Near-infrared spectroscopy of orbitofrontal cortex during odorant stimulation Norio Kokan, M.D.,1 Nobuyuki Sakai, Ph.D.? Kiyoshi Doi, M.D., Ph.D.,1 Hisami Fujio, M.D.,1 Shingo Hasegawa, M.D., Ph.D.,1 Hitoshi Tanimoto, M.D., Ph.D.,1 and Ken-ichi Nibu, M.D., Ph.D. I ABSTRACT Background: For olfaction, several studies have reported near-infrared spectroscopy (NIRS) signal changes in the orbitofrontal cortex (OFC) during odor stimulation. However, the roles of human OFC in olfactory cognition are less well understood. This study was designed to better understand the roles of OFC for olfaction. Methods: Hemodynamic responses for phenyl ethyl alcohol or citral in the OFCs were measured with NIRS. After the experiment, participants were asked to describe the characteristics of the odor and to rate odor intensity and hedonic valence. Results: Statistical analysis of all participants' data showed significant changes in the concentration of total hemoglobin in the left OFC during the trial (p = 0.04). The total hemoglobin signal increased significantly in the right OFC (p = 0.0008) of the participants who successfully identified the odorant stimulus. Conclusion: Our findings showed that NIRS combined with a questionnaire is a useful method for studying the functional neuroanatomy of OFC in terms of olfaction. (Am J Rhinol Allergy 25, 163-165,2011; doi: 1O.2500/ajra.2011.25.3634)
N
ear-infrared spectroscopy (NIRS) is an established technology to noninvasively assess the hemodynamic activity of various tissues. Changes in oxyhemoglobin, deoxyhemoglobin, and total hemoglobin in the targeted lesions can be calculated from the differences between absorption of near-infrared rays with wavelengths of 780 and 830 nm. Based on the assumption that an increase in the recorded oxygenated hemoglobin concentration reflects neuronal activation, NIRS has been widely used to study the functional activation of the brain. 1 For olfaction, several studies have reported NIRS signal changes in the orbitofrontal cortex (OFC) during odor stimulation. 2-6 To better understand the roles of human OFC in olfactory cognition, we used NIRS to examine OFC responses to two distinct odorants, l3-phenyl ethyl alcohol (BPEA; rose-like odor) and citral (lemon-like odor). Results were statistically analyzed according to type of stimulation, preference, and identification.
METHODS Because gender differences have been reported in the cognitive process, we performed the present study only on female participants to avoid the erroneous conclusion caused by the possible gender gap? Fourteen female university students with normal olfaction participated in this study as volunteers. Their average age was 19.6 years, ranging from 18 to 23 years. All participants were right handed. All experiments were approved by the Ethics Committee of Kobe Uni-
From the 1 Department of Otolaryngology-Head and Neck Surgery, Kobe University Graduate School of Medicine, Kobe, Japan, and 2 Department of Living Sciences, Faculty of Human Sciences, Kobe Shoin Women's University, Kobe, Japan Presented at the annual meeting of the American Academy of Otolaryngology-Head and Neck Surgery, San Diego, California, October 4-8, 2009 Work was performed at Kobe University Graduate School of Medicine; all experiments were approved by the Ethics Committee of Kobe University Graduate School of Medicine and were performed with informed consent from the participants Supported in part by grants from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government (K. Nibu [18390458J and K. Doi [20791192]); the authors have no other financial disclosures The authors had no conflicts to declare pertaining to this article Address correspondence and reprint requests to Ken-ichi Nibu, M.D., PhD., Department of Otolaryngology-Head and Neck Surgery, Kobe University Graduate School of Medicine, Kusunoki-cho 7-5-1, Chuo-Ku, Kobe 650-0017, Japan E-mail address:
[email protected] Copyright © 2011, OceanSide Publications, Inc., U.S.A.
