- Email: [email protected]
A difference exists in somatosensory processing between the anterior and posterior parts of the tongue
Neuroscience Research 66 (2010) 173–179
Contents lists available at ScienceDirect
Neuroscience Research journal homepage: www.elsevier.com/locate/neures
A difference exists in somatosensory processing between the anterior and posterior parts of the tongue Kiwako Sakamoto a,c,d,*, Hiroki Nakata a,c, Koji Inui c, Mauro Gianni Perrucci a,b, Cosimo Del Gratta a,b, Ryusuke Kakigi c,d, Gian Luca Romani a,b a
ITAB-Institute for Advanced Biomedical Technologies, University of Chieti, Chieti, Italy Department of Clinical Sciences and Bio-imaging, University of Chieti, Chieti, Italy Department of Integrative Physiology, National Institute for Physiological Sciences, Okazaki, Japan d Department of Physiological Sciences, School of Life Sciences, The Graduate University for Advanced Studies, Hayama, Kanagawa, Japan b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 January 2009 Received in revised form 27 October 2009 Accepted 27 October 2009 Available online 6 November 2009
The somatic sensation of the tongue is necessary for daily life, but it is difficult to know the underlying neural mechanisms. In particular, because of the vomiting reflex and several morphological problems, no neuroimaging studies have examined somatosensory processing by stimulating the posterior part of the tongue, except for two magnetoencephalographic studies (Sakamoto et al., 2008a,b). This is the first study to clarify the human cortical processing for sensory perception by the posterior part of the tongue with a newly developed device and functional magnetic resonance imaging (fMRI). Stimulation of the left and right postero-lateral parts of the tongue induced significant activity in the primary somatosensory cortex (SI) and Brodmann area 40 in the right hemisphere and the anterior cingulate cortex (ACC). In contrast, antero-lateral stimulation produced activity only in the right SI. The activated region in SI was significantly larger following stimulation of the posterior than anterior part. These results indicate that a clear difference exists in somatosensory processing between stimulation of the antero-lateral and postero-lateral parts of the tongue, and a right hemisphere is dominant for the stimulation of both antero-lateral and postero-lateral areas. The activity in BA 40 and ACC may imply that the posterior of the tongue belongs to the visceral system. ß 2009 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.
Keywords: ACC Anterior cingulate cortex fMRI Functional magnetic resonance imaging Intraoral region Human
1. Introduction The tongue is essential for daily life; thus it is a wellcharacterized sensory organ. The tongue is an epithelial sac filled with muscles and connective tissue; these muscles can be controlled willfully and are generally referred to as skeletal muscles or voluntary striated muscles, which are divided into intrinsic and extrinsic muscles (Brand and Isselhard, 2003). In addition, the tongue has various functions: preservation of the position of the teeth and expression of feelings, speech, swallowing, and mastication. However, there are only few neuroimaging and neurophysiological studies focusing on several functions of the tongue. There are some problems with the investigation of the somatosensory processing of the tongue; that is, it is technically difficult to stimulate the posterior of the tongue. The first problem is the choice of a stimulator that can stimulate the tongue while the
* Corresponding author at: Department of Integrative Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan. Tel.: +81 564 55 7810; fax: +81 564 52 7913. E-mail address: [email protected] (K. Sakamoto).
subject is under a scanning coil without causing noise or technical problems. The second problem is that it is extremely difficult to fix the stimulator on the tongue stably, since the tongue itself is made of soft tissue and is convex in shape. The third problem is that tactile stimulation in this area frequently triggers the vomiting reflex. Recently, several studies using functional magnetic resonance imaging (fMRI), positron emission tomography (PET) has reported human brain activities evoked by tactile stimulation of the tongue, to clarify its somatotopic representation (Sakai et al., 1995; Pardo et al., 1997; Miyamoto et al., 2006). These studies showed that the postcentral gyrus was activated by mechanical stimulation of the tip of tongue. However, the precise regions and neural projections to the tongue remain unclarified, because in these studies, cortical activities following stimulation of the posterior part of the tongue were not recorded and the anatomical and histological aspects of the tongue were not focused on. Sakamoto et al. (2008a,b) stimulated both anterior and posterior parts of the tongue and examined brain activities by magnetoencephalography (MEG). They detected brain activities in hemispheres both contralateral and ipsilateral to the stimulation, which were estimated to be located around the tongue SI by electrical tactile stimulation of the
0168-0102/$ – see front matter ß 2009 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved. doi:10.1016/j.neures.2009.10.013
174
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
tongue. In addition, the latency of the contralateral hemisphere was significantly shorter than that of the ipsilateral hemisphere for all components, independent of the area stimulated. However, in these studies, only somatosensory evoked magnetic fields (SEFs) were recorded and researchers did not refer to regional cerebral blood flow in the postcentral gyrus. In the present study, we used a newly developed device that has a small projection on its lingual side. When the tongue was moved voluntarily, it was stimulated steadily. With this device, we aimed to investigate the somatosensory processing of the tongue using fMRI, and compared the brain activities following stimulation of the postero-lateral part of the tongue with those following stimulation of the antero-lateral part. Anatomically, the anterior two-thirds and posterior one-third of the tongue are innervated by different afferent fibers (Kandel et al., 1991). Thus, we hypothesize that a difference in brain activity exists in somatosensory processing, depending on the area of the tongue stimulated. 2. Materials and methods 2.1. Subjects Twelve normal right-handed subjects (six females and six males; mean age 23.6 years, range 20–32 years) participated in this study. All of them were according to the Edinburgh Inventory (Oldfield, 1971). The subjects did not have a history of neurological or psychiatric disorders. They had no pathological problems with the tongue as determined by a dentist (K.S.). The protocol was approved by the Institutional Ethics Committee of the Gabriele D’annunzio University, Chieti, Italy. Before the experiment, the subjects were informed in detail about the experiment, and gave their written informed consent for the study.
Fig. 1. Device for the stimulation of the tongue. The red arrow indicates a projection to stimulate the tongue. This projection has an elliptical shape and is 3 mm in diameter and 3 mm in height. It is easily attached and detached, but very stable during the experiments. The blue arrow shows bite blocks, which were made for each individual using a hydrophilic vinyl silicone material. In this figure, the projection is set to stimulate the right postero-lateral part of the tongue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
2.2. Sensory stimulation To stimulate different areas of the tongue, we fabricated an intraoral device for each individual using hydrophilic vinyl silicone impression material (EXAFAST/Putty Type, GC, Japan). The subject bit bilaterally into the EXAFAST, which was mixed uniformly and formed into two blocks. The jaws of the subject were positioned based on centric occlusion and opened about 5 mm between the upper and lower teeth to make a small space that was important to build the projection for stimulating the tongue. These blocks were used to create a wide space from the right to left canine teeth to allow comfortable frontal movement of the tongue. Then, we made four grooves on the lingual side of this device, which were positioned on the lingual cusp of the first premolar of the lower jaw and the distal-lingual cusp of the second molar of the lower jaw bilaterally. Next, we made a projection by polymethylmethacrylate (UNIFAST II, GC, Japan) on each groove (Fig. 1). This projection has an elliptical shape and is 3 mm in diameter and 3 mm in height. This projection is easily attached and detached, but very stable during the experiments. 2.3. Tasks The subjects performed one control task and four stimulation tasks. They were presented with a green-filled or red-filled circle cue via a PC and projector system (Fig. 2). The screen was located at an optical distance of 140 cm from the eyes of the subjects. Each cue was 30 mm in diameter and presented at the center of the visual display with a gray background. The green-filled circle was presented for 1 s and then the background only was shown for 1 s. When the green cue was presented, the subjects were instructed to perform a protruding movement to the front, that is, to move their tongue from a relaxed position at the bottom of the oral cavity to
Fig. 2. Simplified scheme of experimental design. Each cue was presented at the center of the visual display with a gray background. The green-filled circle was presented for 1 s and then only the gray background was shown for 1 s. When the green cue was presented, the subjects were instructed to protrude their tongue to the front, that is, to move their tongue from a relaxed position at the bottom of the oral cavity to the anterior teeth in each task to scratch the lateral side of their tongue as they protruded it (tongue-protrusion movement). When only the gray background was presented, the tongue was moved back to its original position. This alternating presentation was repeated for 21 s. After alternately presenting the green cues and the gray background, red-filled circles were presented for 21 s. The subjects were instructed to relax their tongue when the red cue was presented. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
the anterior teeth in each task so as to scratch the lateral side of their tongue with the projection. While only the gray background was presented, the tongue was moved back to its original position. This alternate presentation was repeated for 21 s. Therefore, the subjects repeated the tongue-protruding movement a total of 11 times. After the alternate green cue was presented, the red-filled circle was presented for 21 s. The subjects were asked to relax the tongue during the presentation of the red cue. In one session, the blocks were performed as follows: (red-cue block)–(green-cue block)–(red-cue block)–(green-cue block)–(red-cue block)– (green-cue block)–(red-cue block)–(green-cue block)–(red-cue block)–(green-cue block)–(red-cue block)–(green-cue block)– (red-cue block)–(green-cue block)–(red-cue block) (Fig. 2). One session lasted about 5 min. As a control task, the subjects were required to perform the tongue-protruding movement while no projection was set on the device. In the other four tasks, the subjects performed the movement with a projection. This projection was set at four different positions to stimulate specific areas of the tongue: the left antero-lateral, left postero-lateral, right antero-lateral, and right postero-lateral areas (Fig. 3). In each task, only one projection was set for the target area (Figs. 1 and 3). After each session, this projection was replaced with another groove; which was attached and detached by the operator (K.S.) out of the MR gantry. Before
