Cerebral Cortex October 2007;17:2268--2275 doi:10.1093/cercor/bhl135 Advance Access publication December 5, 2006 Interactions between Olfaction and the Johannes Frasnelli1,2, Benno Schuster2 and Thomas Hummel2 Trigeminal System: What Can Be Learned 1Cognitive Unit, Montreal Neurological from Olfactory Loss Institute, Montreal, Quebec, Canada and 2Smell and Clinic, University of Dresden Medical School, Dresden, Germany

The olfactory and the trigeminal systems have a close relationship. trigeminal stimuli activate the trigeminal system on the level of Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021 Most odorants also stimulate the trigeminal . Further, subjects the respiratory (Kobal 1985; Thu¨rauf et al. 1991, with no of smell exhibit a decreased trigeminal sensitivity 1993; Hummel, Schiessl, et al. 1996; Lo¨tsch et al. 1997; Hummel with unclear underlying mechanisms. Previous studies indicated et al. 2000; Wendler et al. 2001). In addition, investigators can that single stages of trigeminal processing may differently be examine trigeminal activation on a central level by means of affected by olfactory loss. A better knowledge of adaptive and trigeminal event-related potentials (tERPs), which are electro- compensatory changes in the trigeminal system of subjects with encephalographic responses generated in the cortex (Kobal acquired (AA) will improve the understanding of interac- et al. 1990; Hummel et al. 1994; Lo¨tsch et al. 1997). Previously, tive processes between the 2 sensory systems. Thus, we aimed to we have used a parallel application of both techniques—which assess trigeminal function on different levels of processing in permits the localization of processes within the trigeminal subjects with AA. Subjects with AA showed larger electrophysio- pathway (Frasnelli and Hummel 2003; Dalton et al. 2006)—to logical responses to irritants obtained from the mucosa than investigate subjects born without a (idiopathic healthy controls. On central levels, however, they exhibited smaller congenital anosmia—ICA). ICA subjects had larger peripheral event-related potentials and psychophysical measures to irritants. responses to intranasal trigeminal stimuli than healthy controls Over 9 months, they exhibited an increase in trigeminal sensitivity. (Frasnelli et al. 2006). This finding led us to the development of Subjects with recovering olfactory function showed an even more a model where compensatory adaptive processes occur in the increased peripheral responsiveness to irritants. These data periphery of the trigeminal system of ICA subjects. However, we suggest dynamic mechanisms of mixed sensory adaptation/com- do not know if subjects with congenital anosmia are compara- pensation in the interaction between the olfactory and trigeminal ble with subjects with acquired anosmia (AA subjects). First, systems, where trigeminal activation is increased on mucosal representing a minor subpopulation of anosmic subjects (ap- levels in subjects with AA and amplified on central levels in proximately 1%), ICA subjects, born without a sense of smell, subjects with a functioning . have a fundamentally different history than subjects with anosmia acquired during lifetime. Second, ICA subjects ex- Keywords: anosmia, chemosensory, event-related potentials, hibit similar central electrophysiological responses as healthy negative mucosal potentials, smell controls, contrasting markedly the findings in AA subjects (Hummel, Barz, et al. 1996). Thus, it is unclear whether results from ICA subjects can be generalized to all subjects with Introduction olfactory dysfunction. Most odorants also stimulate the (Doty 1975; Therefore, here we aimed to perform a combined investiga- Doty et al. 1978; Wysocki et al. 2003). Therefore, even anosmic tion of peripheral and central responses to trigeminal stimula- subjects are able to distinguish between odorants based on their tion in subjects with acquired olfactory dysfunction and to trigeminally mediated sensitivity (Laska et al. 1997). Thus, compare their results with those of healthy controls. We anosmic subjects are frequently investigated to assess the hypothesized that AA subjects would exhibit larger peripheral trigeminal impact of stimulants without concomitant olfactory and smaller central responses to irritants than healthy controls. stimulation (Cometto-Muniz et al. 1998, 1999). However, Furthermore, the longitudinal design enabled us to examine the anosmic subjects show reduced trigeminal sensitivity when long-term impact of olfactory dysfunction (Hummel, Barz, et al. compared with healthy controls (Hummel, Barz, et al. 1996; 1996; Hummel et al. 2003) and the effect of recovery in the Gudziol et al. 2001; Kendal-Reed et al. 2001; Walker et al. 2001; olfactory system (Reden et al. 2006) on trigeminally mediated Hummel et al. 2003). This suggests that, in addition to the responses. known mutual interactions between the olfactory and the trigeminal chemosensory systems in healthy subjects (Stone et al. 1968; Cain and Murphy 1980; Bouvet et al. 1987; Livermore Materials and Methods et al. 1992; Livermore and Hummel 2004), even the absence or The study was conducted according to the Declaration of Helsinki on presence of a functioning olfactory system influences trigeminal Biomedical Research Involving Subjects and was approved by . Anatomical and functional characteristics of the the local Ethics Committee. During enrollment, subjects were given underlying mechanisms are largely unknown. detailed information about all testing procedures. Written consent was obtained from all subjects prior to the study. Recently established electrophysiological methods enable us A total of 123 subjects participated in the study, divided in groups of to investigate the trigeminal chemosensory system in subjects with olfactory dysfunction and healthy controls. The group of with high sensitivity and temporal resolution. The negative subjects with olfactory dysfunction (AA subjects) consisted of 50 mucosal potential (NMP) allows researchers to investigate how women and 23 men. Olfactory dysfunction was caused either by a closed