American Journal of Rhinology & Allergy
versity Graduate School of Medicine and were performed with informed consent from the participants. BPEA (2-phenylethanol; Wako Pure Chemical Industries, Ltd., Osaka, Japan) was presented as an odor stimulus to seven of the participants and citral (Citral 95%; Aldrich, Milwaukee, WI) was presented to the other seven participants. The odor stimuli were presented at a flow rate of 4 L/min by means of an olfactometer equipped with two flow controllers (Flow Gentle Plus; Koike Medical Co., Ltd., Tokyo, Japan) and Teflon tubes (Koike Medical Co.). Odorant-containing or odor-free air was delivered through distilled water and alternately with the aid of a flow controller. For each experiment, odorant-containing air was delivered during the first 30 seconds, and odor-free air was delivered during the next 30 seconds in the same manner as our previous study on brain responses to odors using functional MRI (fMRI).8 Experiments were performed consecutively up to eight times for each participant. Hemodynamic changes in the bilateral OFCs were measured by means of NIRS using two sets comprising one emitter and two detectors (Omega Monitor BOM-LlW; Omega Wave Co., Ltd., Tokyo, Japan). Near-infrared emitters were positioned on the bilateral supraorbital frontal scalps and near-infrared detectors were placed on either side, 20 and 40 mm apart from the emitters (Fig. 1). Changes in total hemoglobin were measured every 0.1 seconds; 1.0 V indicates -1000 pieces/mm3 or 0.045ILmol/100 mL of hemoglobin. Values thus measured were then imported to a personal computer by means of PowerLab 16/30 (PowerLab, Bella Vista, NSW, Australia). Participants were presented with one of the two odorants, BPEA or citral, and during the experiment were asked to push a hand switch when they detected an odor. Participants were also asked to pay careful attention to the odor while data were being recorded. Immediately after completion of the experiment, participants were asked to describe the characteristics of the odor (or guess the name of the odorant) and to rate odor intensity on a 6-point scale from 0 (none) to 5 (strong) as well as odor hedonic valence on a 7-point-scale from -3 (strongly unpleasant) to 3 (strongly pleasant). The participants' nasal cavities were examined. For statistical analysis, Fisher's exact probability test, Wilcoxon signed-ranks test, or repeated-measure ANOV A was used in conjunction with JMP ve.7.02 (SAS Institute, Tokyo, Japan). A value of p < 0.05 was considered significant.
163
0.06
+--------------1+- - - - - - - - - -
0.04
-hr-- - - - - - --.-HI=I\+\I--\-I+- - - - - - -
0.02
+J...!.-llJ!1rft-----:-1f-..ll..- - - -----liI'\- - - --I\--r-
·0.04
+-------''''--'If - - - - - - - - - - - - - ' - - - - -
.0.06
..1.-_-""'-"'-_ _ _ _ _ _ _ _ _ _ _ _ _ _ _---'==='--_
Figure 3. Changes of total hemoglobin concentration signal of left orbitofrontal cortex (OFC). The average values of total hemoglobin concentration at each point of measurement in the left OFC of all participants was graphed. Total hemoglobin signal of left OFC dynamically fluctuated during odor stimulation. The pale and dark gray zones indicate the first and last half of the detection stage. 0. 0 6 , - - - - - - - - - - - - - , ._ _ _ _ _ _- - - -
Figure 1. Placement of emitters and detectors.
0.02
+-+-- --f'..--::I-- - - - - /
0 .025 , - - - - - - -- - - - - - ,......- - - - - : -- - - - 0.02
-l------------;------;.------
0 .015 +..----,AHC"ft.r-~\_---!-~-----:-----0.01 +-+-I-------\-~--i'-----+_----0 .005
·0.01 '0.015
+ - - - - - - - - --\, - - -
- l - - - - - - -- - - - -- - - -=-- l - - --=- " - - --
-\-- - - -
-"'-"'' - - - - ! - -= ' - - -+ --'tJ''--; .r--:
.0.02 .L_ _ _ _ _ _ _ _ _ _ _~~~'---''''''''--_---''''-'''''"''-_
Figure 2. Typical near-infrared spectroscopy (NIRS)-detected response. Total hemoglobin signal in the right orbitofrontal cortex (OFC) showed significant increase during odorant stimulation (f3-phenyl ethyl alcoho/). The pale and dark gray zones indicate the first and last half of the detection stage.