175
starting each session, the head was positioned accurately. In addition, the lower jaw was kept in the same position using an intraoral device. Before the experiment, we marked the projection with red edible ink and checked whether the projection could stimulate each target area precisely. For each area, the anterolateral and postero-lateral parts were marked about 2 and 4.5 cm from the tip of the tongue, respectively, and the lengths of these areas were all about 0.5 cm along the rostro-caudal axis of the tongue. In addition, each subject practiced the movement of protruding the tongue for 3 min to understand the experimental procedure and objective before the recordings. Through these experiments, the subjects felt a clear mechanical cutaneous stimulation of the tongue without pain. They felt only tactile sensation arising from the scratching of the tongue, and did not complain of any discomfort such as pain or a feeling of sickness. The order of tasks was randomized for each subject and counterbalanced across all subjects. 2.4. fMRI data acquisition All images were acquired using a 1.5 T MR scanner (Magnetom Vision; Siemens, Erlangen, Germany). For each functional imaging sequence, the subjects performed a block design alternating seven tongue movement blocks of 21 s (green cue) with eight rest blocks (red cue) having the same duration. Blood oxygen level dependent (BOLD) contrast functional images were acquired using T2*weighted echo planar imaging (EPI) free induction decay (FID) sequences with the following parameters: TR 3096 ms, TE 60 ms, matrix size 64 64, FOV 256 mm, in-plane voxel size 4 mm 4 mm, flip angle 908, slice thickness 4 mm and no gap. For anatomical reference, a high resolution structural volume was acquired at the end of the session via a 3D magnetizationprepared rapid gradient echo (MPRAGE) sequence (TR 9.7 ms, TE 4 ms, sagittal, matrix 256 256, FOV 256 mm, slice thickness 1 mm, no gap, in-plane voxel size 1 mm 1 mm, flip angle 128, slice thickness 1 mm) for each subject. 2.5. Data analysis
Fig. 3. Schematic drawings of the relationship between tongue movement and a protrusion in each task. Tongue movement itself is the same in the control and all four tasks. Each part of the tongue is stimulated with a protrusion by a protruded movement of the tongue. The shaded areas of the tongue are stimulated by a protrusion. LA = left antero-lateral stimulation; LP = left postero-lateral stimulation; RA = right antero-lateral stimulation; RP = right postero-lateral stimulation
Raw data were analyzed using Statistical Parametric Mapping (SPM2, Wellcome Department of Cognitive Neurology, London, UK) (Friston et al., 1995) implemented in Matlab (Mathworks, Sherborn, Massachusetts, USA). The first five scans of each run were discarded from the analysis due to unsteady magnetization. The effect of head motion was corrected by realigning all scans to the first one. After being coregistered with a T1-weighted structural volume, they were normalized in the functional scans to a standard stereotaxic space (Montreal Neurological Institute (MNI) template). Then, the images were spatially smoothed using an isotropic Gaussian kernel of 8 mm full width at half maximum (FWHM) in the x, y and z axes. Temporal filters were also applied and low frequency noise and global changes in the signals were removed. Statistical analysis was performed at two levels. First, individual task-related activation was evaluated. Second, the summary data for each individual were incorporated into the second-level analysis using a random-effect model (Friston et al., 1999) to make inferences at a population level. In the first phase of an individual analysis, we analyzed the active areas during each task compared with those during the rest periods of the same session. However, the tongue could be stimulated not only by scratch stimuli but also by the device itself and/or intraoral structures such as the palate and teeth during the protruding movement. Then, to investigate only the somatosensory-related activation by removing the motor-related activation and somatosensory-related activation for the device and/or intraoral structures, we analyzed the subtraction images obtained from the
176
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
contrasts as follows: the task in which the projection was on the left antero-lateral of the tongue (left antero-lateral) minus nonprojection task (Control) (LA), left postero-lateral minus Control (LP), right antero-lateral minus Control (RA), and right posterolateral minus Control (RP). Group analysis (random-effects model) of each task was performed by entering contrast images into a one-sample t-test. The statistical threshold was set at P < 0.001, uncorrected (no significant activation could be found in all conditions when we set our threshold at P < 0.05, corrected). Activation clusters <10 voxels were removed. The locations of brain activity were transformed MNI coordinates into the Talairach standard brain coordinates (Talairach and Tournoux, 1988). As a further analysis, the threshold of the individual data was set P < 0.005, uncorrected to identify the activation of the difference among conditions in detail.
right hemispheres, because the left and right ACC are very close to each other. In addition to SI, stimulation of the posterior part of the tongue activated a slightly more lateral region in the inferior parietal lobule (BA 40), which was located at coordinates (X, Y, Z = 61, 28, 33) in LP and (63, 28, 33) in RP. The ACC activities were located at coordinates (X, Y, Z = 0, 2, 41) in LP and ( 4, 3, 49) in RP. In the individual data, SI activation of the right hemisphere was recorded in all subjects. In the left hemisphere, this activation was found in LP and RP from three subjects. This coordinates for each subject was at (X, Y, Z = 63, 28, 33), ( 63, 28, 33) and ( 63, 28, 31) in LP, showing BA 40, and at (X, Y, Z = 61, 26, 31), ( 61, 28, 33) and ( 61, 26, 32) in RP, indicating BA 40.
3. Results
In the group analysis, SI activation evoked by LA and RA stimulation converged at BA 2. On the other hand, posterior stimulation, LP and RP, indicated a broad activation in BA 2 of SI and BA 40 (Fig. 4). In addition, in order to examine these results in more detail, we analyzed the individual data about the number of active voxels around SI. This procedure was allowed to avoid the canceling out the spatial inter-individual variability in a group analysis (Arienzo et al., 2006). Statistical analysis was performed by a paired t-test (SPSS 14.0 for Windows). SI had significantly larger activation with posterior stimulation than with anterior stimulation (LA vs. LP and RA vs. RP, p < 0.05, respectively) (Fig. 6).
3.1. Anterior stimulation In the group analysis, the LA and RA conditions produced brain activity around the right primary somatosensory cortex (SI), but there was no significant activity in the left hemisphere for each condition (Fig. 4). Significant SI activation was found in the crown of the postcentral gyrus (Brodmann’s area 2; BA 2), which is located at (X, Y, Z = 61, 24, 34) in LA and (63, 24, 34) in RA. In the individual data, this SI activation of the right hemisphere was recorded from all 12 subjects, whereas the SI activation of the left hemisphere was observed from two subjects in LA and none in RA. The location of SI activation from these two subjects was at (X, Y, Z = 63, 24, 34) and ( 61, 25, 36), respectively, showing BA 2. 3.2. Posterior stimulation As for the group data, LP and RP stimulation also induced significant activation at SI in the right hemisphere and the anterior cingulate cortex (ACC) (Figs. 4 and 5). No significant activation was found around the SI of the left hemisphere. It was difficult to determine an asymmetry in ACC activation between the left and
3.3. Comparison of the individual data between anterior and posterior sensory processing of the tongue around SI
4. Discussion In the present study, we investigated the somatosensory processing of the tongue using a newly developed device, and identified the cortical somatosensory representation of the tongue in the human brain in more detail than had previous reports (Sakai et al., 1995; Pardo et al., 1997; Miyamoto et al., 2006). Our results suggested that the somatosensory representation of the posterior part of the tongue was larger than that of the anterior part in SI, and expanded to BA 40. Furthermore, the ACC was only activated by stimulation of the posterior tongue.
Fig. 4. Group activation map showing activated brain regions in four conditions. The figures around SI areas were enlarged in all conditions. Using the SPM2 template, areas showing an increase in BOLD-signal are superimposed on a 3D-rendered standard brain. Although our statistical threshold was P < 0.001 (uncorrected), the threshold was lowered at P < 0.05 (uncorrected) for this figure for display purposes. LA = left antero-lateral stimulation; LP = left postero-lateral stimulation; RA = right antero-lateral stimulation; RP = right postero-lateral stimulation
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
177
2006). They showed the SI activity in the postcentral gyrus, which is consistent with the homunculus reported by Penfield and Boldrey (1937). The present study obtained new findings by stimulating the postero-lateral area of the tongue. Until now, there had been crucial problems investigating the somatosensory representation related to the posterior part of the tongue (see Section 1). As a result, the somatosensory representation of this area existed in SI and BA 40, whereas the antero-lateral part of the tongue converged at the SI (Figs. 4 and 6). Neuroanatomically, the anterior two-thirds of the tongue are innervated by the afferent fibers that travel in a branch of the trigeminal nerve (V) called the lingual nerve. The posterior onethird of the tongue is innervated by the afferent fibers that travel in the lingual branch of the glossopharyngeal nerve (IX) (Kandel et al., 1991). From the result of the present study, therefore, the anterolateral part of the tongue should be innervated by the trigeminal nerve, but the postero-lateral part may be innervated by both the trigeminal and glossopharyngeal nerves, which may underlie the clear differences in brain activities, although the border of the innervation zone and pattern between the trigeminal nerve and the glossopharyngeal nerve is vague and unclear anatomically. Fitzgerald and Law (1958) reported lingual-hypoglossal connexion between the lingual nerve and hypoglossal nerve in the tongue, so we speculate that the same connection is formed between the lingual nerve and glossopharyngeal nerve. Doty et al. (2009) also suggested that branches of the glossopharyngeal nerve extend anteriorly beyond the sulcus terminalis and circumvallate papillae, with extensions occurring along the lateral lingual margin anterior to the foliate papillae. In addition, anastomoses were identified between the glossopharyngeal nerve and the lingual nerve, raising the possibility of functional interactions between the trigeminal nerve and the glossopharyngeal nerve. This notion was supported by a recent anatomical study (Zur et al., 2004). 4.2. Visceral sensation
Fig. 5. Neural activation in the ACC overlaid on an anatomically normalized MRI. Only LP and RP stimulation produced these activities. A brighter color represents a higher statistical significance. The map is threshold at P < 0.001 (uncorrected) in LP and P < 0.005 (uncorrected), respectively.