Ó The Author 2006. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] head trauma (posttraumatic, 16 males, 20 females, mean age [MA]: 45.7 (80% relative humidity; total flow 8 L/min; 36 °C) (Kobal 1981). [standard error of mean (SEM) 2.1] years) or by an upper respiratory Recordings were made with a DC amplifier (low pass 30 Hz; Toennies, tract infection (postinfectious, 7 males, 30 females, MA: 59.3 [1.5] years). Germany). After analog-to-digital conversion (sampling rate 62.5 Hz), Subjects were identified based on their medical history. Only subjects records of 8192 ms were obtained. The pretrigger period was 500 ms. who provided clear anamnestic evidence that their olfactory dysfunc- Using a video system, subjects were monitored for movements during tion was caused by a trauma or a viral infection were included in the the recording periods. Records that might have been contaminated by study. This included the occurrence of olfactory dysfunction directly/ movements were excluded from further analysis. After averaging, the shortly after a trauma/viral infection and the absence of other factors amplitude and latency of the negativity (peak N1) was measured. known to cause olfactory dysfunction (e.g., nasal polyposis). Other than the olfactory dysfunction, subjects were in a good-health state as The tERP Measurements revealed by the otorhinolaryngological examination and a detailed The tERPs were recorded following CO2 stimuli of 40, 50, and 60% v/v. history. duration was 200 ms. Each stimulus concentration was The duration of olfactory dysfunction had a median of 19 months (1-- presented 15 times. The ISI was 30 s. 300 months). Subjects with olfactory dysfunction were compared with This resulted in a duration of the session of 25 min. Stimuli were 50 healthy controls (26 females, 24 males, MA: 48.6 [2.5] years). The

presented to the same as in the NMP measurement. Subjects Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021 participants’ age was distributed bimodally with a minimum at 42 years. were seated comfortably in an air-conditioned room. They received Thus, participants were divided into age groups (young: <42 years; white noise through headphones to mask switching clicks of the older: >42 years). stimulation device. To stabilize vigilance, subjects performed a simple Subjects were investigated in 2 sessions, separated by at least 9 tracking task on a computer screen. Using a joystick, they had to keep months and at most 15 months. All participating subjects were invited a small square inside a larger one, which moved unpredictably (Hummel for the second session; all controls returned for the second session, and Kobal 2002). whereas 12 of the subjects with olfactory dysfunction opted not to The tERPs were recorded at position Cz of the international 10-20 continue the experiment. System. Eye movements were monitored via the Fp2 lead. The tERPs In each session, tests of both olfactory and trigeminal sensitivity were were referenced against linked earlobes (A1--A2). The sampling fre- performed. Olfactory function was assessed by means of the ‘‘Sniffin’ quency was 250 Hz, and the pretrigger period was 500 ms with Sticks’’ test kit; trigeminal function was assessed by the NMP, tERP, a recording time of 2048 ms per record (bandpass, 0.02--30 Hz). intensity ratings of CO2 stimuli, and a lateralization task. Recordings were filtered additionally off-line (low pass, 15 Hz). After records contaminated through motor artifacts or blink artifacts had Test of Olfactory Function been discarded, tERPs were averaged and analyzed for amplitudes and All subjects underwent extensive tests of olfactory function. Olfactory latencies of the major peaks N1 and P2. function was measured with the ‘‘Sniffin’ Sticks’’ olfactory test kit (Hummel et al. 1997; Kobal et al. 2000), where odorants are presented Intensity Ratings in commercially available felt-tip pens (Burghart, Wedel, Germany). The After each stimulus presentation during the tERP session, subjects rated kit consists of separate tests for phenyl ethyl alcohol thresholds, stimulus intensity on a visual analog scale displayed on the computer odor discrimination, and identification. Results of the 3 subtests are screen. Using a joystick, subjects moved a marker on the scale. The right presented as a composite threshold discrimination identification (TDI) end of the scale indicated an ‘‘extremely strong’’ intensity (100%); the score. Based on a multicenter investigation, subjects with a TDI score left end indicated that no stimulus had been perceived (0%) (Hummel below 16 are considered functionally anosmic, with a score between 16 and Kobal 2002). Data for each stimulus condition were averaged and 31 subjects are considered hyposmic, and with a score equal to or separately to obtain a mean rating. above 31 subjects are identified as normosmic (Kobal et al. 2000). Lateralization Task NMP Measurements Trigeminal sensitivity was quantified by the subjects’ ability to lateralize NMPs were measured as previously described (Frasnelli et al. 2006). In stimuli presented to either the left or the right nostril. Based on previous short, NMPs were recorded from one nostril by means of tubular studies, neat (99%) eucalyptol (Aldrich-Chemie, Steinheim, Germany) electrodes (Teflonä tubing filled with 1% Ringer agar containing was used for the odor localization paradigm (Doty et al. 1978; Berg et al. a silver-chlorided silver wire) (Ottoson 1956). Placement of the 1998; Hummel et al. 2003; Frasnelli et al. 2006). The odor was presented electrode was controlled by nasal endoscopy (Leopold et al. 2000). to either one nostril in a high-density polyethylene squeeze bottle (total For reference, a silver-chlorided silver electrode was attached to the volume 250 mL) filled with 30 mL of the odorant; at the same time, an bridge of the nose. The recording sites were selected according to identical bottle filled with 30 mL of odorless propylene glycol was accessibility, low signal-to-noise ratio, and/or absence of stimulus- presented to the other nostril. The bottles had a pop-up spout that was induced artifacts (Hummel, Schiessl, et al. 1996). To keep the electrode placed into either nostril. A puff of approximately 15 mL air was in place, it was attached to clips mounted on a frame similar to lensless delivered by pressing the 2 bottles at the same time by means of a hand- glasses. held squeezing device. The subjects held onto the spouts to prevent In order to test whether at a given position a NMP could be recorded their movement inside the nostril, which accompanied squeezing of the or not, CO2 stimuli of 500-ms duration and 60% v/v were applied after bottles; movement of the spouts might produce mechanical irritation placing the electrode on the mucosa. Criteria for a reliable NMP were as that, in turn, might interfere with the subject’s ability to localize the follows: 1) response of the typical shape within the time range of NMP odor. A total of 40 stimuli were applied to the blindfolded subjects/ (Kobal 1985); 2) reproducible responses through 3 consecutive stimuli. patients at an ISI of approximately 40 s; stimulation of the left or right If no reliable NMP could be recorded at a given position, the electrode nostril followed a pseudorandomized counterbalanced sequence. After was placed at another position. This procedure was repeated up to 6 each stimulus, subjects/patients were asked to identify the nostril times. If still no reliable NMP could be recorded, the procedure was where the odorant had been presented. The sum of correct identifica- repeated on another day. If no NMP could be recorded on the second tions was used for further statistical analyses (Berg et al. 1998; Hummel day, the measurements were considered as ‘‘no reliable NMP record- et al. 2003). Testing required approximately 30min. able.’’ Subjects received CO2 stimuli of 500-ms duration and concentrations First and Second Session of 40, 50, and 60% v/v. The interstimulus interval (ISI) was 40 s. Stimuli AA subjects without recovery of olfactory function were expected to were presented to a randomly selected nostril by means of a computer- exhibit an increase of trigeminal sensitivity. In order to investigate the controlled air dilution (OM 6B; Burghart). This stimulator influence of a significant increase of olfactory function on trigeminal allows for application of chemical stimuli with a very rapid onset and sensitivity, the group of the subjects with olfactory dysfunction was offset. Mechanical stimulation is avoided by embedding these stimuli in divided into 2 groups in the second session. In line with previous reports a constant flow of odorless, humidified air of controlled temperature (Gudziol et al. 2006), an increase in the olfactory test result (TDI score)