RESULTS The intensity of the odor stimulus was sufficient for detection by all participants. Ten participants correctly identified (named or described) the odorant, and four did not. Citral was correctly identified by six of seven participants (p = 0.23). BPEA was correctly identified by four of seven participants. With hedonic valence scores of 0 or higher defined as representing a pleasant odor and scores of -lor lower as an unpleasant odor, the odorant was found pleasant by eight participants and unpleasant by six. Interestingly, none of the participants who failed to correctly identify the odorant found the odor pleasant (p = 0.0025). No significant differences were observed between the hedonic valence scores for BPEA and citra!. Typical signal changes in NIRS for BPEA stimulation are shown in Fig. 2. For statistical analysis, each experiment was divided into a predetection stage (Pre), a detection stage (D), and a postdetection stage (Post). The detection stage was further divided into the first half (D1) and the second half (D2). The average value for each stage was used as representative of the corresponding stage for further analysis of individual participant's evaluation. Zero was the average for predetection and postdetection signal levels. First, we examined whether odor stimulus could generally produce hemodynamic changes in OFCs. Statistical analysis of all participants' data by Wilcoxon signed-rank test showed significant changes during
164
·0.04 +---------~
-0.06
+-----------~
· O.OS
+------------/
-0.1
+--1-'''-'"- - - - - '......- - - /
-0.12
+ - - - - - - -- - - - - /
·0.14 . 1 - - - - - - - - - - - - - - - ' = = = = = = - - - - -
Figure 4. Changes of total hemoglobin concentration signal in right orbitofrontal cortex (OFC) according to odor identification. The average values of total hemoglobin concentration at each point of measurement in the right OFC of the subjects who correctly identified the odor and who incorrectly identified the odor were graphed. Significant differences were observed in the total hemoglobin concentration signal changes in the right OFC between the participants who successfully identified the odor (solid line) and those who failed to identify the odor (dotted line). Average predetection data and postdetection signal levels were zero. The pale and dark gray zones indicate the first and last half of the detection stage. Solid line indicates average signal for the participants who successfully identified the odor. Dotted line indicates average signal for the participants who failed to identify the odor.
the experiment in the concentration of total hemoglobin in the left OFC (p = 0.04) but not in the right OFC (p = 0.33). Total hemoglobin concentration in the left OFC of all but two participants dynamically fluctuated in a similar manner during odor stimulation (Fig. 3). Next, we analyzed whether preference for or perception of odors can make a difference in the hemodynamics of OFC. We found significant differences in the total hemoglobin signal changes (Fig. 4; P = 0.0008) of the right OFC between the participants who correctly identified the odorant stimulus and those who did not. No significant differences were observed between the "pleasant odor group" and the "unpleasant odor group" or between the "citral group" and the "BPEA group" (data not shown).
DISCUSSION To date, robust efforts have been made to explore the mechanism of odor perception by using fMRI,9,1O PET,l1 NIRS,I-6 or magnetoencephalography.12 However, most studies were performed based on the assumption that so-called pleasant odors, such as citra!, must
May-June 2011, Vol. 25, NO.3
generate pleasant feelings, and the opposite for unpleasant odorS.13,14 The activated regions were thus analyzed according to the type of task and/ or odor stimulation given to the participants but not in terms of their actual feelings, reactions, or recognition. However, as the findings of our study show, "pleasant" odor stimulation is not necessarily recognized as pleasant odor. It has been reported that OFCs are significantly activated during the odor familiarity, hedonicity, and intensity judgment tasks rather than during the detection task. l s Generally speaking, the OFC region is related to tasks involving semantic association or encoding. 16 Thus, activation of the OFCs is regarded as the final step in odor identification since it provides the basis for verbalization and naming of odors as shown in our previous study using fMRI.8 However, measurement of OFC activation by fMRI has its limitations because of severe artifacts such as signal loss or distortion induced by intravoxel phase dispersion at the skull baseY To address this issue, we used NIRS combined with a questionnaire to further investigate the functional role of OFCs in olfactory processes. In line with the findings of previous studies,5,l S statistical analysis of all participants' data showed significant hemodynamic changes in the left OFC produced by the odorant stimulation, further supporting the idea that the left OFC is generally involved in the olfactory system, i.e., the process of smelling. In addition, the fact that a similar pattern of signal change was observed in the majority of participants suggests that, as proposed by Ishimaru et al.,2,3 NIRS can function as an objective test for olfaction. Furthermore, NIRS disclosed a significant increase in the signal in the right OFC of the participants who correctly identified the odor in line with our previous study.s These findings hint at the notion that the right OFC may be involved in the odor familiarity and hedonicity judgment tasks. 15,1 6 Although our present study could not show the significant signal changes, our previous study showed that the left middle OFC was significantly more often activated in the participants who failed to name the odor correctly and in those who perceived the odor stimulation as unpleasant,S suggesting the hypothesis that the left OFC may be involved in efforts to name or identify odors and be associated with the unpleasant aspects of these stimuli. 19 ,20
CONCLUSIONS Our findings indicate that NIRS combined with a questionnaire is a useful method for studying the functional neuroanatomy of OFC in terms of olfaction. The left OFC is generally involved in olfaction, and the right OFC is involved in the odor familiarity judgment tasks. Additional studies using NIRS in combination with questionnaires with variety of odors on normal subjects and patients with olfactory disorders caused by various diseases 21 ,22 can be expected to provide a better understanding of the functional role of various brain lesions in odor information processing.