4.1. The cortical somatosensory representation Several studies using neuroimaging methods have found a somatosensory representation after stimulating the tip of the tongue (Sakai et al., 1995; Pardo et al., 1997; Miyamoto et al.,
Fig. 6. Comparison of the individual data on the number of voxels around SI. Error bars denote the standard error (SE) across subjects. Stimulation of the posterior part of the tongue activated SI in wider regions than that of the anterior part. The statistical analysis was performed with a paired t-test. *P < 0.05.
In addition to SI, BA 40 immediately lateral to SI was activated in LP and RP conditions. Indeed, it seems difficult to determine why this area was activated following stimulation of the posterior part of the tongue. However, we assumed that activation of BA 40 included neuronal activation associated with visceral sensation following the stimulation of the posterior tongue. Previous studies demonstrated that the cortical representation of visceral organs differs from that of the somatosensory system, using fMRI (Aziz et al., 2000; Hobday et al., 2001; Lotze et al., 2001; Strigo et al., 2003; Eickhoff et al., 2006; Ladabaum et al., 2007), and MEG (Schnitzler et al., 1999). For instance, Hobday et al. (2001) observed activation in the inferior part of SI and BA 40 following visceral rectal stimulation, but only in SI following somatic anal stimulation. Ladabaum et al. (2007), who recorded brain activity following gastric distension, found no evidence of activation of S1, but found activation in a broad region of BA 40. Taking these studies into consideration, it is likely that BA 40 was activated following visceral stimulation. However, further studies would be necessary to determine the relationship between the neural activity of BA 40 and visceral sensation and to clarify the characteristics of this area with respect to tongue stimulation. In general, the ACC plays an important role in sensory, motor, cognitive and emotional information (Bush et al., 2000) and pain processing (Schnitzler and Ploner, 2000; Vogt, 2005; Qiu et al., 2006). Our results demonstrated that the ACC was activated only during the postero-lateral stimulation (Fig. 5). Some studies showed that the ACC was often concerned with visceral sensation (see Table 1, for findings of previous studies of visceral sensation). For example, Hobday et al. (2001) noted that the ACC was activated by visceral stimulation, not by somatic stimulation. It appears that
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
178 Table 1 Previous neuroimaging studies for visceral sensation. Author name
Stimulated area
Recording
ACC
L-insula
R-insula
L-SI
R-SI
L-BA 40
R-BA 40
Aziz et al. (1997) Binkofski et al. (1998) Aziz et al. (2000) Hobday et al. (2001) Ladabaum et al. (2001) Lotze et al. (2001) Gregory et al. (2003) Bittorf et al. (2006) Coen et al. (2007) Ladabaum et al. (2007) Coen et al. (2008)
Esophagus Esophagus Esophagus Rectum Gastric distention Rectum Esophagus Rectum Esophagus Gastric distention Esophagus
PET fMRI fMRI fMRI PET fMRI fMRI fMRI fMRI fMRI fMRI
O O O O O O O O O
O O O O O O O O
O O O O O O O O O
O O O O O O
O O O O O O O
O O
O O O
These studies were identified by a thorough search of literature, using PubMed (http://www.ncbi.nlm.nih.gov/pubmed/). fMRI = functional magnetic resonance imaging; PET = positron emission tomography; L = left; R = right; ACC = anterior cingulate cortex; SI = primary somatosensory cortex; BA 40 = Brodmann’s area 40.
their results are consistent with our findings. Thus, we considered that our ACC activation might reflect the attribution of the viscera, because the viscera have a complex peripheral nervous system that allows for a wide variety of autonomic functions (Ness and Gebhart, 1990). On the other hand, previous studies of visceral stimulation often showed activation in insula, suggesting that it plays some important roles in visceral processing (Table 1), but our data did not show activation in this region. There are several possible explanations for this. The first is a difference in characteristics between the ACC and insula within the neural networks concerning visceral sensation. For instance, Gregory et al. (2003) reported that selective attention to the esophageal (visceral) target stimuli activated the SI, secondary somatosensory cortex (SII), and ACC rather than insula, suggesting that the ACC and insula do not always show similar activation during visceral stimulation. Our tasks did not belong to general passive tasks that elicit somatosensory activation, and the subjects were required to move their tongue ‘actively’. Therefore, the enhanced activation in the ACC during attention to the stimuli for tongue scratching might be due to the visceral sensation under LP and RP conditions. Second, insula may not always be activated following visceral sensation, even if both ACC and insula were involved in the limbic system. In our literature search shown in Table 1, several studies showed that the activation of ACC and insula was sometimes not detected. Under LP and RP conditions of the present study, stimulation of the posterior part of the tongue elicited ACC activation, which might be insufficient for achieving insula activation. Indeed, it might be difficult to clarify why the insula was not activated during stimulation of the posterior part of the tongue. Further study is needed. 4.3. Hemisphere lateralization Our results showed the cortical representation on the right hemisphere, but not the left hemisphere. The cortical responses to stimulation of the tongue have been a matter of debate. Anatomically, previous studies in monkeys reported that some neurons were activated in both the contralateral and ipsilateral hemispheres following stimulation of the tongue (Manger et al., 1996; Jain et al., 2001). In humans, some neurophysiological studies using electroencephalography (EEG) and MEG also showed bilateral activation in the SI following the tongue’s stimulation (Ishiko et al., 1980; Altenmuller et al., 1990; Karhu et al., 1991; Disbrow et al., 2003). In our previous MEG studies, we fabricated individual intraoral devices, and recorded SEFs (Sakamoto et al., 2008a,b). The tongue was stimulated with a concentric bipolar electrode in four areas: the right and left antero-lateral margins, and the right and left postero-lateral
margins. Bilateral activations were observed under all conditions, which stimulated four different areas of the tongue. However, our previous data did not show the right dominant response in some components of SEFs. There are three possible explanations for the discrepancy between the present neuroimaging findings and some previous neurophysiological studies including our own MEG studies. The first possibility is that somatosensory processing includes asymmetric neural activation. That is, as several neuroimaging studies already showed (Perlmutter et al., 1987; Fox and Applegate, 1988; Naito et al., 2005; Nihashi et al., 2005; Eickhoff et al., 2008), the brain’s response should be stronger in the right hemisphere than the left for somatosensory processing. We believe that the present study also indicated this asymmetric neural activation. Indeed, our method of stimulation may be unable to elicit clear activation in the left hemisphere, compared to general electrical stimulation. If so, it might be difficult to detect the response in the left hemisphere. The second possibility is a negative motor effects on the left somatosensory areas. Some neuroimaging studies have also provided evidence that activation of the sensorimotor cortex representing the oral and facial regions during volitional swallowing and mastication showed left hemispheric preference (Martin et al., 2004, 2007; Shinagawa et al., 2004). From these studies, there is a possibility that active movement of the tongue affects SI activity in the left hemisphere. A third explanation is that the above two possibilities may be interrelated. 5. Conclusion In conclusion, we could record the human brain response after stimulating the postero-lateral part of the tongue, and compared it with the antero-lateral part of the tongue. We showed that a difference existed in the somatosensory processing of the tongue, particularly around the SI and ACC. These findings also demonstrate the possibility that stimulating the posterior of the tongue elicited a visceral sensation. Acknowledgements We would like to thank Dr. A. Ferretti for general assistance during the measurements, Ms. T.L. Chen and N. Savini for the help they gave in recruiting the subjects and Dr. H.C. Tanabe, Dr. D.N. Saito and Dr. Y. Morito for analyzing the data. References Altenmuller, E., Cornelius, C.P., Buettner, U.W., 1990. Somatosensory evoked potentials following tongue stimulation in normal subjects and patients with lesions of the afferent trigeminal system. Electroencephalogr. Clin. Neurophysiol. 77, 403–415.