Cerebral Cortex October 2007, V 17 N 10 2269 of at least 6 points was considered as a significant improvement. Subjects controls (F1,49 = 4.8; P = 0.034). Post hoc tests revealed group showing such recovery were compared with those who did not exhibit differences to be significant at higher concentrations. The NMP improvement (no recovery). In order to investigate the development of amplitude was found to be significantly different for the 60% v/v trigeminal sensitivity, the difference (D) between second and first stimulus in both sessions and for the 50% v/v stimulus in the session was calculated and used for statistical analyses. second session (P < 0.05; see Fig. 1). No significant group Statistical Analysis difference was observed in the other conditions. Statistical analyses were performed using SPSS 14.0 (SPSS Inc., Chicago, IL). Repeated measures analyses of variance were computed with variables of interest (between subject factors ‘‘age group’’ [young, old], Trigeminal Event-Related Potential. With regard to tERP ‘‘status’’ [olfactory dysfunction, control], and ‘‘sex’’ [female, male]; within measurements, subjects with olfactory dysfunction exhibited subject factors ‘‘session’’ [first, second] and ‘‘concentration’’ [low, smaller base-to-peak amplitudes P2 and peak-to-peak ampli- medium, high; where applicable]). For post hoc analyses, t-tests were tudes N1P2 when compared with controls (both F1,98 > 9.9; P < calculated. The alpha level was set at 0.05. Means are indicated together 0.003). Post hoc tests revealed significant group differences for with SEMs in square brackets. P2 for each concentration and session (all P < 0.025) with the Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021 exception of the 50% v/v concentration on the first session and Results the 60% v/v concentration on the second session (see Fig. 2). The posttraumatic subjects with olfactory dysfunction were With regard to N1P2, post hoc tests detected significant significantly younger than both postinfectious subjects with differences for each comparison except for 60% v/v on the olfactory dysfunction and controls (P < 0.03). However, no second session (all P < 0.043). significant difference was detected between subjects anosmic Younger subjects were found to have larger amplitudes P2 after trauma and subjects anosmic after an infection with regard and N1P2 than older subjects (both F1,98 > 9.2; P < 0.004). In to any trigeminal measure. Therefore, for further analysis, addition, women were found to have larger amplitudes N1P2 subjects with olfactory dysfunction were grouped together than men (F1,98 = 6.2; P = 0.014). Furthermore, there was and compared with healthy controls. We will, however, report a significant interaction between ‘‘age group’’ and ‘‘status’’ for the descriptive statistics for both groups of olfactory dysfunc- N1P2 (F1,98 = 6.8; P = 0.01). This indicated that differences in tion and healthy controls (Table 1). N1P2 were detected between subjects with olfactory dysfunc- tion within the young participants, but not in the older group. Comparisons across All Subjects The same interaction was detected for amplitude N1 (F1,98 = 4.38; P = 0.039), indicating that large differences for N1 Electrophysiological Measurements appeared between young subjects with olfactory dysfunction and young controls; in older subjects, no such group difference Negative Mucosal Potential. In the first session, a NMP could be was detected. recorded in 37 of 50 (74%) controls and 45 of 73 (61%) subjects with olfactory disorder. In the second session, the numbers Psychophysical Measurements were 37 of 50 (74%) and 46 of 58 (79%) for controls and subjects with olfactory loss, respectively. AA subjects were Intensity Ratings. With regard to intensity ratings, no main found to show higher levels of peripheral activation than effect of status, sex, or age group could be detected.

