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FIGURE 10.1 Macroscopic view of the human ventral forebrain and medial temporal lobes, depicting the olfactory tract, its primary projections, and surrounding nonolfactory structures. The right medial temporal lobe has been resected horizontally through the mid-portion of the amygdala (AM) to expose the olfactory cortex. AON, anterior olfactory nucleus; CP, cerebral peduncle; EA, entorhinal area; G, gyrus ambiens; L, limen insula; los, lateral olfactory sulcus; MB, mammillary body; mos, medial olfactory sulcus; olf, olfactory sul-cus; PIR-FR, frontal piriform cortex; OB, olfactory bulb; OpT, optic tract; OT, olfactory tract; tos, transverse olfactory sulcus; Tu, olfactory tubercle; PIR-TP, temporal piriform cortex. Figure prepared with the help of Dr. Eileen H. Bigio, Department of Pathology, Northwestern University Feinberg School of Medicine, Chicago, Illinois. (From Gottfried, J.A. and D.A. Zald, 2005. Brain Res. Rev., 50: 287-304. With permission.)

as odor valence (Baron and Thomley, 1994), on semantic cues, for example, memories and experiences associated with a particular odor, as well as on placebo effects related to the expectation of certain effects (Jellinek, 1997). None of the latter mechanisms is substance, that is, odorant, specific but their effectiveness depends on cognitive mediation and control.

Many odorants stimulate not only the olfactory system via the first cranial nerve (N. olfactorius) but also the trigeminal system via the fifth cranial nerve (N. trigeminus), which enervates the nasal mucosa. The trigeminal system is part of the body's somatosensory system and mediates mechanical and temperature-related sensations, such as itching and burning or warmth and cooling sensations. Trigeminal information reaches the brain via the trigeminal ganglion and the ventral posterior nucleus of the thalamus. The primary cortical projection area of the somatosensory system is the contralateral postcentral gyrus of the parietal lobe (Zilles and Rehkampfer, 1998). The reticular formation in the brain stem, which is part of the reticular activating system (RAS) (Figure 10.2), receives collaterals from the trigeminal system. Thus, trigeminal stimuli have direct effects on arousal. Utilizing this direct connection, highly potent trigeminal stimulants, such as ammonia and menthol, have been used in the past in smelling salts to awaken people who fainted.

It has been shown in experimental animals that, due to their lipophilic properties, fragrances not only penetrate the skin (Hotchkiss, 1998) but also the blood-brain barrier (Buchbauer et al., 1993). Also, odorants have been found to bind to several types of brain receptors (Aoshima and Hamamoto, 1999; Elisabetsky et al., 1999; Okugawa et al., 2000), and it has been suggested

Basal ganglia Cortex

Basal ganglia Cortex

Cell Axon bodies terminals

Cell Axon bodies terminals

4-----< Noradrenergic neurons

Serotonergic neurons

FIGURE 10.2 Schematic of the RAS with noradrenergic and serotonergic connections. (From Grilly, D.M., 2002. Drugs & Human Behavior. Boston: Allyn & Bacon. With permission.)

that these odorant-receptor interactions are responsible for psychoactive effects of fragrances in experimental animals. With regard to these findings, it is important to note that Heuberger et al. (2001) have observed differential effects of fragrances as a function of chirality. It seems likely that such differences in effectiveness are related to enantiomeric selectivity of receptor proteins. However, the question whether effects of fragrances on human arousal and cognition rely on a similar psycho-pharmacological mechanism remains to be answered.

10.1.2 Activation and Arousal: Definition and Neuroanatomical Considerations

Activation, or arousal, refers to the ability of an organism to adapt to internal and external challenges (Schandry, 1989). Activation is an elementary process that serves in the preparation for overt activity. Nevertheless, it does not necessarily result in overt behavior (Duffy, 1972). Activation varies in degree and can be described along a continuum from deep sleep to overexcitement. Early theoretical accounts of activation have emphasized physiological responses as the sole measurable correlate of arousal. Current models, however, consider physiological, cognitive, and emotional activity as observable consequences of activation processes. It has been shown that arousal processes within each of these three systems, that is, physiological, cognitive, and emotional, can occur to varying degrees so that the response of one system need not be correlated linearly to that of the other systems (Baltissen and Heimann, 1995).