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179 Arienzo, D., Babiloni, C., Ferretti, A., Caulo, M., Grarra, C., Tartaro, A., Rossini, P.M., Romani, G.L., 2006. Somatotopy of anterior cingulate cortex (ACC) and supplementary motor area (SMA) for electric stimulation of the median and tibial nerves: an fMRI study. NeuroImage 33, 700–705. Aziz, Q., Andersson, J.L., Valind, S., Sundin, A., Hamdy, S., Jones, A.K., Foster, E.R., La˚ngstro¨m, B., Thompson, D.G., 1997. Identification of human brain loci processing esophageal sensation using positron emission tomography. Gastroenterology 113, 50–59. Aziz, Q., Thompson, D.G., Ng, V.W., Hamdy, S., Sarkar, S., Brammer, M.J., Bullmore, E.T., Hobson, A., Tracey, I., Gregory, L., Simmons, A., Williams, S.C., 2000. Cortical processing of human somatic and visceral sensation. J. Neurosci. 20, 2657–2663. Binkofski, F., Schnitzler, A., Enck, P., Frieling, T., Posse, S., Seitz, R.J., Freund, H.J., 1998. Somatic and limbic cortex activation in esophageal distention: a functional magnetic resonance imaging study. Ann. Neurol. 44, 811–815. Bittorf, B., Ringler, R., Forster, C., Hohenberger, W., Matzel, K.E., 2006. Cerebral representation of the anorectum using functional magnetic resonance imaging. Br. J. Surg. 93, 1251–1257. Bush, G., Luu, P., Posner, M.I., 2000. Cognitive and emotional influences in anterior cingulate cortex. Trends Cogn. Sci. 4, 215–222. Brand, R.W., Isselhard, D.E., 2003. Anatomy of Orofacial Structures, 7th ed. Mosby, pp. 300–304. Coen, S.J, Gregory, L.J., Ya´gu¨ez, L., Amaro Jr., E., Brammer, M., Williams, S.C., Aziz, Q., 2007. Reproducibility of human brain activity evoked by esophageal stimulation using functional magnetic resonance imaging. Am. J. Physiol. Gastrointest. Liver Physiol. 293, G188–G197. Coen, S.J., Aziz, Q., Ya´gu¨ez, L., Brammer, M., Williams, S.C., Gregory, L.J., 2008. Effects of attention on visceral stimulus intensity encoding in the male human brain. Gastroenterology 135, 2065–2074. Disbrow, E.A., Hinkley, L.B., Roberts, T.P., 2003. Ipsilateral representation of oral structures in human anterior parietal somatosensory cortex and integration of inputs across the midline. J. Comp. Neurol. 467, 487–495. Doty, R.L., Cummins, D.M., Shibanova, A., Sanders, I., Mu, L., 2009. Lingual distribution of the human glossopharyngeal nerve. Acta Otolaryngol. 129, 52–56. Eickhoff, S.B., Lotze, M., Wietek, B., Amunts, K., Enck, P., Zilles, K., 2006. Segregation of visceral and somatosensory afferents: an fMRI and cytoarchitectonic mapping study. NeuroImage 31, 1004–1014. Eickhoff, S.B., Grefkes, C., Fink, G.R., Zilles, K., 2008. Functional lateralization of face, hand, and trunk representation in anatomically defined human somatosensory areas. Cereb. Cortex 18, 2820–2830. Fitzgerald, M.J., Law, M.E., 1958. The peripheral connexions between the lingual and hypoglossal nerves. J. Anat. 92, 178–188. Fox, P.T., Applegate, C.N., 1988. Right-hemisphere dominance for somatosensory processing in humans. Soc. Neurosci. Abstr. 18, 760. Friston, K.J., Holmes, A.P., Worsley, K.J., Poline, J.B., Frith, C.D., Frackowiak, R.S.J., 1995. Statistical parametric maps in functional imaging: a general linear approach. Hum. Brain Mapp. 2, 189–210. Friston, K.J., Holmes, A.P., Worsley, K.J., 1999. How many subjects constitute a study? NeuroImage 10, 1–5. Gregory, L.J., Ya´gu¨ez, L., Williams, S.C., Altmann, C., Coen, S.J., Ng, V., Brammer, M.J., Thompson, D.G., Aziz, Q., 2003. Cognitive modulation of the cerebral processing of human oesophageal sensation using functional magnetic resonance imaging. Gut 52, 1617–1671. Hobday, D.I., Aziz, Q., Thacker, N., Hollander, I., Jackson, A., Thompson, D.G., 2001. A study of the cortical processing of ano-rectal sensation using functional MRI. Brain 124, 361–368. Ishiko, N., Hanamori, T., Murayama, N., 1980. Spatial distribution of somatosensory responses evoked by tapping the tongue and finger in man. Electroencephalogr. Clin. Neurophysiol. 50, 1–10. Jain, N., Qi, H.X., Catania, K.C., Kaas, J.H., 2001. Anatomic correlates of the face and oral cavity representations in the somatosensory cortical area 3b of monkeys. J. Comp. Neurol. 429, 455–468. Kandel, E., Schwartz, J.H., Jessell, T.M., 1991. Principles of Neural Science, 3rd ed. Appleton & Lange, Connecticut, 519 pp. Karhu, J., Hari, R., Lu, S.T., Paetau, R., Rif, J., 1991. Cerebral magnetic fields to lingual stimulation. Electroencephalogr. Clin. Neurophysiol. 80, 459–468. Ladabaum, U., Minoshima, S., Hasler, W.L., Cross, D., Chey, W.D., Owyang, C., 2001. Gastric distention correlates with activation of multiple cortical and subcortical regions. Gastroenterology 120, 369–376.
179
Ladabaum, U., Roberts, T.P., McGonigle, D.J., 2007. Gastric fundic distension activates fronto-limbic structures but not primary somatosensory cortex: a functional magnetic resonance imaging study. NeuroImage 34, 724–732. Lotze, M., Wietel, B., Birbaumer, N., Ehrhardt, J., Grodd, W., Enck, P., 2001. Cerebral activation during anal and rectal stimulation. NeuroImage 14, 1027–1034. Manger, P.R., Woods, T.M., Jones, E.G., 1996. Representation of face and intra-oral structures in area 3b of macaque monkey somatosensory cortex. J. Comp. Neurol. 371, 513–521. Martin, R.E., MacIntosh, B.J., Smith, R.C., Barr, A.M., Stevens, T.K., Gati, J.S., Menon, R.S., 2004. Cerebral areas processing swallowing and tongue movement are overlapping but distinct: a functional magnetic resonance imaging study. J. Neurophysiol. 92, 2428–2443. Martin, R., Barr, A., MacIntosh, B., Smith, R., Stevens, T., Taves, D., Gati, J., Menon, R., Hachinski, V., 2007. Cerebral cortical processing of swallowing in older adults. Exp. Brain Res. 176, 12–22. Miyamoto, J.J., Honda, M., Saito, D.N., Okada, T., Ono, T., Ohyama, K., Sadato, N., 2006. The representation of the human oral area in the somatosensory cortex: a functional MRI Study. Cereb. Cortex 16, 669–675. Naito, E., Roland, P.E., Grefkes, C., Choi, H.J., Eickhoff, S., Geyer, S., Zilles, K., Ehrsson, H.H., 2005. Dominance of the right hemisphere and role of area 2 in human kinesthesia. J. Neurophysiol. 93, 1020–1034. Ness, T.J., Gebhart, G.F., 1990. Visceral pain: a review of experiment studies. Pain 41, 167–234. Nihashi, T., Naganawa, S., Sato, C., Kawai, H., Nakamura, T., Fukatsu, H., Ishigaki, T., Aoki, I., 2005. Contralateral and ipsilateral responses in primary somatosensory cortex following electrical median nerve stimulation—an fMRI study. Clin. Neurophysiol. 116, 842–848. Oldfield, R.C., 1971. The assessment and analysis of handedness: the Edinburgh inventory. Neuropsychologia 9, 97–113. Pardo, J.V., Wood, T.D., Costello, P.A., Pardo, P.J., Lee, J.T., 1997. PET study of the localization and laterality of lingual somatosensory processing in humans. Neurosci. Lett. 234, 23–26. Penfield, W., Boldrey, E., 1937. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60, 389–443. Perlmutter, J.S., Powers, W.J., Herscovitch, P., Fox, P.T., Raichle, M.E., 1987. Regional asymmetries of cerebral blood flow, blood volume, and oxygen utilization and extraction in normal subjects. J. Cereb. Blood Flow Metab. 7, 64–67. Qiu, Y., Noguchi, Y., Honda, M., Nakata, H., Tamura, Y., Tanaka, S., Sadato, N., Wang, X., Inui, K., Kakigi, R., 2006. Brain processing of the signals ascending through unmyelinated C fibers in humans: an event-related fenctional magnetic resonance imaging study. Cereb. Cortex 16, 1289–1295. Sakai, K., Watanabe, E., Onodera, Y., Itagaki, H., Yamamoto, E., Koizumi, H., Miyashita, Y., 1995. Functional mapping of the human somatosensory cortex with echo-planar MRI. Magn. Reson. Med. 33, 736–743. Sakamoto, K., Nakata, H., Kakigi, R., 2008a. Somatosensory evoked magnetic fields following stimulation of the tongue in humans. Clin. Neurophysiol. 119, 1664– 1673. Sakamoto, K., Nakata, H., Kakigi, R., 2008b. Somatotopic representation of the tongue in human secondary somatosensory cortex. Clin. Neurophysiol. 119, 2125–2134. Schnitzler, A., Volkmann, J., Enck, P., Frieling, T., Witte, O.W., Freund, H.J., 1999. Different cortical organization of visceral and somatic sensation in humans. Eur. J. Neurosci. 11, 305–315. Schnitzler, A., Ploner, M., 2000. Neurophysiology and functional neuroanatomy of pain perception. J. Clin. Neurophysiol. 17, 592–603. Shinagawa, H., Ono, T., Honda, E., Sasaki, T., Taira, M., Iriki, A., Kuroda, T., Ohyama, K., 2004. Chewing-side preference is involved in differential cortical activation patterns during tongue movements after bilateral gum-chewing: a functional magnetic resonance imaging study. J. Dent. Res. 83, 762–766. Strigo, I.A., Duncan, G.H., Boivin, M., Bushnell, M.C., 2003. Differentiation of visceral and cutaneous pain in the human brain. J. Neurophysiol. 89, 3294–3303. Talairach, J., Tournoux, P., 1988. Co-planar Stereotaxic Atlas of the Human Brain. Thieme, Stuttgart. Vogt, B.A., 2005. Pain and emotion interactions in subregions of the cingulate gyrus. Nat. Rev. Neurosci. 6, 533–544. Zur, K.M., Mu, L., Sanders, I., 2004. Distribution pattern of the human lingual nerve. Clin. Anat. 17, 88–92.