Lateralization. In the lateralization task, younger subjects had Table 1 higher results than older ones (F1,103 = 7.6; P = 0.007). There was Mean values and SEM for the most important measures a tendency toward higher scores in controls than in subjects Measure (dimension) Condition Session Olfactory dysfunction Controls with olfactory dysfunction; it failed, however, to reach signifi- Postviral SEM Posttraumatic SEM Healthy SEM cance (F1,103 = 3.4; P = 0.068). On the first session, the difference between AA subjects and controls was significant (P = 0.023); on Lateralization 1 31.9 0.9 33.3 0.9 34.8 0.7 (score) 2 33.1 0.9 33.2 1 34.7 0.7 the second session, there was no significant difference between Intensity ratings 40% v/v CO2 1202162171the 2 groups (see Fig. 3). (units) 2192182181 50% v/v CO2 1333322322 2262272251 60% v/v CO2 1453463492Comparisons within the Group of Subjects with Olfactory 2423433462 Dysfunction NMP amplitude 40% v/v CO2 1 161 30 103 18 120 19 (lV) 2 148 24 111 31 100 15 50% v/v CO2 1 308 60 156 30 158 30 Comparisons between First and Second Session for Subjects 2 276 56 176 47 139 18 60% v/v CO2 1 529 130 278 61 240 44 with Olfactory Dysfunction without Recovery 2 312 64 291 75 184 21 No difference between first and second session in any test of Event-related 40% v/v CO2 1 10.2 0.6 11.1 1.0 13.6 1.9 potential amplitude 2 11.7 0.9 11.6 1.2 15.2 1.0 trigeminal sensitivity could be observed for those subjects who N1P2 ([lV) 50% v/v CO2 1 16.4 1.2 16.4 1.2 19.3 1.3 showed no increase in their olfactory function. 2 14.7 0.9 15.1 1.3 18.4 1.0 60% v/v CO2 1 20.0 1.6 20.0 1.4 23.6 1.4 2 20.6 2.2 20.8 1.4 22.3 1.2 Comparisons of Subjects with Recovery of Olfactory Function Smell function 1 14.9 0.8 12.4 0.6 35.3 0.6 and Subjects without Recovery of Olfactory Function (TDI score) 2 19.1 1.1 14.3 1.0 34.1 0.6 An increase of at least 6 points of the TDI score (recovery of Note: Subjects with olfactory dysfunction are grouped depending on the etiology (postviral or olfactory function) was found for 38% and 12% of the subjects in posttraumatic). the postinfectious and the posttraumatic group, respectively.

2270 Trigeminal Function in Olfactory Loss d Frasnelli et al. Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021

Figure 1. Upper part: Amplitudes (means and SEM) of NMPs following stimuli of 40%, 50%, and 60% v/v CO2 in the first (left) and second (right) session for AA subjects (black bars) and controls (white bars). Lower part: Grand means of NMPs following stimuli of 60% v/v CO2. Graphs are displayed separately for first (left) and second session (right). Results for healthy subjects are indicated by gray lines and results of AA subjects are indicated by black lines. The black horizontal bar indicates the onset and duration of the CO2 stimulus.

Figure 2. Upper part: Amplitudes P2 (means and SEM) of event-related potential (ERP) following stimuli of 40%, 50%, and 60% v/v CO2 in the first (left) and second (right) session for AA subjects (black bars) and controls (white bars). Lower part: Grand means of ERP following stimuli of 60% v/v CO2. Graphs are displayed separately for first (left) and second session (right). Results for healthy subjects are indicated by gray lines and results of AA subjects are indicated by black lines. The black horizontal bar indicates the onset and duration of the CO2 stimulus.

The duration of the olfactory dysfunction was not related to a longer duration of olfactory dysfunction. The groups were not the improvement (DTDI) of olfactory function (r58 = –0.028). significantly different from each other. We further divided the anosmic subjects into 2 groups by using There was a significant effect of ‘‘recovery’’ on DNMP (F1,28 = the median of the duration of olfactory dysfunction (19 12.6; P = 0.001). DNMP was larger in the recovery group than in months). The group with a shorter duration of olfactory loss the no recovery group following stimulation of each concen- showed, on average, an improvement of 3.4 (1.0) points in the tration (all P < 0.035; see Fig. 4), reflecting that NMPs became TDI score; this number was 2.5 (0.9) for the subjects with larger in the recovery group and smaller in the no recovery

Cerebral Cortex October 2007, V 17 N 10 2271 Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021

Figure 3. Upper part: Intensity ratings (means, SEM) for 40%, 50%, and 60% v/v CO2 in the first (left) and second (right) session for AA subjects (black bars) and controls (white bars). Lower part: Results of the lateralization score ratings (means, SEM) in the first and second session for AA subjects (black bars) and controls (white bars). Results are given as absolute numbers, a score of 40 corresponds to 100% correct responses; chance level (50%) is at a score of 20.

