Olfactory perception in dreams

Studying the human olfactory system during sleep is a very interesting research topic for several reasons: First, odors presented during sleep can deepen sleep (Perl et al., 2016) – comparable to the positive effect of aromatherapy (odors presented prior to sleep onset) on sleep (Hwang & Shin, 2015). Second, olfactory stimuli without a trigeminal component, such as hydrogen sulfide, do not cause arousals even when presented in high concentrations during sleep (Stuck et al., 2007) because olfactory information processing largely bypasses the spinal cord, the brain stem, and the thalamus (Gottfried, 2006). This is used in targeted memory reactivation, i.e., a learning task is coupled with a specific odor in waking life and this odor is then presented during sleep in order to enhance the sleep-dependent memory consolidation (Rasch, Buchel, Gais, & Born, 2007; Shanahan, Gjorgieva, Paller, Kahnt, & Gottfried, 2018). Third, Arzi et al. (2012) found that the olfactory sniff response (larger in-breaths while presenting positive odors compared to the in-breaths while presenting negative odors) is preserved in sleep and was able to demonstrate that simple condition paradigms associating different tones with different odors are successful during sleep and that the effects carry over to waking life. In addition, olfactory aversive conditioning during sleep (e.g., coupling cigarette smoke with very unpleasant odors) has reduced smoking behavior during the day (Arzi et al., 2014). Given the specific features of the olfactory system during sleep, the question is whether olfactory stimuli presented during sleep affect dream content like auditory or tactile stimuli (Schredl, 2018) sometimes do. The first systematic study (Trotter, Dallas, & Verdone, 1988) found that about 19% of the stimuli, e.g., lemon, coffee, dirty ashtray, dog feces, and onion, were incorporated into the dreams. However, in a subsequent study (Schredl et al., 2009), purely olfactory stimuli, i.e., olfactory stimuli without a trigeminal component (e.g., H2S and phenyl ethyl alcohol), did not affect dream content while a positive odor was associated with more positive dream emotions compared to an unpleasant odor – commensurate with the neuroanatomical close connection between the olfactory bulb and the limbic system. Rural pictures associated with a specific odor prior to sleep were more often found in dreams after stimulation with the same odor during REM sleep, supporting the idea of targeted memory reactivation during sleep (Schredl, Hoffmann, Sommer, & Stuck, 2014). Despite this intriguing relationship between olfaction, sleep, and dreaming, systematic studies of olfactory dreaming, especially the features of these dreams, are quite rare. Depending on the sample about 20% to 40% of the participants reported having had at least one dream that included an explicit olfactory perception (Arshamian, 2007; Lovati et al., 2014; Stevenson & Case, 2004-05; Weitz, Croy, Seo, Negoias, & Hummel, 2010; Zadra, Nielsen, & Donderi, 1998). Analyzing diary dreams or dreams obtained in the sleep laboratory, yielded relative low figures of olfactory dreams ranging from 1% to 2% (Calkins, 1893; Snyder, 1970; Strauch & Meier, 2004; Zadra et al., 1998) compared to 50% to 100% of the dreams that included visual and auditory perceptions (Schredl, 2018). If participants were presented with categories of sensory perception categories (smell, odor, etc.) after they recorded their dreams (diary), the percentage of olfactory dreams went up to 8.1% (N = 397 dreams) (Carskadon, Wyatt, Etgen, & Rosekind, 1989). This difference suggests that not all olfactory perceptions within a dream might be reported spontaneously. According to the available data, the odors experienced in the dreams are often related to food, body odors, animals, smoke, perfume (Arshamian, 2007; Carskadon et al., 1989; Monroe, 1899; Stevenson & Case, 2004-05; Weitz et al., 2010). Systematic studies of the emotional valence of the olfactory perception were not yet carried out but examples cited in different publications suggest that unpleasant odors prevail, e.g., foul odor of rotten rat carcasses (Wayne & Clinco, 1959), a very disagreeable food odor (Weed & Hallam, 1896), disgusting smell in a house due to a pitbull (Zadra et al., 1998). Persons who report freOlfactory perception in dreams: Analysis of a long dream series