Contents lists available at ScienceDirect
Neuroscience Research journal homepage: www.elsevier.com/locate/neures
A difference exists in somatosensory processing between the anterior and posterior parts of the tongue Kiwako Sakamoto a,c,d,*, Hiroki Nakata a,c, Koji Inui c, Mauro Gianni Perrucci a,b, Cosimo Del Gratta a,b, Ryusuke Kakigi c,d, Gian Luca Romani a,b a
ITAB-Institute for Advanced Biomedical Technologies, University of Chieti, Chieti, Italy Department of Clinical Sciences and Bio-imaging, University of Chieti, Chieti, Italy Department of Integrative Physiology, National Institute for Physiological Sciences, Okazaki, Japan d Department of Physiological Sciences, School of Life Sciences, The Graduate University for Advanced Studies, Hayama, Kanagawa, Japan b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 January 2009 Received in revised form 27 October 2009 Accepted 27 October 2009 Available online 6 November 2009
The somatic sensation of the tongue is necessary for daily life, but it is difficult to know the underlying neural mechanisms. In particular, because of the vomiting reflex and several morphological problems, no neuroimaging studies have examined somatosensory processing by stimulating the posterior part of the tongue, except for two magnetoencephalographic studies (Sakamoto et al., 2008a,b). This is the first study to clarify the human cortical processing for sensory perception by the posterior part of the tongue with a newly developed device and functional magnetic resonance imaging (fMRI). Stimulation of the left and right postero-lateral parts of the tongue induced significant activity in the primary somatosensory cortex (SI) and Brodmann area 40 in the right hemisphere and the anterior cingulate cortex (ACC). In contrast, antero-lateral stimulation produced activity only in the right SI. The activated region in SI was significantly larger following stimulation of the posterior than anterior part. These results indicate that a clear difference exists in somatosensory processing between stimulation of the antero-lateral and postero-lateral parts of the tongue, and a right hemisphere is dominant for the stimulation of both antero-lateral and postero-lateral areas. The activity in BA 40 and ACC may imply that the posterior of the tongue belongs to the visceral system. ß 2009 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.
Keywords: ACC Anterior cingulate cortex fMRI Functional magnetic resonance imaging Intraoral region Human
1. Introduction The tongue is essential for daily life; thus it is a wellcharacterized sensory organ. The tongue is an epithelial sac filled with muscles and connective tissue; these muscles can be controlled willfully and are generally referred to as skeletal muscles or voluntary striated muscles, which are divided into intrinsic and extrinsic muscles (Brand and Isselhard, 2003). In addition, the tongue has various functions: preservation of the position of the teeth and expression of feelings, speech, swallowing, and mastication. However, there are only few neuroimaging and neurophysiological studies focusing on several functions of the tongue. There are some problems with the investigation of the somatosensory processing of the tongue; that is, it is technically difficult to stimulate the posterior of the tongue. The first problem is the choice of a stimulator that can stimulate the tongue while the
* Corresponding author at: Department of Integrative Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan. Tel.: +81 564 55 7810; fax: +81 564 52 7913. E-mail address: [email protected] (K. Sakamoto).
subject is under a scanning coil without causing noise or technical problems. The second problem is that it is extremely difficult to fix the stimulator on the tongue stably, since the tongue itself is made of soft tissue and is convex in shape. The third problem is that tactile stimulation in this area frequently triggers the vomiting reflex. Recently, several studies using functional magnetic resonance imaging (fMRI), positron emission tomography (PET) has reported human brain activities evoked by tactile stimulation of the tongue, to clarify its somatotopic representation (Sakai et al., 1995; Pardo et al., 1997; Miyamoto et al., 2006). These studies showed that the postcentral gyrus was activated by mechanical stimulation of the tip of tongue. However, the precise regions and neural projections to the tongue remain unclarified, because in these studies, cortical activities following stimulation of the posterior part of the tongue were not recorded and the anatomical and histological aspects of the tongue were not focused on. Sakamoto et al. (2008a,b) stimulated both anterior and posterior parts of the tongue and examined brain activities by magnetoencephalography (MEG). They detected brain activities in hemispheres both contralateral and ipsilateral to the stimulation, which were estimated to be located around the tongue SI by electrical tactile stimulation of the
0168-0102/$ – see front matter ß 2009 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved. doi:10.1016/j.neures.2009.10.013
174
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
tongue. In addition, the latency of the contralateral hemisphere was significantly shorter than that of the ipsilateral hemisphere for all components, independent of the area stimulated. However, in these studies, only somatosensory evoked magnetic fields (SEFs) were recorded and researchers did not refer to regional cerebral blood flow in the postcentral gyrus. In the present study, we used a newly developed device that has a small projection on its lingual side. When the tongue was moved voluntarily, it was stimulated steadily. With this device, we aimed to investigate the somatosensory processing of the tongue using fMRI, and compared the brain activities following stimulation of the postero-lateral part of the tongue with those following stimulation of the antero-lateral part. Anatomically, the anterior two-thirds and posterior one-third of the tongue are innervated by different afferent fibers (Kandel et al., 1991). Thus, we hypothesize that a difference in brain activity exists in somatosensory processing, depending on the area of the tongue stimulated. 2. Materials and methods 2.1. Subjects Twelve normal right-handed subjects (six females and six males; mean age 23.6 years, range 20–32 years) participated in this study. All of them were according to the Edinburgh Inventory (Oldfield, 1971). The subjects did not have a history of neurological or psychiatric disorders. They had no pathological problems with the tongue as determined by a dentist (K.S.). The protocol was approved by the Institutional Ethics Committee of the Gabriele D’annunzio University, Chieti, Italy. Before the experiment, the subjects were informed in detail about the experiment, and gave their written informed consent for the study.
Fig. 1. Device for the stimulation of the tongue. The red arrow indicates a projection to stimulate the tongue. This projection has an elliptical shape and is 3 mm in diameter and 3 mm in height. It is easily attached and detached, but very stable during the experiments. The blue arrow shows bite blocks, which were made for each individual using a hydrophilic vinyl silicone material. In this figure, the projection is set to stimulate the right postero-lateral part of the tongue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
2.2. Sensory stimulation To stimulate different areas of the tongue, we fabricated an intraoral device for each individual using hydrophilic vinyl silicone impression material (EXAFAST/Putty Type, GC, Japan). The subject bit bilaterally into the EXAFAST, which was mixed uniformly and formed into two blocks. The jaws of the subject were positioned based on centric occlusion and opened about 5 mm between the upper and lower teeth to make a small space that was important to build the projection for stimulating the tongue. These blocks were used to create a wide space from the right to left canine teeth to allow comfortable frontal movement of the tongue. Then, we made four grooves on the lingual side of this device, which were positioned on the lingual cusp of the first premolar of the lower jaw and the distal-lingual cusp of the second molar of the lower jaw bilaterally. Next, we made a projection by polymethylmethacrylate (UNIFAST II, GC, Japan) on each groove (Fig. 1). This projection has an elliptical shape and is 3 mm in diameter and 3 mm in height. This projection is easily attached and detached, but very stable during the experiments. 2.3. Tasks The subjects performed one control task and four stimulation tasks. They were presented with a green-filled or red-filled circle cue via a PC and projector system (Fig. 2). The screen was located at an optical distance of 140 cm from the eyes of the subjects. Each cue was 30 mm in diameter and presented at the center of the visual display with a gray background. The green-filled circle was presented for 1 s and then the background only was shown for 1 s. When the green cue was presented, the subjects were instructed to perform a protruding movement to the front, that is, to move their tongue from a relaxed position at the bottom of the oral cavity to
Fig. 2. Simplified scheme of experimental design. Each cue was presented at the center of the visual display with a gray background. The green-filled circle was presented for 1 s and then only the gray background was shown for 1 s. When the green cue was presented, the subjects were instructed to protrude their tongue to the front, that is, to move their tongue from a relaxed position at the bottom of the oral cavity to the anterior teeth in each task to scratch the lateral side of their tongue as they protruded it (tongue-protrusion movement). When only the gray background was presented, the tongue was moved back to its original position. This alternating presentation was repeated for 21 s. After alternately presenting the green cues and the gray background, red-filled circles were presented for 21 s. The subjects were instructed to relax their tongue when the red cue was presented. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
the anterior teeth in each task so as to scratch the lateral side of their tongue with the projection. While only the gray background was presented, the tongue was moved back to its original position. This alternate presentation was repeated for 21 s. Therefore, the subjects repeated the tongue-protruding movement a total of 11 times. After the alternate green cue was presented, the red-filled circle was presented for 21 s. The subjects were asked to relax the tongue during the presentation of the red cue. In one session, the blocks were performed as follows: (red-cue block)–(green-cue block)–(red-cue block)–(green-cue block)–(red-cue block)– (green-cue block)–(red-cue block)–(green-cue block)–(red-cue block)–(green-cue block)–(red-cue block)–(green-cue block)– (red-cue block)–(green-cue block)–(red-cue block) (Fig. 2). One session lasted about 5 min. As a control task, the subjects were required to perform the tongue-protruding movement while no projection was set on the device. In the other four tasks, the subjects performed the movement with a projection. This projection was set at four different positions to stimulate specific areas of the tongue: the left antero-lateral, left postero-lateral, right antero-lateral, and right postero-lateral areas (Fig. 3). In each task, only one projection was set for the target area (Figs. 1 and 3). After each session, this projection was replaced with another groove; which was attached and detached by the operator (K.S.) out of the MR gantry. Before
