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  • Research Report

    Yuko Nakamuraa, 1, Tazuko KKoji Kobayashic, Yasuhiko NaYuzo Ninomiyad, Kazunori YoaDepartment of Oral and Maxillofacial RadiolobDepartment of Clinical Radiology, Graduate ScDepartment of Medical Technology, Kyushu UdSection of Oral Neuroscience, Graduate Schoo

    2011 Elsevier B.V. All rights reserved.

    B R A I N R E S E A R C H 1 4 0 6 ( 2 0 1 1 ) 1 8 2 9

    ava i l ab l e a t i enced i r ec t . com

    www.e l sev i e r . com/ loca te /b ra i n respurpose, we developed an original taste delivery system for functional magnetic resonanceimaging (fMRI) studies for umami. Then, we compared the results produced by twoauthorized models, namely, the block design model and event-related design model, todecide the appropriate model for detecting activation by umami. Activation by the umamitaste was well localized in the insular cortex using our new system and block design modelanalysis. The peaks of the activated areas in the middle insular cortex by umami were veryclose to another prototypical taste quality (salty). Although we have to carefully interpretthe perceiving intensities and brain activations by taste from different sessions, this studydesign might be effective for detecting the accession area in the cortex of pure umami tasteon the tongue.

    Insular cortexFunctional MRITaste solution delivery device Corresponding author at: Oral Radiology, OrPhilip Dental Hospital, 34 Hospital Road, Hon

    E-mail addresses: Tokumori), (Y. Naka(Y. Ninomiya), yoshiura@rad.dent.kyushu-u.1 These two authors contributed equally to

    0006-8993/$ see front matter 2011 Elsevidoi:10.1016/j.brainres.2011.06.029effective to minimize somatosensory stimuli, oral movement, and psychological effects in aneuroimaging study to elicit cerebral activity by pure umami on the human tongue. For thisAccepted 11 June 2011Available online 24 June 2011

    Keywords:Umami. Gotoa,, 1, Kenji Tokumoria, Takashi Yoshiurab,kamurac, Hiroshi Hondab,shiuraa

    gy, Faculty of Dental Science, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, Japanchool of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, Japanniversity Hospital, 3-1-1, Maidashi, Higashi-ku, Fukuoka, Japanl of Dental Science, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka, Japan

    A B S T R A C T

    There are no credible data to support the notion that individual taste qualities havededicated pathways leading from the tongue to the end of the pathway in the brain.Moreover, the insular cortex is activated not only by taste but also by non-taste informationfrom oral stimuli. These responses are invariably excitatory, and it is difficult to determinewhether they are sensory, motor, or proprioceptive in origin. Furthermore, umami is a moreunfamiliar and complex taste than other basic tastes. Considering these issues, it may beA R T I C L E I N F O

    Article history:Localization of brain activation by umami taste in humansal Diagnosis & Polyclinics, Faculty of Dentistry, The University of Hong Kong, 1B39A, Princeg Kong. Fax: +852 2858 (Y. Nakamura), (T.K. Goto), Yoshiura), (K. Kobayashi),mura), (H. Honda), (K. Yoshiura).this work.

    er B.V. All rights reserved.

  • the mouth and jaw. These responses are invariably excitatory,

    1 4and it was difficult to determine whether they were sensory,motor, or proprioceptive in origin (Scott and Plata-Salamn,1. Introduction

    Umami, discovered by Ikeda (1909), is elicited by monosodiumL-glutamate (MSG). It exists in various foods, such as soup,fish, meat, breast milk, tomatoes and some vegetables.Umami is now clearly known as the fifth basic taste (Baylisand Rolls, 1991) because it is independent from the other fourbasic tastes (sweet, salty, sour and bitter) in human tastesensation (Yamaguchi and Ninomiya, 2000) and in behavioraland electrophysiological studies in animals (Baylis and Rolls,1991; Kumazawa et al., 1991; Ninomiya and Funakoshi, 1989).In addition, the sequencing and functional expression of ataste receptor has been discovered (Chaudhari et al., 2000; Liet al., 2002; Nelson et al., 2002; Toyono et al., 2002, 2003) andthe independency of umami has been confirmed.

