriously analyzed with special equipment. Radiation exposure for human subjects soon became a concern. After an initial flurry of activity, human studies (CFG) effectively ceased, only to resume in the late 1980s, when videofluorography (VFG), which requires much lower radiation levels, became a standard radiological diagnostic tool. Hiiemae (1967, 1968; Hiiemae and Ardran, 1968) used a 35-mm CFG diagnostic machine to analyze patterns of mandibular motion in rats. That pioneering study was followed by a series with opossums and then other non-human mammals (see Hiiemae and Crompton, 1985; Hiiemae, 2000). The duration of single masticatory cycles in humans ranges from about 450 to 1000 msec, with swallowing cycles the longest. A cycle 600 msec long recorded at 30 fps would include about 18 frames of film, or 36 interlaced videofields. Chewing cycles in small mammals are much faster, i.e., on the order of 250–350 ms; 9 frames or 18 videofields are inadequate for the study of such movements. Many of the early records (see Hiiemae and Palmer, 2001) were jerky and difficult to interpret. If such rapid motion was to be investigated, recording speed had to increase. Cinefluorographic facilities for animal studies were installed, first at the Yale Peabody Museum and then at the Museum of Comparative Zoology at Harvard. Those dedicated systems, filming at 100 fps, provided the basis for a series of studies in which the complete feeding process in a wide variety of mammals, including the role of the tongue in food transport, was described (Hiiemae et al., 1978; Hiiemae and Crompton, 1985, et seq.). Those mammalian studies formed the basis for the Process Model of Feeding in humans [discussed below (Hiiemae and Palmer, 1999)].

Those early efforts highlighted the need for reproducible standardized reference points within a complex system which has all its parts in motion. The first markers for the measurement of jaw movement were simple amalgam fillings on the buccal surface of canines or molars which appeared as black dots in the films. To examine tongue surface motion in animals (opossums, cats, hyraces, and macaques), investigators used a hypodermic needle to insert small metal ‘pellets’ just under the gustatory mucosa in anesthetized animals (see Hiiemae and Crompton, 1985, for specific references). Our recent human studies (e.g., Palmer et al., 1997) have used small lead discs (4 x 0.4 mm) cemented to upper and lower teeth, and to the tongue. Similar markers have also been used by Kuehn (1976), Tomura et al.(1981), and Stone and Lele (1992); also see Gay et al.(1994). Gold pellet tongue markers were used at the Microbeam Facility at the University of Wisconsin (Hamlet, 1989; Westbury et al., 2000; Tasko et al., 2002). However, with that technique, the only images were of actual marker positions (see below).

    Data reduction
To plot movements of markers over time in lateral projection radiographs (Figs. 3A, 3B), one must establish the Cartesian coordinates for each marker and then manipulate them to give its position relative to a reference plane within the orofacial complex (Fig. 2B). We have traditionally used a palatal reference with the X axis defined as the line between upper canine and molar markers (representative of the occlusal plane of both the upper post-canines and of the hard palate). This choice was dictated by the functional relationship between the tongue surface and hard palate in feeding. It works equally well for speaking (Fig. 3), since that also depends on the changing relationship between the tongue and hard-palate articulators. The ‘mandibular plane’ is defined by the line between lower canine and lower molar markers, and is perpendicular to the sagittal plane. This reference plane was used in some of the animal studies as a means of examining tongue movement ‘in isolation’ (see Thexton and McGarrick, 1998 see Thexton and McGarrick, 1999). [Details of the data reduction methods used in VFG studies on non-human mammals can be found in the references in Hiiemae and Crompton (1985) and Hiiemae (2000).]

    Lateral projection motion recording
Lateral projection motion recording of the orofacial complex provides a 2D image of 3D events (Hiiemae et al., 2002). This issue has been discussed by Stone (1990). However, many of the animal studies used a conventional 16-mm cinecamera, synchronized to the fluoroscopic camera, to record the animals in frontal view to provide a measure of medio-lateral jaw motion and to identify active and balancing sides in chewing.

In practice, research with human subjects can use one of two VFG projections: The lateral projection allows movements in the vertical and horizontal planes to be measured; the postero-anterior (P-A) projection, medio-lateral and vertical movements. (It should be noted that our human subjects research review boards approved protocols [Institutional Review Board, IRB] allowing us a lifetime total of 5 min of VFG recording per normal subject.) However, the rate of data acquisition is still 30 fps. This creates a problem when VFG is being recorded with other signals, such as EMG. Each videoframe is acquired over the entire 33.33-ms period as the videocamera tracks across and down the screen. Digital data, usually acquired at minimally 500 Hz, must be manipulated to reconcile with the VFG frame period. This means that an EMG event can be identified with a specific frame but not precisely where it occurs within the frame (see Palmer et al., 1992). High-speed digital cameras are now available but are not yet used for routine diagnostic VFG testing and so are not available for experimental purposes.

    (5) X-RAY MICROBEAM (XRMB)
An important data resource for tongue movement studies was created by the development of the x-ray microbeam. Invented in the early 1970s (Fujimura et al., 1973; Kiritani et al., 1977), this technology uses much lower levels of radiation than VFG. The limitation is that it images only the position of the gold pellets glued to the tongue and teeth. Additional instrumentation is needed to capture tongue surface information—for example, a sagittal ultrasound recording of the same utterance was recorded for each subject immediately after the microbeam record was obtained and matched to the pellet positions (see Stone, 1991). A large database (58 subjects) recorded by means of this instrument is now publicly available (Westbury, 1994). It has been used by Westbury et al.(2000) and Tasko et al.(2002) to examine tongue kinematics during speech and swallowing, respectively. Tasko et al. found so much variability in pellet trajectories among 12 subjects that it was remarkably difficult to develop a generalized description of tongue kinematics in liquid swallows.