175
starting each session, the head was positioned accurately. In addition, the lower jaw was kept in the same position using an intraoral device. Before the experiment, we marked the projection with red edible ink and checked whether the projection could stimulate each target area precisely. For each area, the anterolateral and postero-lateral parts were marked about 2 and 4.5 cm from the tip of the tongue, respectively, and the lengths of these areas were all about 0.5 cm along the rostro-caudal axis of the tongue. In addition, each subject practiced the movement of protruding the tongue for 3 min to understand the experimental procedure and objective before the recordings. Through these experiments, the subjects felt a clear mechanical cutaneous stimulation of the tongue without pain. They felt only tactile sensation arising from the scratching of the tongue, and did not complain of any discomfort such as pain or a feeling of sickness. The order of tasks was randomized for each subject and counterbalanced across all subjects. 2.4. fMRI data acquisition All images were acquired using a 1.5 T MR scanner (Magnetom Vision; Siemens, Erlangen, Germany). For each functional imaging sequence, the subjects performed a block design alternating seven tongue movement blocks of 21 s (green cue) with eight rest blocks (red cue) having the same duration. Blood oxygen level dependent (BOLD) contrast functional images were acquired using T2*weighted echo planar imaging (EPI) free induction decay (FID) sequences with the following parameters: TR 3096 ms, TE 60 ms, matrix size 64 64, FOV 256 mm, in-plane voxel size 4 mm 4 mm, flip angle 908, slice thickness 4 mm and no gap. For anatomical reference, a high resolution structural volume was acquired at the end of the session via a 3D magnetizationprepared rapid gradient echo (MPRAGE) sequence (TR 9.7 ms, TE 4 ms, sagittal, matrix 256 256, FOV 256 mm, slice thickness 1 mm, no gap, in-plane voxel size 1 mm 1 mm, flip angle 128, slice thickness 1 mm) for each subject. 2.5. Data analysis
Fig. 3. Schematic drawings of the relationship between tongue movement and a protrusion in each task. Tongue movement itself is the same in the control and all four tasks. Each part of the tongue is stimulated with a protrusion by a protruded movement of the tongue. The shaded areas of the tongue are stimulated by a protrusion. LA = left antero-lateral stimulation; LP = left postero-lateral stimulation; RA = right antero-lateral stimulation; RP = right postero-lateral stimulation
Raw data were analyzed using Statistical Parametric Mapping (SPM2, Wellcome Department of Cognitive Neurology, London, UK) (Friston et al., 1995) implemented in Matlab (Mathworks, Sherborn, Massachusetts, USA). The first five scans of each run were discarded from the analysis due to unsteady magnetization. The effect of head motion was corrected by realigning all scans to the first one. After being coregistered with a T1-weighted structural volume, they were normalized in the functional scans to a standard stereotaxic space (Montreal Neurological Institute (MNI) template). Then, the images were spatially smoothed using an isotropic Gaussian kernel of 8 mm full width at half maximum (FWHM) in the x, y and z axes. Temporal filters were also applied and low frequency noise and global changes in the signals were removed. Statistical analysis was performed at two levels. First, individual task-related activation was evaluated. Second, the summary data for each individual were incorporated into the second-level analysis using a random-effect model (Friston et al., 1999) to make inferences at a population level. In the first phase of an individual analysis, we analyzed the active areas during each task compared with those during the rest periods of the same session. However, the tongue could be stimulated not only by scratch stimuli but also by the device itself and/or intraoral structures such as the palate and teeth during the protruding movement. Then, to investigate only the somatosensory-related activation by removing the motor-related activation and somatosensory-related activation for the device and/or intraoral structures, we analyzed the subtraction images obtained from the
176
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
contrasts as follows: the task in which the projection was on the left antero-lateral of the tongue (left antero-lateral) minus nonprojection task (Control) (LA), left postero-lateral minus Control (LP), right antero-lateral minus Control (RA), and right posterolateral minus Control (RP). Group analysis (random-effects model) of each task was performed by entering contrast images into a one-sample t-test. The statistical threshold was set at P < 0.001, uncorrected (no significant activation could be found in all conditions when we set our threshold at P < 0.05, corrected). Activation clusters <10 voxels were removed. The locations of brain activity were transformed MNI coordinates into the Talairach standard brain coordinates (Talairach and Tournoux, 1988). As a further analysis, the threshold of the individual data was set P < 0.005, uncorrected to identify the activation of the difference among conditions in detail.
right hemispheres, because the left and right ACC are very close to each other. In addition to SI, stimulation of the posterior part of the tongue activated a slightly more lateral region in the inferior parietal lobule (BA 40), which was located at coordinates (X, Y, Z = 61, 28, 33) in LP and (63, 28, 33) in RP. The ACC activities were located at coordinates (X, Y, Z = 0, 2, 41) in LP and ( 4, 3, 49) in RP. In the individual data, SI activation of the right hemisphere was recorded in all subjects. In the left hemisphere, this activation was found in LP and RP from three subjects. This coordinates for each subject was at (X, Y, Z = 63, 28, 33), ( 63, 28, 33) and ( 63, 28, 31) in LP, showing BA 40, and at (X, Y, Z = 61, 26, 31), ( 61, 28, 33) and ( 61, 26, 32) in RP, indicating BA 40.
3. Results
In the group analysis, SI activation evoked by LA and RA stimulation converged at BA 2. On the other hand, posterior stimulation, LP and RP, indicated a broad activation in BA 2 of SI and BA 40 (Fig. 4). In addition, in order to examine these results in more detail, we analyzed the individual data about the number of active voxels around SI. This procedure was allowed to avoid the canceling out the spatial inter-individual variability in a group analysis (Arienzo et al., 2006). Statistical analysis was performed by a paired t-test (SPSS 14.0 for Windows). SI had significantly larger activation with posterior stimulation than with anterior stimulation (LA vs. LP and RA vs. RP, p < 0.05, respectively) (Fig. 6).
3.1. Anterior stimulation In the group analysis, the LA and RA conditions produced brain activity around the right primary somatosensory cortex (SI), but there was no significant activity in the left hemisphere for each condition (Fig. 4). Significant SI activation was found in the crown of the postcentral gyrus (Brodmann’s area 2; BA 2), which is located at (X, Y, Z = 61, 24, 34) in LA and (63, 24, 34) in RA. In the individual data, this SI activation of the right hemisphere was recorded from all 12 subjects, whereas the SI activation of the left hemisphere was observed from two subjects in LA and none in RA. The location of SI activation from these two subjects was at (X, Y, Z = 63, 24, 34) and ( 61, 25, 36), respectively, showing BA 2. 3.2. Posterior stimulation As for the group data, LP and RP stimulation also induced significant activation at SI in the right hemisphere and the anterior cingulate cortex (ACC) (Figs. 4 and 5). No significant activation was found around the SI of the left hemisphere. It was difficult to determine an asymmetry in ACC activation between the left and
3.3. Comparison of the individual data between anterior and posterior sensory processing of the tongue around SI
4. Discussion In the present study, we investigated the somatosensory processing of the tongue using a newly developed device, and identified the cortical somatosensory representation of the tongue in the human brain in more detail than had previous reports (Sakai et al., 1995; Pardo et al., 1997; Miyamoto et al., 2006). Our results suggested that the somatosensory representation of the posterior part of the tongue was larger than that of the anterior part in SI, and expanded to BA 40. Furthermore, the ACC was only activated by stimulation of the posterior tongue.
Fig. 4. Group activation map showing activated brain regions in four conditions. The figures around SI areas were enlarged in all conditions. Using the SPM2 template, areas showing an increase in BOLD-signal are superimposed on a 3D-rendered standard brain. Although our statistical threshold was P < 0.001 (uncorrected), the threshold was lowered at P < 0.05 (uncorrected) for this figure for display purposes. LA = left antero-lateral stimulation; LP = left postero-lateral stimulation; RA = right antero-lateral stimulation; RP = right postero-lateral stimulation
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
177
2006). They showed the SI activity in the postcentral gyrus, which is consistent with the homunculus reported by Penfield and Boldrey (1937). The present study obtained new findings by stimulating the postero-lateral area of the tongue. Until now, there had been crucial problems investigating the somatosensory representation related to the posterior part of the tongue (see Section 1). As a result, the somatosensory representation of this area existed in SI and BA 40, whereas the antero-lateral part of the tongue converged at the SI (Figs. 4 and 6). Neuroanatomically, the anterior two-thirds of the tongue are innervated by the afferent fibers that travel in a branch of the trigeminal nerve (V) called the lingual nerve. The posterior onethird of the tongue is innervated by the afferent fibers that travel in the lingual branch of the glossopharyngeal nerve (IX) (Kandel et al., 1991). From the result of the present study, therefore, the anterolateral part of the tongue should be innervated by the trigeminal nerve, but the postero-lateral part may be innervated by both the trigeminal and glossopharyngeal nerves, which may underlie the clear differences in brain activities, although the border of the innervation zone and pattern between the trigeminal nerve and the glossopharyngeal nerve is vague and unclear anatomically. Fitzgerald and Law (1958) reported lingual-hypoglossal connexion between the lingual nerve and hypoglossal nerve in the tongue, so we speculate that the same connection is formed between the lingual nerve and glossopharyngeal nerve. Doty et al. (2009) also suggested that branches of the glossopharyngeal nerve extend anteriorly beyond the sulcus terminalis and circumvallate papillae, with extensions occurring along the lateral lingual margin anterior to the foliate papillae. In addition, anastomoses were identified between the glossopharyngeal nerve and the lingual nerve, raising the possibility of functional interactions between the trigeminal nerve and the glossopharyngeal nerve. This notion was supported by a recent anatomical study (Zur et al., 2004). 4.2. Visceral sensation
Fig. 5. Neural activation in the ACC overlaid on an anatomically normalized MRI. Only LP and RP stimulation produced these activities. A brighter color represents a higher statistical significance. The map is threshold at P < 0.001 (uncorrected) in LP and P < 0.005 (uncorrected), respectively.