Figure 4. Amplitudes (means and SEM) of NMPs following stimuli of 40%, 50%, and 60% v/v CO2 in the first (left) and second (right) session for AA subjects who showed recovery of olfactory function at second session (black bars) and AA subjects who did not show recovery of olfactory function at second session (white bars). group. No other measure of trigeminal sensitivity revealed an that the olfactory system and the trigeminal system interact on effect of recovery. central levels (Cain and Murphy 1980; Livermore et al. 1992). Further, trigeminal chemosensory stimulation and olfactory Discussion stimulation lead to considerable overlap in their activation Subjects with olfactory dysfunction showed larger peripheral, patterns in the , for example, in the ventral insula, the but smaller central electrophysiological, responses than con- middle frontal gyrus, and supplemental motor areas (Hummel trols. With regard to psychophysical tests, subjects with et al. 2005). In addition, trigeminal stimulation produces olfactory dysfunction tended toward lower lateralization activation in the , superior temporal gyrus, scores. and the caudate nucleus (Hummel et al. 2005); these areas are involved in the processing of olfactory sensations (e.g., Jones- Central Measures Gotman and Zatorre 1988; Zatorre et al. 1992; Kettenmann et al. Subjects with olfactory dysfunction exhibited smaller tERP 1996; Savic et al. 2000; Gottfried et al. 2002; for review, see Zald amplitudes than healthy controls. This is in line with earlier and Pardo 2000). Thus, data from the present study suggest that studies, where subjects with AA showed smaller central elec- olfactory loss produces central nervous changes leading to trophysiological responses compared with controls (Hummel, a reduced responsiveness following trigeminal stimulation. To Barz, et al. 1996). Similarly, they have been reported to exhibit investigate this further and to localize the centers of interaction lower scores in the lateralization task (Hummel et al. 2003) in between olfactory and trigeminal systems, fMRI studies in line with the results in the first session of the present study. anosmic subjects are currently underway. These data indicate that, for full functionality, the trigeminal system relies on a functioning olfactory system (Hummel, Barz, Temporal Effects et al. 1996; Kendal-Reed et al. 1998; Gudziol et al. 2001; Walker In earlier studies, the duration of the olfactory dysfunction and et al. 2001; Hummel et al. 2003; Frasnelli et al. 2006). We know both tERP amplitudes (Hummel, Barz, et al. 1996) and scores in

2272 Trigeminal Function in Olfactory Loss d Frasnelli et al. the lateralization task (Hummel et al. 2003) were positively described that reenter the and reach the correlated. Based on cross-sectional data, the authors concluded and terminate in its glomerular layer (Finger and that trigeminal sensitivity caused by an olfactory loss would Bottger 1993; Schaefer et al. 2002). We do not know in which improve over time. Further, ICA patients, who can be seen as direction information is transferred within these collaterals, but subjects with the longest imaginable absence of olfactory it is possible that they receive information from neurons of the function, showed a similar ability to lateralize eucalyptol as olfactory bulb (Schaefer et al. 2002). Furthermore, in a recent healthy controls, though they had a tendency toward lower study, Christie and Westbrook (2006) found mitral cells in the scores (Frasnelli et al. 2006). The authors therefore speculated same to excite each other laterally. The authors that with time, trigeminal sensitivity in subjects with acquired hypothesized that electrical coupling and spillover creates olfactory dysfunction could reach the level of controls. In line a lateral excitatory network within the glomerulus. This with earlier work (Hummel et al. 2003), in the present study, mechanism may also activate the trigeminal collaterals within subjects with olfactory dysfunction also scored lower in the the olfactory bulb and thus continuously stimulate trigeminal lateralization task in the first session than healthy controls. In neurons in subjects with a normal sense of smell. Ongoing the second session, however, both groups had similar scores. stimulation—due to chronic exposure to trigeminal agents—in Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021 Thus, subjects with olfactory dysfunction reached the levels of turn, has been shown to reduce the overall and especially the normal controls over time. Although their lateralization score peripheral responsiveness of the trigeminal chemosensory did not significantly increase, these data suggest that the effect system, perhaps via a transient downregulation of receptors of olfactory dysfunction on the ability to lateralize trigeminal (Dalton et al. 2006). It is, however, not clear, whether the stimulants tends to get less pronounced over time. density or the responsiveness of trigeminal nerve endings Temporal factors also affected the results of the tERP changes. One could hypothesize that in a damaged olfactory measurements. Group differences were most pronounced in system, no such excitatory network exists. Thus, trigeminal the younger subjects group. Thus, older subjects with olfactory collaterals in the bulb are not excited continuously, and no dysfunction and older controls appear to exhibit similar levels of reactive reduction of responsiveness in the trigeminal system central trigeminal activation. This, however, seems not to be would occur. In other words, missing inhibition via the tri- due to regeneration in the olfactory dysfunction group because geminal collaterals in the olfactory bulb would lead to a com- there was no change from the first to the second session. pensatory functional up regulation in the periphery of the Moreover, it seems to be caused by the age-related decline of trigeminal nerve in the case of olfactory loss. This would be trigeminal sensitivity in healthy subjects (Frasnelli and Hummel followed by increased peripheral responsiveness, as reflected in 2003). Thus, creating additional distortion (olfactory loss) in an enlarged NMP amplitudes. On central levels, however, this already dysfunctional system (age-related decline of trigeminal increased input would not sufficiently compensate for the sensitivity) does not produce much reduction of overall missing amplification of the trigeminal signal, which seems to function (trigeminal sensitivity). In other words, young subjects occur in healthy subjects (Fig. 5). This could be tested in animal have more trigeminal sensitivity to lose, compared with elderly. models where we predict that both bulbectomy and/or Over time, as subjects get older, the difference between transection of intrabulbar trigeminal collaterals would lead to subjects with olfactory dysfunction and controls becomes less higher peripheral responses to irritants. pronounced. AA subjects had larger NMPs than healthy controls. This is in agreement with an earlier study where congenitally anosmic subjects had larger NMPs than healthy controls (Frasnelli et al. Peripheral Measures 2006). Congenital anosmia represents a developmental disorder On a peripheral level, other than on a central level, subjects with olfactory dysfunction responded stronger than controls. This is in line with an earlier report, where subjects with congenital anosmia exhibited larger NMP amplitudes in response to strong CO2 stimuli than controls (Frasnelli et al. 2006). As in the present study, this was most pronounced for stronger stimuli. Therefore, increased peripheral trigeminal chemosensory sus- ceptibility seems to be a general feature of subjects without a functioning sense of smell because the trigeminal system’s periphery responds comparably in both subjects with congen- ital anosmia and subjects with acquired olfactory dysfunction. The finding of higher peripheral trigeminal susceptibility in subjects with a loss of olfactory function (Frasnelli et al. 2006, present study) may lead to the hypothesis that a working olfactory system would inhibit the trigeminal system on peripheral levels. A similar mechanism occurs on central levels in the gustatory system (release of inhibition: Halpern and Nelson 1965). Here, input from the inhibits that Figure 5. Proposed model of interaction between olfactory (gray arrows) and of the glossopharyngeal nerve. Anesthesia or damage of the trigeminal (black arrows) systems. (A) Normal conditions. Peripheral responsiveness is chorda tympani abolishes this inhibition that increases the input decreased due to constant activation of intrabulbar trigeminal collaterals and consequent functional downregulation in the periphery of the trigeminal system. from areas innervated by the glossopharyngeal nerve. As Functional integration of olfactory and trigeminal processes leads to augmented a possible site of interaction between the olfactory and tri- cortical signal. (B) Olfactory loss. Increased NMP due to top--down regulation; geminal systems, collaterals of the trigeminal nerve have been decreased event-related potential due to missing olfactory augmentation.