It has long been established that the RAS, which comprises the reticular formation with its sensory afferents and widespread hypothalamic, thalamic, and cortical projections, plays a crucial role in the control of both phasic and tonic activation processes (Becker-Carus, 1981; Schandry, 1989). Pribram and McGuinness (1975) distinguish three separate but interacting neural networks in the control of activation (Figure 10.3). The arousal network involves amygdalar and related frontal cortical structures and regulates phasic physiological responses to novel incoming information. The activation network centers on the basal ganglia of the forebrain and controls the tonic physiological readiness to respond. Finally, the effort network, which comprises hippocampal circuits, coordinates the arousal and activation networks. Noradrenergic projections from the locus ceruleus, which is located within the dorsal wall of the rostral pons, are particularly important in the regulation of

FIGURE 10.3 Control of activation processes. OFC, orbitofrontal cortex; AM, amygdala; MDT, medial dorsal thalamus; AH, anterior hypothalamus; RF, reticular formation; SC, spinal cord; HI, hippocampus; CC, cingulate cortex; S, septum; ADT, anterior dorsal thalamus; PH, posterior hypothalamus; SMP, sensory-motor projections; CS, corpus striatum; LDT, lateral dorsal thalamus; SR, subthalamic regions; T, tectum. Left, structures of the arousal network; middle, structures of the effort network; right, structures of the activation network. (Adapted from Pribram, K. H. and D. McGuinness, 1975. Psychol. Rev, 82(2): 116-149.)

FIGURE 10.3 Control of activation processes. OFC, orbitofrontal cortex; AM, amygdala; MDT, medial dorsal thalamus; AH, anterior hypothalamus; RF, reticular formation; SC, spinal cord; HI, hippocampus; CC, cingulate cortex; S, septum; ADT, anterior dorsal thalamus; PH, posterior hypothalamus; SMP, sensory-motor projections; CS, corpus striatum; LDT, lateral dorsal thalamus; SR, subthalamic regions; T, tectum. Left, structures of the arousal network; middle, structures of the effort network; right, structures of the activation network. (Adapted from Pribram, K. H. and D. McGuinness, 1975. Psychol. Rev, 82(2): 116-149.)

circadian alertness, the sleep-wake rhythm, and the sustenance of alertness (alerting) (Pedersen et al., 1998; Aston-Jones et al., 2001). On the other hand, tonic alertness seems to be dependent on cholinergic (Baxter and Chiba, 1999; Gill et al., 2000) frontal and inferior parietal thalamic structures of the right hemisphere (Sturm et al., 1999). Other networks that are involved in the control of arousal and attentional functions are found in posterior parts of the brain, for example, the parietal cortex, superior colliculi, and posterior-lateral thalamus, as well as in anterior regions, for example, the cingular and prefrontal cortices (Posner and Petersen, 1990; Paus, 2001).

10.1.3 Influence of EOs and Fragrances on Brain Potentials Indicative of Arousal 10.1.3.1 Spontaneous Electroencephalogram Activity

Recordings of spontaneous electroencephalogram (EEG) activity during the administration of EOs and fragrances have widely been used to assess stimulant and sedative effects of these substances. Particular attention has been paid to changes within the a and b bands, sometimes also the 0 band of the EEG in response to olfactory stimulation since these bands are thought to be most indicative of central arousal processes. Alpha waves are slow brain waves within a frequency range of 8-13 Hz and amplitudes between 5 and 100 mV, which typically occur over posterior areas of the brain in an awake but relaxed state, especially with closed eyes. The a rhythm disappears immediately when subjects open their eyes and when cognitive activity is required, for example, when external stimuli are processed or tasks are solved. This phenomenon is often referred to as a block or desynchroni-zation. Simultaneously with the a block, faster brain waves occur, such as b waves with smaller amplitudes (2-20 mV) and frequencies between 14 and 30 Hz. The b rhythm, which is most evident frontally, is characteristic of alertness, attention, and arousal. In contrast, 0 waves are very slow brain waves occurring in fronto-temporal areas with amplitudes between 5 and 100 mV in the frequency range between 4 and 7 Hz. Although the 0 rhythm is most commonly associated with drowsiness and light sleep, some researchers found 0 activity to correlate with memory processes (Grunwald et al., 1999; Hoedlmoser et al., 2007) and creativity (Razumnikova, 2007). Other authors found correlations between 0 activity and ratings of anxiety and tension (Lorig and Schwartz, 1988). With regard to animal olfaction, it has been proposed that the 0 rhythm generated by the hippocampus is concomitant to sniffing and allows for encoding and integration of olfactory information with other cognitive and motor processes (Kepecs et al., 2006).

A large number of measures can be derived from recordings of the spontaneous EEG. Time (index) or voltage (power)-based rates of typical frequency bands, as well as ratios between certain frequency bands (e.g., between the a and b band or the 0 and b band) within a selected time interval are most commonly used to quantify EEG patterns. Period analysis quantifies the number of waves that occur in the various frequency bands within a distinct time interval of the EEG record and is supposed to be more sensitive to task-related changes than spectral analysis (Lorig, 1989). Other parameters that describe the covariation of a given signal at different electrodes are coherence and neural synchrony. These measures inform on the functional link between brain areas (Oken et al., 2006).