4.1. The cortical somatosensory representation Several studies using neuroimaging methods have found a somatosensory representation after stimulating the tip of the tongue (Sakai et al., 1995; Pardo et al., 1997; Miyamoto et al.,
Fig. 6. Comparison of the individual data on the number of voxels around SI. Error bars denote the standard error (SE) across subjects. Stimulation of the posterior part of the tongue activated SI in wider regions than that of the anterior part. The statistical analysis was performed with a paired t-test. *P < 0.05.
In addition to SI, BA 40 immediately lateral to SI was activated in LP and RP conditions. Indeed, it seems difficult to determine why this area was activated following stimulation of the posterior part of the tongue. However, we assumed that activation of BA 40 included neuronal activation associated with visceral sensation following the stimulation of the posterior tongue. Previous studies demonstrated that the cortical representation of visceral organs differs from that of the somatosensory system, using fMRI (Aziz et al., 2000; Hobday et al., 2001; Lotze et al., 2001; Strigo et al., 2003; Eickhoff et al., 2006; Ladabaum et al., 2007), and MEG (Schnitzler et al., 1999). For instance, Hobday et al. (2001) observed activation in the inferior part of SI and BA 40 following visceral rectal stimulation, but only in SI following somatic anal stimulation. Ladabaum et al. (2007), who recorded brain activity following gastric distension, found no evidence of activation of S1, but found activation in a broad region of BA 40. Taking these studies into consideration, it is likely that BA 40 was activated following visceral stimulation. However, further studies would be necessary to determine the relationship between the neural activity of BA 40 and visceral sensation and to clarify the characteristics of this area with respect to tongue stimulation. In general, the ACC plays an important role in sensory, motor, cognitive and emotional information (Bush et al., 2000) and pain processing (Schnitzler and Ploner, 2000; Vogt, 2005; Qiu et al., 2006). Our results demonstrated that the ACC was activated only during the postero-lateral stimulation (Fig. 5). Some studies showed that the ACC was often concerned with visceral sensation (see Table 1, for findings of previous studies of visceral sensation). For example, Hobday et al. (2001) noted that the ACC was activated by visceral stimulation, not by somatic stimulation. It appears that
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179
178 Table 1 Previous neuroimaging studies for visceral sensation. Author name
Stimulated area
Recording
ACC
L-insula
R-insula
L-SI
R-SI
L-BA 40
R-BA 40
Aziz et al. (1997) Binkofski et al. (1998) Aziz et al. (2000) Hobday et al. (2001) Ladabaum et al. (2001) Lotze et al. (2001) Gregory et al. (2003) Bittorf et al. (2006) Coen et al. (2007) Ladabaum et al. (2007) Coen et al. (2008)
Esophagus Esophagus Esophagus Rectum Gastric distention Rectum Esophagus Rectum Esophagus Gastric distention Esophagus
PET fMRI fMRI fMRI PET fMRI fMRI fMRI fMRI fMRI fMRI
O O O O O O O O O
O O O O O O O O
O O O O O O O O O
O O O O O O
O O O O O O O
O O
O O O
These studies were identified by a thorough search of literature, using PubMed (http://www.ncbi.nlm.nih.gov/pubmed/). fMRI = functional magnetic resonance imaging; PET = positron emission tomography; L = left; R = right; ACC = anterior cingulate cortex; SI = primary somatosensory cortex; BA 40 = Brodmann’s area 40.
their results are consistent with our findings. Thus, we considered that our ACC activation might reflect the attribution of the viscera, because the viscera have a complex peripheral nervous system that allows for a wide variety of autonomic functions (Ness and Gebhart, 1990). On the other hand, previous studies of visceral stimulation often showed activation in insula, suggesting that it plays some important roles in visceral processing (Table 1), but our data did not show activation in this region. There are several possible explanations for this. The first is a difference in characteristics between the ACC and insula within the neural networks concerning visceral sensation. For instance, Gregory et al. (2003) reported that selective attention to the esophageal (visceral) target stimuli activated the SI, secondary somatosensory cortex (SII), and ACC rather than insula, suggesting that the ACC and insula do not always show similar activation during visceral stimulation. Our tasks did not belong to general passive tasks that elicit somatosensory activation, and the subjects were required to move their tongue ‘actively’. Therefore, the enhanced activation in the ACC during attention to the stimuli for tongue scratching might be due to the visceral sensation under LP and RP conditions. Second, insula may not always be activated following visceral sensation, even if both ACC and insula were involved in the limbic system. In our literature search shown in Table 1, several studies showed that the activation of ACC and insula was sometimes not detected. Under LP and RP conditions of the present study, stimulation of the posterior part of the tongue elicited ACC activation, which might be insufficient for achieving insula activation. Indeed, it might be difficult to clarify why the insula was not activated during stimulation of the posterior part of the tongue. Further study is needed. 4.3. Hemisphere lateralization Our results showed the cortical representation on the right hemisphere, but not the left hemisphere. The cortical responses to stimulation of the tongue have been a matter of debate. Anatomically, previous studies in monkeys reported that some neurons were activated in both the contralateral and ipsilateral hemispheres following stimulation of the tongue (Manger et al., 1996; Jain et al., 2001). In humans, some neurophysiological studies using electroencephalography (EEG) and MEG also showed bilateral activation in the SI following the tongue’s stimulation (Ishiko et al., 1980; Altenmuller et al., 1990; Karhu et al., 1991; Disbrow et al., 2003). In our previous MEG studies, we fabricated individual intraoral devices, and recorded SEFs (Sakamoto et al., 2008a,b). The tongue was stimulated with a concentric bipolar electrode in four areas: the right and left antero-lateral margins, and the right and left postero-lateral
margins. Bilateral activations were observed under all conditions, which stimulated four different areas of the tongue. However, our previous data did not show the right dominant response in some components of SEFs. There are three possible explanations for the discrepancy between the present neuroimaging findings and some previous neurophysiological studies including our own MEG studies. The first possibility is that somatosensory processing includes asymmetric neural activation. That is, as several neuroimaging studies already showed (Perlmutter et al., 1987; Fox and Applegate, 1988; Naito et al., 2005; Nihashi et al., 2005; Eickhoff et al., 2008), the brain’s response should be stronger in the right hemisphere than the left for somatosensory processing. We believe that the present study also indicated this asymmetric neural activation. Indeed, our method of stimulation may be unable to elicit clear activation in the left hemisphere, compared to general electrical stimulation. If so, it might be difficult to detect the response in the left hemisphere. The second possibility is a negative motor effects on the left somatosensory areas. Some neuroimaging studies have also provided evidence that activation of the sensorimotor cortex representing the oral and facial regions during volitional swallowing and mastication showed left hemispheric preference (Martin et al., 2004, 2007; Shinagawa et al., 2004). From these studies, there is a possibility that active movement of the tongue affects SI activity in the left hemisphere. A third explanation is that the above two possibilities may be interrelated. 5. Conclusion In conclusion, we could record the human brain response after stimulating the postero-lateral part of the tongue, and compared it with the antero-lateral part of the tongue. We showed that a difference existed in the somatosensory processing of the tongue, particularly around the SI and ACC. These findings also demonstrate the possibility that stimulating the posterior of the tongue elicited a visceral sensation. Acknowledgements We would like to thank Dr. A. Ferretti for general assistance during the measurements, Ms. T.L. Chen and N. Savini for the help they gave in recruiting the subjects and Dr. H.C. Tanabe, Dr. D.N. Saito and Dr. Y. Morito for analyzing the data. References Altenmuller, E., Cornelius, C.P., Buettner, U.W., 1990. Somatosensory evoked potentials following tongue stimulation in normal subjects and patients with lesions of the afferent trigeminal system. Electroencephalogr. Clin. Neurophysiol. 77, 403–415.