Cerebral Cortex October 2007, V 17 N 10 2273 that is in contrast to AA, where olfactory loss is linked to tion to occur in the interactions between olfactory and the a noxious event as trauma/viral infection. The similarity of the trigeminal system. changes in at least the periphery of the trigeminal system supports the hypothesis that trauma/viral infection do not have Notes a direct effect on the trigeminal system, but rather an indirect Research described in this article was supported by Philip Morris USA impact via the alterations of the olfactory system. This is further Inc. and Philip Morris International. Conflict of Interest: None declared. supported by the fact that, in contrast to the very thin olfactory Address correspondence to Johannes Frasnelli, Cognitive Neurosci- nerve bundles, the trigeminal nerve is the thickest cranial nerve, ence Unit, Montreal Neurological Institute, 3801 University Street, which makes it less susceptible to a trauma. In addition, the Montreal, Quebec H3A 2B4, Canada. Email: [email protected]. and the trigeminal nerve have very distinct anatomical pathways. References We could measure a NMP signal in 61--79% of our subjects. In Berg J, Hummel T, Huang G, Doty RL. 1998. Trigeminal impact of other studies, similar numbers were reached. Scheibe et al. odorants assessed with lateralized stimulation. Chem . 23:587. (2006) successfully recorded a NMP in 62% healthy subjects. Bouvet JF, Delaleu JC, Holley A. 1987. cell function is Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021 affected by trigeminal nerve activity. Neurosci Lett. 77:181--186. Similarly, Frasnelli et al. (2006) report to have obtained a NMP in Cain WS, Murphy CL. 1980. Interaction between chemoreceptive 44--84% of subjects. In the present study, the percentages of modalities of odour and irritation. Nature. 284:255--257. successful NMP recordings were similar across the subject Christie JM, Westbrook GL. 2006. Lateral excitation within the olfactory groups; the fact that a NMP was not recordable in every subject bulb. J Neurosci. 26:2269--2277. should therefore not have impacted on the outcome of this Cometto-Muniz JE, Cain WS, Abraham MH, Gola JM. 1999. Chemo- study. sensory detectability of 1-butanol and 2-heptanone singly and in binary mixtures. Physiol Behav. 67:269--276. Further, we optimized stimulus delivery parameters for NMP Cometto-Muniz JE, Cain WS, Abraham MH, Kumarsingh R. 1998. and event-related potential paradigms, respectively. In the tERP Trigeminal and olfactory chemosensory impact of selected terpenes. measurements, both stimulus duration (tERP: 200 ms; NMP: 500 Pharmacol Biochem Behav. 60:765--770. ms) and the ISI were shorter (tERP: 30 s; NMP: 40 s). We chose Dalton P, Dilks D, Hummel T. 2006. Effects of long-term exposure to these parameters because they have been shown to be optimal volatile irritants on sensory thresholds, negative mucosal potentials, stimuli for the respective measurements (Hummel and Kobal and event-related potentials. Behav Neurosci. 120:180--187. 2002; Frasnelli and Hummel 2003; Frasnelli et al., 2003). How- Doty RL. 1975. Intranasal trigeminal detection of chemical vapors by humans. Physiol Behav. 14:855--859. ever, as tERP mainly depends on stimulus onset and stimulus Doty RL, Brugger WPE, Jurs PC, Orndorff MA, Snyder PJ, Lowry LD. 1978. concentration (Frasnelli et al., 2003), we do not expect the Intranasal trigeminal stimulation from odorous volatiles: psychomet- differences in stimulus characteristics to cause a major bias in ric responses from anosmic and normal humans. Physiol Behav. the results (Frasnelli et al. 2006). 20:175--185. Finger TE, Bottger B. 1993. Peripheral peptidergic fibers of the trigeminal nerve in the olfactory bulb of the rat. J Comp Neurol. 334:117--124. The Effect of Recovery of Olfactory Function Frasnelli J, Hummel T. 2003. Age-related decline of intranasal trigeminal sensitivity: is it a peripheral event? Brain Res. 987:201--206. Reden et al. (2006) found that the sense of smell of 32% of the Frasnelli J, Lotsch J, Hummel T. 2003. Event-related potentials to subjects with olfactory dysfunction following an infection of the intranasal trigeminal stimuli change in relation to stimulus concen- upper respiratory tract recovered over the range of 1 year. In tration and stimulus duration. J Clin Neurophysiol. 20:80--86. the group of subjects with posttraumatic olfactory dysfunction, Frasnelli J, Schuster B, Hummel T. 2006. Subjects with congenital only 10% exhibited improvement of their olfactory function. We anosmia have larger peripheral but similar central trigeminal responses. Cereb Cortex. Advance Access published March 8, found similar percentages, with 38% of the postinfectious group 2006, doi:10.1093/cercor/bhj154. and 12% of the posttraumatic group showing a significant Gottfried JA, Deichmann R, Winston JS, Dolan RJ. 2002. Functional increase in their olfactory test score over the range of 9--15 heterogeneity in human olfactory cortex: an event-related functional months. With regard to trigeminal sensitivity, subjects with magnetic resonance imaging study. J Neurosci. 22:10819--10828. regenerated olfactory function had relatively larger NMP Gudziol H, Schubert M, Hummel T. 2001. Decreased trigeminal amplitudes than those without recovery. If the model proposed sensitivity in anosmia. ORL J Otorhinolaryngol Relat Spec. 63:72--75. above applies, regained activity in the olfactory bulb via lateral Gudziol V, Lotsch J, Hahner A, Zahnert T, Hummel T. 2006. Clinical significance of results from olfactory testing. Laryngoscope. excitation (Christie and Westbrook 2006) would add to the 116:1858--1863. overall input to the trigeminal nerve, leading to an even higher Halpern BP, Nelson LM. 1965. Bulbar gustatory responses to anterior and peripheral responsiveness. However, over time, we expect NMP to posterior tongue stimulation in the rat. Am J Physiol. 209:105--110. responses of subjects recovering from olfactory loss to decrease Hummel T, Barz S, Lotsch J, Roscher S, Kettenmann B, Kobal G. 1996. to the levels of normal controls. Loss of olfactory function leads to a decrease of trigeminal sensitivity. Thus, data from the present study support the model of Chem Senses. 21:75--79. Hummel T, Doty RL, Yousem DM. 2005. Functional MRI of intranasal mixed sensory adaptation/compensation in the interactions chemosensory trigeminal activation. Chem Senses. 30:i205--i206. between the olfactory and the trigeminal system (Frasnelli et al. Hummel T, Futschik T, Frasnelli J, Huttenbrink KB. 2003. Effects of 2006). In this dynamic model, healthy subjects exhibit a reduced olfactory function, age, and gender on trigeminally mediated primary trigeminal activation. On a central level, the trigeminal sensations: a study based on the lateralization of chemosensory signal is amplified in subjects with a functioning olfactory stimuli. Toxicol Lett. 140--141:273--280. system. Hummel T, Hummel C, Friedel I, Pauli E, Kobal G. 1994. Assessment of antinociceptive effects in man: comparison of imipramine, tramadol, In conclusion, subjects with olfactory dysfunction exhibited and anpirtoline. Br J Clin Pharmacol. 37:325--333. larger peripheral, but smaller central electrophysiological, Hummel T, Kobal G. 2002. Olfactory event related potentials. In: Simon responses when compared with healthy controls. These data S, Nicolelis M, editors. Methods in chemosensory research. Boca indicate mixed mechanisms of sensory adaptation/compensa- Raton (FL): CRC press. p. 429--464

