The contingent negative variation (CNV) is a slow, negative event-related brain potential, which is generated when an imperative stimulus is preceded by a warning stimulus and reflects expectancy and preparation (Walter et al., 1964). The amplitude of the CNV is correlated to attention and arousal (Tecce, 1972). Since changes of the magnitude and latency of CNV components have long been associated with the effects of psychoactive drugs (Kopell et al., 1974; Ashton et al., 1977), measurement of the CNV has also been used to evaluate psychostimulant and sedating effects of EOs and fragrances. In a pioneering investigation, Torii et al. (1988) measured CNV magnitude changes evoked by a variety of EOs, such as jasmine, lavender, and rose oil, in male subjects. CNV was recorded at the frontal, central, and parietal sites after the presentation of an odorous or blank stimulus in the context of a cued reaction time paradigm. In addition, physiological markers of arousal, that is, skin potential level and heart rate, were simultaneously measured. Results showed that at frontal sites the amplitude of the early negative shift of the CNV was significantly altered after the presentation of odor stimuli, and that these changes were mostly congruent with stimulating and sedative properties reported for the tested oils in the traditional aromatherapy literature. In contrast to other psychoactive substances, such as caffeine or benzodiazepines, presentation of the EOs neither affected physiological parameters nor reaction times. The authors concluded that the EOs tested influenced brain waves "almost exclusively" while having no effects on other indicators of arousal.
Subsequently, CNV recordings have been used by a number of researchers on a variety of EOs and fragrances to establish effects of odors on the human brain along the activation-relaxation continuum. For instance, Sugano (1992) in the aforementioned study demonstrated that a-pinene (1), sandalwood, and lavender odor increased the magnitude of the CNV in healthy young adults, whereas eucalyptus reduced it. It is interesting to note, however, that all of these odors—despite their differential influence on the CNV—increased spontaneous a activity in the same experiment. An increase of CNV magnitude was also observed with the EO from pine needles (Manley, 1993), which was interpreted as having a stimulating effect. Aoki (1996) investigated the influence of odors from several coniferous woods, that is, hinoki [Chamaecyparis obtusa (Siebold & Zucc.) Endl. (Cupressaceae)], sugi [Cryptomeria japonica D.Don (Cupressaceae)], akamatsu [Pinus den-siflora Siebold & Zucc. (Pinaceae)], hiba [Thujopsis dolabrata var. hondai Siebold & Zucc. (Cupressaceae)], Alaska cedar [Chamaecyparis nootkatensis (D.Don) Spach (Cupressaceae)], Douglas fir [Pseudotsuga manziesii (Mirbel) Franco (Pinaceae)], and Western red cedar [Thuja plicata Donn (Cupressaceae)], on the CNV and found conflicting effects: the amplitude of the early CNV component at central sites was decreased by these wood odors, and the a/b-wave ratio of the EEG increased. Moreover, the decrease of CNV magnitude was correlated with the amount of a-pinene (1) in the tested wood odors. Also Sawada et al. (2000) measured changes of the early component of the CNV in response to stimulation with terpenes found in the EO of woods and leaves. These authors noticed a reduction of the CNV magnitude after the administration of a-pinene (1), A-3-carene (9), and bornyl acetate (10). However, a more recent investigation (Hiruma et al., 2002) showed that hiba [Thujopsis dolabrata Siebold & Zucc. (Cupressaceae)] odor increased the CNV magnitude at frontal and central sites and shortened reaction times to the imperative stimulus in female subjects. These authors thus concluded that the odor of hiba heightened the arousal level of the CNS.
Although the CNV is believed to be largely independent of individual differences, such as age, sex, or race (Manley, 1997), there seem to be cognitive influences that must not be neglected when interpreting the effects of odor stimuli on the CNV. Lorig and Roberts (1990) repeated the study by Torii et al., (1988) and investigated cognitive factors by introducing a manipulation of their subjects as an additional variable into the paradigm. In this experiment subjects were exposed to the original two odors, that is, lavender and jasmine, referred to as odor A and odor B, respectively, as well as to a mixture of the two fragrances. However, in half of the trials in which the mixture was administered subjects were led to believe that they received a low concentration of odor A, whereas in the other half of trials they thought that they would be exposed to a low concentration of odor B. In none of the four conditions were the subjects given the correct odor names. As in the Torii et al. study, lavender reduced the amplitude of the CNV whereas jasmine increased it. When the mixture was administered, however, the CNV magnitude decreased when subjects believed to receive a low concentration of lavender, but increased when they thought they were inhaling a low concentration of jasmine. This means that the alteration of the CNV amplitude was not solely related to the substance that had been administered but also related to the expectation of the subjects. Another point made by Lorig and Roberts (1990) is that in their study self-report data indicated that lavender was actually rated as more arousing than jasmine. Since low CNV amplitudes are not only associated with low arousal but also with high arousal in the context of distraction (Travis and Tecce, 1998), the lavender odor might in fact have led to higher arousal levels than jasmine even though the CNV magnitude was smaller with lavender. Other authors have noted that CNV changes might not only reflect effects of odor stimuli but also the anticipation, expectancy, and the emotional state of the subjects who are exposed to these odorants (Hiruma et al., 2005). The involvement of these and other cognitive factors might well explain why the findings of CNV changes in response to odorants are rather inconsistent.
10.1.4 Effects of EOs and Fragrances on Selected Basic and Higher Cognitive Functions
Psychoactive effects of odorants at the cognitive level have been explored in humans using a large number of methods. A variety of testing procedures ranging from simple alertness or mathematical tasks to tests that assess higher cognitive functions, such as memory or creativity, have been employed to study stimulant or relaxing/sedating effects of EOs and fragrances. Nevertheless, the efficiency of odorants is commonly defined by changes in performance in such tasks as a function of the exposure to fragrances.
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