K. Sakamoto et al. / Neuroscience Research 66 (2010) 173–179 Arienzo, D., Babiloni, C., Ferretti, A., Caulo, M., Grarra, C., Tartaro, A., Rossini, P.M., Romani, G.L., 2006. Somatotopy of anterior cingulate cortex (ACC) and supplementary motor area (SMA) for electric stimulation of the median and tibial nerves: an fMRI study. NeuroImage 33, 700–705. Aziz, Q., Andersson, J.L., Valind, S., Sundin, A., Hamdy, S., Jones, A.K., Foster, E.R., La˚ngstro¨m, B., Thompson, D.G., 1997. Identification of human brain loci processing esophageal sensation using positron emission tomography. Gastroenterology 113, 50–59. Aziz, Q., Thompson, D.G., Ng, V.W., Hamdy, S., Sarkar, S., Brammer, M.J., Bullmore, E.T., Hobson, A., Tracey, I., Gregory, L., Simmons, A., Williams, S.C., 2000. Cortical processing of human somatic and visceral sensation. J. Neurosci. 20, 2657–2663. Binkofski, F., Schnitzler, A., Enck, P., Frieling, T., Posse, S., Seitz, R.J., Freund, H.J., 1998. Somatic and limbic cortex activation in esophageal distention: a functional magnetic resonance imaging study. Ann. Neurol. 44, 811–815. Bittorf, B., Ringler, R., Forster, C., Hohenberger, W., Matzel, K.E., 2006. Cerebral representation of the anorectum using functional magnetic resonance imaging. Br. J. Surg. 93, 1251–1257. Bush, G., Luu, P., Posner, M.I., 2000. Cognitive and emotional influences in anterior cingulate cortex. Trends Cogn. Sci. 4, 215–222. Brand, R.W., Isselhard, D.E., 2003. Anatomy of Orofacial Structures, 7th ed. Mosby, pp. 300–304. Coen, S.J, Gregory, L.J., Ya´gu¨ez, L., Amaro Jr., E., Brammer, M., Williams, S.C., Aziz, Q., 2007. Reproducibility of human brain activity evoked by esophageal stimulation using functional magnetic resonance imaging. Am. J. Physiol. Gastrointest. Liver Physiol. 293, G188–G197. Coen, S.J., Aziz, Q., Ya´gu¨ez, L., Brammer, M., Williams, S.C., Gregory, L.J., 2008. Effects of attention on visceral stimulus intensity encoding in the male human brain. Gastroenterology 135, 2065–2074. Disbrow, E.A., Hinkley, L.B., Roberts, T.P., 2003. Ipsilateral representation of oral structures in human anterior parietal somatosensory cortex and integration of inputs across the midline. J. Comp. Neurol. 467, 487–495. Doty, R.L., Cummins, D.M., Shibanova, A., Sanders, I., Mu, L., 2009. Lingual distribution of the human glossopharyngeal nerve. Acta Otolaryngol. 129, 52–56. Eickhoff, S.B., Lotze, M., Wietek, B., Amunts, K., Enck, P., Zilles, K., 2006. Segregation of visceral and somatosensory afferents: an fMRI and cytoarchitectonic mapping study. NeuroImage 31, 1004–1014. Eickhoff, S.B., Grefkes, C., Fink, G.R., Zilles, K., 2008. Functional lateralization of face, hand, and trunk representation in anatomically defined human somatosensory areas. Cereb. Cortex 18, 2820–2830. Fitzgerald, M.J., Law, M.E., 1958. The peripheral connexions between the lingual and hypoglossal nerves. J. Anat. 92, 178–188. Fox, P.T., Applegate, C.N., 1988. Right-hemisphere dominance for somatosensory processing in humans. Soc. Neurosci. Abstr. 18, 760. Friston, K.J., Holmes, A.P., Worsley, K.J., Poline, J.B., Frith, C.D., Frackowiak, R.S.J., 1995. Statistical parametric maps in functional imaging: a general linear approach. Hum. Brain Mapp. 2, 189–210. Friston, K.J., Holmes, A.P., Worsley, K.J., 1999. How many subjects constitute a study? NeuroImage 10, 1–5. Gregory, L.J., Ya´gu¨ez, L., Williams, S.C., Altmann, C., Coen, S.J., Ng, V., Brammer, M.J., Thompson, D.G., Aziz, Q., 2003. Cognitive modulation of the cerebral processing of human oesophageal sensation using functional magnetic resonance imaging. Gut 52, 1617–1671. Hobday, D.I., Aziz, Q., Thacker, N., Hollander, I., Jackson, A., Thompson, D.G., 2001. A study of the cortical processing of ano-rectal sensation using functional MRI. Brain 124, 361–368. Ishiko, N., Hanamori, T., Murayama, N., 1980. Spatial distribution of somatosensory responses evoked by tapping the tongue and finger in man. Electroencephalogr. Clin. Neurophysiol. 50, 1–10. Jain, N., Qi, H.X., Catania, K.C., Kaas, J.H., 2001. Anatomic correlates of the face and oral cavity representations in the somatosensory cortical area 3b of monkeys. J. Comp. Neurol. 429, 455–468. Kandel, E., Schwartz, J.H., Jessell, T.M., 1991. Principles of Neural Science, 3rd ed. Appleton & Lange, Connecticut, 519 pp. Karhu, J., Hari, R., Lu, S.T., Paetau, R., Rif, J., 1991. Cerebral magnetic fields to lingual stimulation. Electroencephalogr. Clin. Neurophysiol. 80, 459–468. Ladabaum, U., Minoshima, S., Hasler, W.L., Cross, D., Chey, W.D., Owyang, C., 2001. Gastric distention correlates with activation of multiple cortical and subcortical regions. Gastroenterology 120, 369–376.
179
Ladabaum, U., Roberts, T.P., McGonigle, D.J., 2007. Gastric fundic distension activates fronto-limbic structures but not primary somatosensory cortex: a functional magnetic resonance imaging study. NeuroImage 34, 724–732. Lotze, M., Wietel, B., Birbaumer, N., Ehrhardt, J., Grodd, W., Enck, P., 2001. Cerebral activation during anal and rectal stimulation. NeuroImage 14, 1027–1034. Manger, P.R., Woods, T.M., Jones, E.G., 1996. Representation of face and intra-oral structures in area 3b of macaque monkey somatosensory cortex. J. Comp. Neurol. 371, 513–521. Martin, R.E., MacIntosh, B.J., Smith, R.C., Barr, A.M., Stevens, T.K., Gati, J.S., Menon, R.S., 2004. Cerebral areas processing swallowing and tongue movement are overlapping but distinct: a functional magnetic resonance imaging study. J. Neurophysiol. 92, 2428–2443. Martin, R., Barr, A., MacIntosh, B., Smith, R., Stevens, T., Taves, D., Gati, J., Menon, R., Hachinski, V., 2007. Cerebral cortical processing of swallowing in older adults. Exp. Brain Res. 176, 12–22. Miyamoto, J.J., Honda, M., Saito, D.N., Okada, T., Ono, T., Ohyama, K., Sadato, N., 2006. The representation of the human oral area in the somatosensory cortex: a functional MRI Study. Cereb. Cortex 16, 669–675. Naito, E., Roland, P.E., Grefkes, C., Choi, H.J., Eickhoff, S., Geyer, S., Zilles, K., Ehrsson, H.H., 2005. Dominance of the right hemisphere and role of area 2 in human kinesthesia. J. Neurophysiol. 93, 1020–1034. Ness, T.J., Gebhart, G.F., 1990. Visceral pain: a review of experiment studies. Pain 41, 167–234. Nihashi, T., Naganawa, S., Sato, C., Kawai, H., Nakamura, T., Fukatsu, H., Ishigaki, T., Aoki, I., 2005. Contralateral and ipsilateral responses in primary somatosensory cortex following electrical median nerve stimulation—an fMRI study. Clin. Neurophysiol. 116, 842–848. Oldfield, R.C., 1971. The assessment and analysis of handedness: the Edinburgh inventory. Neuropsychologia 9, 97–113. Pardo, J.V., Wood, T.D., Costello, P.A., Pardo, P.J., Lee, J.T., 1997. PET study of the localization and laterality of lingual somatosensory processing in humans. Neurosci. Lett. 234, 23–26. Penfield, W., Boldrey, E., 1937. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60, 389–443. Perlmutter, J.S., Powers, W.J., Herscovitch, P., Fox, P.T., Raichle, M.E., 1987. Regional asymmetries of cerebral blood flow, blood volume, and oxygen utilization and extraction in normal subjects. J. Cereb. Blood Flow Metab. 7, 64–67. Qiu, Y., Noguchi, Y., Honda, M., Nakata, H., Tamura, Y., Tanaka, S., Sadato, N., Wang, X., Inui, K., Kakigi, R., 2006. Brain processing of the signals ascending through unmyelinated C fibers in humans: an event-related fenctional magnetic resonance imaging study. Cereb. Cortex 16, 1289–1295. Sakai, K., Watanabe, E., Onodera, Y., Itagaki, H., Yamamoto, E., Koizumi, H., Miyashita, Y., 1995. Functional mapping of the human somatosensory cortex with echo-planar MRI. Magn. Reson. Med. 33, 736–743. Sakamoto, K., Nakata, H., Kakigi, R., 2008a. Somatosensory evoked magnetic fields following stimulation of the tongue in humans. Clin. Neurophysiol. 119, 1664– 1673. Sakamoto, K., Nakata, H., Kakigi, R., 2008b. Somatotopic representation of the tongue in human secondary somatosensory cortex. Clin. Neurophysiol. 119, 2125–2134. Schnitzler, A., Volkmann, J., Enck, P., Frieling, T., Witte, O.W., Freund, H.J., 1999. Different cortical organization of visceral and somatic sensation in humans. Eur. J. Neurosci. 11, 305–315. Schnitzler, A., Ploner, M., 2000. Neurophysiology and functional neuroanatomy of pain perception. J. Clin. Neurophysiol. 17, 592–603. Shinagawa, H., Ono, T., Honda, E., Sasaki, T., Taira, M., Iriki, A., Kuroda, T., Ohyama, K., 2004. Chewing-side preference is involved in differential cortical activation patterns during tongue movements after bilateral gum-chewing: a functional magnetic resonance imaging study. J. Dent. Res. 83, 762–766. Strigo, I.A., Duncan, G.H., Boivin, M., Bushnell, M.C., 2003. Differentiation of visceral and cutaneous pain in the human brain. J. Neurophysiol. 89, 3294–3303. Talairach, J., Tournoux, P., 1988. Co-planar Stereotaxic Atlas of the Human Brain. Thieme, Stuttgart. Vogt, B.A., 2005. Pain and emotion interactions in subregions of the cingulate gyrus. Nat. Rev. Neurosci. 6, 533–544. Zur, K.M., Mu, L., Sanders, I., 2004. Distribution pattern of the human lingual nerve. Clin. Anat. 17, 88–92.