    There are no credible data that individual taste qualitieshave dedicated pathways leading from the tongue to the endof the pathway in the brain. Especially with regard to umami,it is difficult to recognize umami itself because we do not tasteit independently in daily life. Moreover, the physiologicalsignificance of the activation of the brain by umami ispresented not only in the mouth but also in the process ofdigestion, absorption,metabolism, andother functions (Kondohet al., 2009). In addition, it is clearly known that the pathwaysfrom the tongue to the central nervous system to perceiveumami are different among species. There is evidence thatsome non-primate species (e.g., rodents) do not respondelectrophysiologically and behaviorally to umami in the sameway as humans, or they do not differentiate between MSG andNaCl (Yamamoto et al., 1988), or MSG and sweet (Heyer et al.,2003, 2004). Even anatomically, the primate taste systemmay beorganized in a different manner from that of non-primates(Beckstead et al., 1980; Norgren and Leonard, 1973). For example,inmacaques there exists an obligatory relay from the nucleus ofthe solitary tract via the taste thalamus to the taste cortex,although in rodents, there is an obligatory relay from thesolitary tract nucleus to the pontine taste area, which in turnprojects to the thalamus (Norgren and Leonard, 1973). Even inthe primary taste cortex of macaques, single neurons werefound that were tuned to respond best to glutamate (umamitaste) (Baylis and Rolls, 1991); on the other hand, in rhesusmonkeys, cluster analysis did not indicate that MSG washandled as a separate taste quality (Hellekant et al., 1997).Considering the differences across mammalian orders, we mayassume that the human taste system would differ from eventhat of monkeys.

    Focusing on the primary taste cortices in the non-humanprimate, there are valuable studies which traced a singleneuron from the mouth to the central nervous system. Thesestudies showed that the primary taste cortices in the non-human primate are mainly involved in functions other thangustation. An article states that only 823 (6.5%) of the neuronstestedwere reliably responsive to one ormore of the four basictastes. The other 3014 (23.8%) responded duringmovements of

    B R A I N R E S E A R C H1999). In addition, the primary taste cortex in primatescontains not only taste neurons, but also other neurons thatencode oral somatosensory stimuli including viscosity, fattexture, temperature, and capsaicin inputs (Rolls, 2006;Verhagen et al., 2004).

    The human insula represents not only taste but also thenon-taste information of oral stimuli as the results offunctional neuroimaging studies show. They are odorants(de Araujo et al., 2003c; Lombion et al., 2009), texture (DeAraujo and Rolls, 2004), and temperature (Guest et al., 2007).Umami and odor (McCabe and Rolls, 2007) in particularinfluence each other strongly. These functional pathwaysand localization in the taste cortex are more complex inhumans. However, since the single neuron experimentscannot be conducted on humans, precise brain mapping byfunctional magnetic resonance imaging (fMRI) is one of themost promisingmethods for extracting the result of activationin the brain by stimuli only from umami taste on the tongue.In most previous fMRI studies of umami, small volumes ofsolutions (0.75 or 2.0 ml) have been delivered to the partici-pant's mouth through some tubes, and then the participantwas cued by visual or tone signals to move his or her tongueand swallow the taste solution (de Araujo et al., 2003a;Grabenhorst and Rolls, 2008; Grabenhorst et al., 2008; McCabeand Rolls, 2007; Schoenfeld et al., 2004). These tasks are wellestablished for the experiments that are designed to investi-gate umamiwith odor, attention, the combination of taste andpsychological effects, and then to prove the natural andscientific features of umami in humans. In fact, it is knownthat a participant's attention and psychological effect modu-late brain activation (Bender et al., 2009; Grabenhorst andRolls, 2008, 2010; Veldhuizen et al., 2007).

    From another physiological standpoint, if we performed asimple experiment that minimized the somatosensory stim-uli, oral movement, and psychological effects as much aspossible in a neuroimaging study, it might be effective indetecting the accession area in the cortex of pure umami tasteon the tongue. For this, wewould like to avoid the participant'sswallowing to reduce head movement, tongue movement,and brain activation by umami in the gastrointestinal tract(Kondoh et al., 2009; Tsurugizawa et al., 2009).

    Our previous taste delivery system, which prevented theparticipant's swallowing, solved these difficulties (Kami et al.,2008); however, the system delivered the taste solution only toa small area on the tongue tip. The system was valuable forsweet; however, it can be assumed that the stimulated areawas too small for umami, and it is necessary to stimulate thelateral and posterior area of the tongue also. Therefore, it isindispensable to develop a new system to investigate the brainactivation caused by pure umami taste on the tongue.

    After we develop the system, we then need to investigatewhich typeofmethodof analysis ismore adequate for acquiringmore significant results. For this, the activatedareas in the braincortex by taste should be compared using two authorizedmodels: a block design model and an event-related designmodel. The b