We say that olfaction is a very powerful thing, that we recognize some smells from our childhood when we find them again later in life, but we almost never dream smells.

Olfactory perception in dreams: Analysis of a long dream series. Michael Schredl. International Journal of Dream Research,     Vol 12, No 1 (April 2019)
https://doi.org/10.11588/ijodr.2019.1.57845

Abstract: The present study analyzed the frequency of olfactory perceptions in a long dream series (N = 11,180 dreams) reported by a single participant. Overall, about 0.30% of the dreams included references to olfactory perceptions with unpleasant odors outweighing positive ones. Moreover, most of the olfactory perceptions are uncommon with regard to typical everyday life. To expand these findings, it would be very interesting to study larger samples using a diary paradigm including explicit questions about type, quality, and commonness of the dream odors.


4. Discussion

The present findings confirm previous results showing that spontaneously mentioned olfactory perceptions in dreams are rare since the frequency found in this male dreamer (0.30%) was close to the figure of 0.11% diary dreams re-ported by males that include references to olfactory percep-tions (Zadra et al., 1998). However, one has to keep in mind that the dreamer was not aware of the study’s rationale, i.e., analyzing olfactory perceptions, and, thus, the spontaneously reported olfactory perceptions might be an underestimation compared to a paradigm with explicit questions about sensory perceptions after recording the dreams (Carskadon et al., 1989).

From a methodological viewpoint it has to be considered that the codings were done by the dreamer himself alone; there was no second external judge so that interrater reliabilities could be ascertained. However, previous research indicated that for simple scales measuring the presence or absence of a specific topic have shown high interrater reli-ability (Schredl, Burchert, & Grabatin, 2004). As reported in previous studies (Arshamian, 2007; Carska-don et al., 1989; Monroe, 1899; Stevenson & Case, 2004-05; Weitz et al., 2010) the olfactory perception included a large variety of topics. However, a closer look showed that only one third of the contexts in which olfactory perceptions in the dreams were experienced also occurred in the dream-er’s everyday life, most of the olfactory perception contexts were unusual for the dreamer or even bizarre (ketchup with fish smell) – indicating the dreams are not simple replays of waking life experiences (Fosse, Fosse, Hobson, & Stick-gold, 2003; Malinowski & Horton, 2014). This contributes to the discussion whether dream content is continuous and/
discontinuous to waking life (Hobson & Schredl, 2011). In this context, it would be interesting to study odor experts (perfumers, chefs, oenologists etc.) who focus quite often on olfactory perceptions during the day.

Most of the spontaneously reported odors in this dream series were unpleasant. One might speculate that there might be a report bias similar to the underreporting of positive emotions compared to negative emotions (Röver & Schredl, 2017; Schredl & Doll, 1998; Sikka, Feilhauer, Valli, & Revon-suo, 2017). To test this hypothesis, it would be interesting to ask explicitly for olfactory perceptions in the dream and their emotional quality after the dream is recorded. Although the reported dream examples in the literature are mainly nega-tive (Wayne & Clinco, 1959; Weed & Hallam, 1896; Zadra et al., 1998), the preponderance of negative odors in dreams might be characteristic for this special dreamer, i.e., larger samples are necessary to corroborate this finding. It would also be interesting to test pleasantness/unpleasantness in the context of other sensory modalities like visual or audi-tory experiences. As the olfactory system is closely linked to the limbic system (Gottfried, 2006), one would not expect a strong relationship between perception per se and emo-tions.

To summarize, the analysis of this dream series indicate that olfactory perceptions in dreams are quite rare, mainly unpleasant und uncommon regarding the context compared to everyday life. Due to the design (single case study), the findings raise a lot of questions as to what olfactory per-ceptions look like in larger dream samples and/or in other subjects, for example, in odor experts. It would be very in-teresting to carry out an experimental dream study using ambulatory assessment techniques with one group regularly reminded to focus on olfaction during the day and answer-ing questions about type, pleasantness and commonness of the odor. According to the continuity hypothesis (Schredl, 2018), the percentage of olfactory dreams should increase and the pleasant/unpleasant ratio of waking olfactory per-ception should be reflected in the participants’ dreams. These studies might help to elucidate the interesting rela-tionship between olfaction, sleep, and dreams.