REFERENCES 1.
2.
Harada H, Tanaka M, and Kato T. Brain olfactory activation measured by near-infrared spectroscopy in humans. J Laryngol Otol 120:638-643, 2006. Ishimaru T, Yata T, and Hatanaka-Ikeno S. Hemodynamic response of the frontal cortex elicited by intravenous thiamine propyldisulphide administration. Chern Senses 29:247-251, 2004.
American Journal of Rhinology & Allergy
3. Ishimaru T, Yata T, Horikawa K, and Hatanaka S. Near-infrared spectroscopy of the adult human olfactory cortex. Acta Otolaryngol Suppl 553:95-98, 2004. 4. Fladby T, Bryhn G, Halvorsen 0, et al. Olfactory response in the temporal cortex of the elderly measured with near-infrared spectroscopy: A preliminary feasibility study. J Cereb Blood Flow Metab 24:677-680, 2004. 5. Kobayashi E, Kusaka T, Karaki M, et al. Functional optical hemodynamic imaging of the olfactory cortex. Laryngoscope 117:541-546, 2007. 6. Kobayashi E, Karaki M, Kusaka T, et al. Functional optical hemodynamic imaging of the olfactory cortex in normosmia subjects and dysosmia subjects. Acta Otolaryngol Suppl 562:79-84, 2009. 7. Koch K, Pauly K, Kellermann T, et al. Gender differences in the cognitive control of emotion: An fMRI study. Neuropsychologia 45: 2744-2754,2007. 8. Katata K, Sakai N, Doi K, et al. Functional MRI of regional brain responses to "pleasant" and "unpleasant" odors. Acta Otolaryngol Suppl 562:85-90, 2009. 9. Koizuka I, Yano H, Nagahara M, et al. Functional imaging of the human olfactory cortex by magnetic resonance imaging. ORL J Otorhinolaryngol Relat Spec 56:273-275, 1994. 10. Atighechi S, Salari H, Baradarantar MH, et al. A comparative study of brain perfusion single-photon emission computed tomography and magnetic resonance imaging in patients with post-traumatic anosmia. Am J Rhinol Allergy 23:409-412, 2009. 11. Dade LA, Jones-Gotman M, Zatorre RJ, and Evans AC. Human brain function during odor encoding and recognition. A PET activation study. Ann N Y Acad Sci 855:572-574, 1998. 12. Sakuma K, Kakigi R, Kaneoke Y, et al. Odorant evoked magnetic fields in humans. Neurosci Res 27:115-122, 1997. 13. Rolls ET, Kringelbach ML, and de Araujo IE. Different representations of pleasant and unpleasant odours in the human brain. Eur J Neurosci 18:695-703, 2003. 14. Grabenhorst F, Rolls ET, Margot C, et al. How pleasant and unpleasant stimuli combine in different brain regions: Odor mixtures. J Neurosci 27:13532-13540, 2007. 15. Petersen SE, van Mier H, Fiez JA, and Raichle ME. The effects of practice on the functional anatomy of task performance. Proc Natl Acad Sci USA 95:853-860, 1998. 16. PIa illy J, Bensafi M, Pachot-Clouard M, et al. Involvement of right piriform cortex in olfactory familiarity judgments. Neuroimage 24: 1032-1041,2005. 17. Osterbauer RA, Wilson JL, Calvert GA, and Jezzard P. Physical and physiological consequences of passive intra-oral shimming. Neuroimage 29:245-253, 2006. 18. Kobayashi M, Sasabe T, Takeda M, et al. Functional anatomy of chemical senses in the alert monkey revealed by positron emission tomography. Eur J Neurosci 16:975-980, 2002. 19. Gottfried JA, Deichmann R, Winston JS, and Dolan RJ. Functional heterogeneity in human olfactory cortex: An event-related functional magnetic resonance imaging study. J Neurosci 22:10819-10828, 2002. 20. Kringelbach ML, O'Doherty J, Rolls ET, and Andrews C. Activation of the human orbitofrontal cortex to a liquid food stimulus is correlated with its subjective pleasantness. Cereb Cortex 13:1064-1071, 2003. 21. Fleiner F, Dahlslett SB, Schmidt F, et al. Olfactory and gustatory function in patients with multiple sclerosis. Am J Rhinol Allergy 24:e93-e97, 2010. 22. Gaines AD. Anosmia and hyposmia. Allergy Asthma Proc 31:185189,2010. 0
165