2274 Trigeminal Function in Olfactory Loss d Frasnelli et al. Hummel T, Schiessl C, Wendler J, Kobal G. 1996. Peripheral electro- Livermore A, Hummel T, Kobal G. 1992. Chemosensory event-related physiological responses decrease in response to repetitive painful potentials in the investigation of interactions between the olfactory stimulation of the human nasal mucosa. Neurosci Lett. 212:37--40. and the somatosensory (trigeminal) systems. Electroencephalogr Hummel T, Schiessl C, Wendler J, Kobal G. 2000. Peripheral and Clin Neurophysiol. 83:201--210. central nervous changes in patients with rheumatoid arthritis in Lo¨tsch J, Hummel T, Kraetsch HG, Kobal G. 1997. The negative mucosal response to repetitive painful stimulation. Int J Psychophysiol. potential: separating central and peripheral effects of NSAIDs in man. 37:177--183. Eur J Clin Pharmacol. 52:359--364. Hummel T, Sekinger B, Wolf SR, Pauli E, Kobal G. 1997. ‘‘Sniffin’ sticks’’: Ottoson D. 1956. Analysis of the electrical activity of the olfactory olfactory performance assessed by the combined testing of odor epithelium. Acta Physiol Scand. 35:1--83. identification, odor discrimination and olfactory threshold. Chem Reden J, Mueller A, Mueller C, Konstantinidis I, Frasnelli J, Landis BN, Senses. 22:39--52. Hummel T. 2006. Recovery of olfactory function following closed Jones-Gotman M, Zatorre RJ. 1988. Olfactory identification deficits head injury or infections of upper respiratory tract. Arch Otolaryngol in patients with focal cerebral excision. Neuropsychologia. Head Surg. 132:265--269. 26:387--400. Savic I, Gulyas B, Larsson M, Roland P. 2000. Olfactory functions Kendal-Reed M, Walker JC, Morgan WT. 2001. Investigating sources of are mediated by parallel and hierarchical processing. Neuron. response variability and neural mediation in human nasal irritation. 26:735--745. Downloaded from https://academic.oup.com/cercor/article/17/10/2268/309112 by guest on 01 October 2021 Indoor Air. 11:185--191. Schaefer ML, Bottger B, Silver WL, Finger TE. 2002. Trigeminal collaterals Kendal-Reed M, Walker JC, Morgan WT, LaMacchio M, Lutz RW. 1998. in the nasal epithelium and olfactory bulb: a potential route for direct Human responses to propionic acid. I. Quantification of within- and modulation of olfactory information by trigeminal stimuli. J Comp between-participant variation in perception by normosmics and Neurol. 444:221--226. anosmics. Chem Senses. 23:71--82. Scheibe M, Zahnert T, Hummel T. 2006. Topographical differences in Kettenmann B, Jousmaki V, Portin K, Salmelin R, Kobal G, Hari R. 1996. the trigeminal sensitivity of the human nasal mucosa. Neuroreport. Odorants activate the human superior temporal sulcus. Neurosci 17:1417--1420. Lett. 203:143--145. Stone H, Williams B, Carregal EJ. 1968. The role of the trigeminal nerve in Kobal G. 1981. Elektrophysiologische Untersuchungen des menschli- olfaction. Exp Neurol. 21:11--19. chen Geruchssinns. Stuttgart (Germany): Thieme Verlag. Thu¨rauf N, Friedel I, Hummel C, Kobal G. 1991. The mucosal potential Kobal G. 1985. -related electrical potentials of the human nasal elicited by noxious chemical stimuli: is it a peripheral nociceptive mucosa elicited by chemical stimulation. Pain. 22:151--163. event. Neurosci Lett. 128:297--300. Kobal G, Hummel T, Hoesl M. 1990. Pain-related electrical evoked Thu¨rauf N, Hummel T, Kettenmann B, Kobal G. 1993. Nociceptive and potentials by chemical stimuli: effects of fentanyl. In: Lipton S, Tunks reflexive responses recorded from the human nasal mucosa. Brain E, Zoppi M, editors. Advances in pain research and therapy. New Res. 629:293--299. York: Raven Press. p. 95--98 Walker JC, Kendal-Reed M, Hall SB, Morgan WT, Polyakov VV, Lutz RW. Kobal G, Klimek L, Wolfensberger M, Gudziol H, Temmel A, Owen CM, 2001. Human responses to propionic acid. II. Quantification of Seeber H, Pauli E, Hummel T. 2000. Multicenter investigation of breathing responses and their relationship to perception. Chem 1,036 subjects using a standardized method for the assessment of Senses. 26:351--358. olfactory function combining tests of odor identification, odor Wendler J, Hummel T, Reissinger M, Manger B, Pauli E, Kalden JR, Kobal discrimination, and olfactory thresholds. Eur Arch Otorhinolaryngol. G. 2001. Patients with rheumatoid arthritis adapt differently to 257:205--211. repetitive painful stimuli compared to healthy controls. J Clin Laska M, Distel H, Hudson R. 1997. Trigeminal perception of odorant Neurosci. 8:272--277. quality in congenitally anosmic subjects. Chem Senses. 22:447--456. Wysocki CJ, Cowart BJ, Radil T. 2003. Nasal trigeminal chemosensitivity Leopold DA, Hummel T, Schwob JE, Hong SC, Knecht M, Kobal G. 2000. across the adult life span. Percept Psychophys. 65:115--122. Anterior distribution of human . Laryngoscope. Zald DH, Pardo JV. 2000. Functional neuroimaging of the olfactory 110:417--421. system in humans. Int J Psychophysiol. 36:165--181. Livermore A, Hummel T. 2004. The influence of training on chemo- Zatorre RJ, Jones-Gotman M, Evans AC, Meyer E. 1992. Functional sensory event-related potentials and interactions between the localization and lateralization of human olfactory cortex. Nature. olfactory and trigeminal systems. Chem Senses. 29:41--51. 360:339--340.

Cerebral Cortex October 2007, V 17 N 10 2275