Generally speaking, the pattern of the spontaneous EEG varies with the arousal level of the CNS. Thus, different states of consciousness, such as sleep, wakefulness, or meditation, can be distinguished by their characteristic EEG patterns. For instance, an increase in central activation is typically characterized by a decrease in a and an increase in b activity (Schandry, 1989). More precisely, a decline of a and b power together with a decrease of a index and an increase of b and 0 activity have been observed under arousing conditions, that is, in a mental calculation and a psychosocial stress paradigm (Walschburger, 1976). Also when subjects are maximally attentive, frequencies in the a band are attenuated and activity in the b and even higher frequency bands can be observed. Fatigue and performance decrements in situations requiring high levels of attention are often associated with increases in 0 and decreases in b activity (Oken et al., 2006). On the other hand, drowsiness and the onset of sleep are characterized by an increase in slow and a decrease in fast EEG waves. However, high activity in the a band, particularly in the range between 7 and 10 Hz, is not indicative of low arousal states of the brain, such as relaxation, drowsiness, and the onset of sleep, but rather seems to be a component of selective neural inhibition processes that are necessary for a number of cognitive processes, such as perception, attention, and memory (Miller and O'Callaghan, 2006; Palva and Palva, 2007).

Because changes of spontaneous EEG activity accompany a wide range of cognitive as well as emotional brain processes and "EEG measurements [...] do not tell investigators what the brain is doing" (Lorig 1989, p. 93), it is somewhat naive to interpret changes that are induced by the application of an odorant as a result of a single and specific process, particularly when no other correlates of the process of interest are assessed. Nevertheless, this is exactly the approach that has been taken by many researchers to identify stimulant or relaxing effects of odors. The simplest, but in terms of interpretation of the results probably most problematic setup for such experiments is the comparison of spontaneous EEG activity in response to odorants with a no-odor baseline. Using this design, Sugano (1992) observed increased EEG a activity after inhalation of a-pinene (1), 1,8-cineole (2), lavender, sandalwood, musk, and eucalyptus odors. Considering that traditional aromatherapy discriminates these fragrances by their psychoactive effects, for example, lavender is assigned relaxing properties while eucalyptus is supposedly stimulant (Valnet, 1990); these findings are at least rather curious. Also Ishikawa et al. (2002) recorded the spontaneous EEG in 13 Japanese subjects while drinking either lemon juice with a supplement of lemon odor or lemon juice without it. It was shown that a power was enhanced by supplementation with lemon odor, an indicative of increased relaxation. Again, with regard to aromatherapeutical accounts of the lemon EO this result is somewhat counterintuitive, even more so as in the same study the juice supplemented with lemon odor increased spontaneous locomotion in experimental animals. Haneyama and Kagatani (2007) tested a fragrant spray made from extracts of Chinese spikenard roots [Nardostachys chinensis Batalin (Valerianaceae)] in butylene glycol and found increased a activity in subjects under stress. This finding was interpreted by the authors as demonstrating a sedative effect of the extract. However, it is unknown how stress was induced in the subjects and how it was measured. Similarly, Ishiyama (2000) concluded from measurements of the frequency fluctuation patterns of the a band that smelling a blend of terpene compounds, typically found in forests, induced feelings of refreshment and relaxation in human subjects without proper description of how these feelings were assessed.

Inconclusive findings as those described above are not unexpected with such simple experimental designs as there are several problems associated with this kind of experiments. First, a no-odor baseline is often inappropriate as it does not control for cognitive activity of the subject. For instance, subjects might be puzzled by the fact that they do not smell anything eventually focusing attention to the search for an odor. This may lead to quite high arousal levels rather than the intended resting brain state. This was the case with Lorig and Schwarzt (1988), who tested changes of spontaneous EEG in response to spiced apple, eucalyptus, and lavender fragrances diluted in an odorless base: contrary to the authors' expectations it was found that a activity in the no-odor condition was less than that during odor presentation.

Considering the various mechanisms outlined by Jellinek (1997) by which fragrances influence human arousal and behavior, another inherent problem of such simple designs is that little is known about how subjects process stimulus-related information, for example, the pleasantness or intensity of an odorant, and whether or not higher cognitive processes related to the odorant are initiated by the stimulation. For instance, subjects might be able to identify and label some odors but might fail to do so with others; similarly, some odors might trigger the recall of associated memories while others might not. In order to assess psychoactive effects of EOs and fragrances, it seems necessary to control for these factors, for instance by assessing additional variables that inform on the subject's perception of primary and secondary stimulus features and that correlate to the subject's cognitive or emotional arousal state. In the above-mentioned study, Lorig and Schwarzt (1988) collected ratings of intensity and pleasantness of the tested fragrances as well as subjective ratings of a number of affective states in addition to the EEG recordings. Analysis of the amount of EEG 0 activity revealed that the spiced apple odor produced more relaxation than lavender and eucalyptus; the analysis of the secondary variables suggested that this relaxing effect was correlated with subjective estimates of anxiety and tension. As to the EEG patterns, similar results were observed when subjects imagined food odors and practiced relaxation techniques. Thus, the authors conclude that the relaxing effect of spiced apple was probably related to its association with food. These cognitive influences also seem to be a plausible explanation for the increase in a power by lemon odor in the Ishikawa et al. (2002) study. Other studies related to food odors were conducted by Kaneda et al. (2005, 2006). These authors investigated the influence of smelling beer flavors on the frequency fluctuation of a waves in frontal areas of the brain. The results showed relaxing effects of the aroma of hop extracts as well as of linalool (3). In addition, in the right hemisphere these fluctuations were correlated with subjective estimates of arousal and with the intensity of the hop aroma. Lee et al. (1994) also found evidence for differential EEG patterns as a function of odor intensity for citrus, lavender, and a floral odor. A 10-min exposure to the weaker intensity of the citrus fragrance in comparison to lavender odor increased the rate of occipital a. Moreover, there was a general trend for citrus to be rated as more comfortable than other fragrances. In contrast, the higher intensity of the floral fragrance increased the rate of occipital b more than the lavender odor.

Several authors have shown influences of odor pleasantness and familiarity on changes of the spontaneous EEG. For instance, Kaetsu et al. (1994) reported that pleasant odors increased the a activity, while unpleasant ones decreased it. In a study on the effects of lavender and jasmine odor on electrical brain activity (Yagyu, 1994), it was shown that changes in the a, b, and 0 bands in response to these fragrances were similar when subjects rated them as pleasant, while lavender and jasmine odor led to distinct patterns when they were rated as unpleasant. Increases of a activity in response to pleasant odors might be explained by altered breathing patterns since it has been demonstrated that pleasant odors induce deeper inhalations and exhalations than unpleasant odors, and that this form of breathing by itself increases the activity in the a band (Lorig, 2000). Masago et al. (2000) tested the effects of lavender, chamomile, sandalwood, and eugenol (4) fragrances on ongoing EEG activity and self-ratings of comfort and found a significant positive correlation between the degree of comfort and the odorants' potency to decrease the a activity in parietal and posterior temporal regions. In relation to the previously described investigations, this finding is rather difficult to explain, although it differs from the other studies in that it differentiated between electrode sites rather than reporting merely global changes in electrical brain activity. Therefore, this result suggests that topographical differences in electrical brain activity induced by fragrances may be important and need further investigation. In fact, differences in hemispheric localization of spontaneous EEG activity in response to pleasant and unpleasant fragrances seem to be quite consistent. While pleasant odors induced higher activation in left frontal brain regions, unpleasant ones led to bilateral and widespread activation (Kim and Watanuki, 2003) or no differences were observed when an unpleasant odor (valerian) was compared to a no-odor control condition (Kline et al., 2000). Another interesting finding in the study of Kim and Watanuki (2003) was that EEG activity in response to the tested fragrances was observed when subjects were at rest but vanished after performing a mental task. The importance of distinguishing EEG activity arising from different areas of the brain is highlighted by an investigation by Van Toller et al. (1993). These authors recorded a wave activity at 28 sites of the scalp immediately after the exposure to a number of fragrances covering a range of different odor types and hedonic tones at iso-intense concentrations; the odorants had to be rated in terms of pleasantness, familiarity, and intensity. It was shown that in posterior regions of the brain changes in a activity in response to these odors compared to an odorless blank were organized in distinct topographical maps. Moreover, a activity in a set of electrodes at frontal and temporal sites correlated with the psychometric ratings of the fragrances.

As to influences of the familiarity of or experience with fragrances, Kawano (2001) reported that the odors of lemon, lavender, patchouli, marjoram, rosemary, and sandalwood increased a activity over occipital electrode sites in subjects to whom these fragrances were well known. On the one hand, lag times between frontal and occipital a phase were shorter in subjects less experienced with the fragrances, indicating that these subjects were concentrating more on smelling—and probably identifying—the odorants. These findings were confirmed in an investigation comparing professional perfume researchers, perfume salespersons, and general workers (Min et al., 2003). This study showed that measures of cortico-cortical connectivity, that is, the averaged cross mutual information content, in odor processing were more pronounced in frontal areas with perfume researchers, whereas with perfume salespersons and general workers a larger network of posterior temporal, parietal, and frontal regions was activated. These results could result from a greater involvement of orbitofrontal cortex neurons in perfume researchers, who exhibit high sophistication in discriminating and identifying odors. Moreover, it was shown that the value of the averaged cross mutual information content was inversely related to preference in perfume researchers and perfume salespersons, but not in general workers.

As pointed out above, the administration of EO and fragrances to naive subjects can lead to cognitive processes that are unknown to the investigator, and sometimes even the subject, but that nevertheless affect spontaneous EEG activity. Some researchers have sought to solve this problem by engaging subjects in a secondary task while the influence of the odorant of interest is assessed. This procedure does not only draw the subject's attention away from the odor stimulus, but also provides the desired information about his/her arousal state. Another benefit of such experimental designs is that the task may control for the subject's arousal state if a certain amount of attention is required to perform it. Measurement of changes of a and 0 activity in the presence or absence of 1,8-cineole (2), methyl jasmonate (5), and trans-jasmin lactone (6) in subjects who performed a simple visual task showed that the increase in slow wave activity was attenuated by 1,8-cineole (2) and methyl jasmonate (5), while augmented by trans-jasmin lactone (6) (Nakagawa et al. 1992). At least in the case of 1,8-cineole (2) these findings are supported by results from experimental animals and humans, indicating activating effects of this odorant (Kovar et al., 1987; Nasel et al., 1994; Bensafi et al., 2002). An investigation of the effects of lemon odor EEG a, b, and 0 activity during the administration of lemon odor showed that the odor reduced power in the lower a range while it increased power in the higher a, lower b, and lower 0 bands (Krizhanovs'kii et al., 2004). These findings of increased arousal were in agreement with better performance in a cognitive task. In addition, it was shown that inhalation of the lemon fragrance was most effective during rest and in the first minutes of the cognitive task, but wore off after less than 10 min. In several experiments, the group of Sugawara demonstrated complex interactions between electrical brain activity induced by the exposure to fragrances, sensory profiling, and various types of tasks (Sugawara et al., 2000; Satoh and Sugawara, 2003). In one study they showed that the odor of peppermint, in contrast to basil, was rated less favorable on a number of descriptors and reduced the magnitude of b waves after as compared to before performance of a cognitive task. In a similar investigation, it was shown that the sensory evaluation as well as changes in spontaneous EEG activity in response to the odors of the linalool enantiomers differed as a function of the molecular structure and the kind of task. For instance, R-(-)-linalool (7) was rated more favorable and led to larger decreases of b activity after listening to natural sounds than before. In contrast, after as compared to before cognitive effort R-(-)-linalool (7) was rated as less favorable and tended to increase b power. A similar pattern was found for RS-(+)-linalool (3), whereas for S-(+)-linalool (8) the pattern was different, particularly with regard to EEG activity.

In the study of Yagyu (1994), the effects of lavender and jasmine fragrances on the performance in a critical flicker fusion and an auditory reaction time task were assessed in addition to changes of the ongoing EEG. It was demonstrated that in contrast to the EEG findings, lavender decreased performance in both tasks independent of its hedonic evaluation. Jasmine, however, had no effect on task performance. The EEG changes in response to these odorants might well explain their effects on performance: lavender induced decreases of activity in the b band, which is associated with states of low attention regardless of being rated as pleasant or unpleasant; jasmine, on the other hand, increased EEG b activity when it was judged unpleasant but lowered b activity when judged pleasant, so that overall its effect on performance levelled out. The effects of lavender and rosemary fragrances on electrical brain activity, mood states, and math computations were investigated by Diego, Field, and co-workers (Diego et al., 1998; Field et al., 2005). These investigations showed that the exposure to lavender increased b power, elevated feelings of relaxation, reduced feelings of depression, and improved both speed and accuracy in the cognitive task. In contrast, rosemary odor decreased frontal a and b power, decreased feelings of anxiety, increased feelings of relaxation and alertness, and increased speed in the math computations. The EEG results were interpreted as indicating increased drowsiness in the lavender group and increased alertness in the rosemary group; however, the behavioral data showed performance improvement and similar mood ratings in both groups. These findings suggest that different electrophysiological arousal patterns may still be associated with similar behavioral arousal patterns emphasizing the importance of collecting additional endpoints to evaluate psychoactive effects of EO and fragrances.

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