The nasal cycle is a shift of nasal airflow mediated by alternating dilation and constriction of veins in the nasal mucosa, by action of the sympathetic nervous system (Hanif et al., 2000). The nose exhibits an asymmetrical airflow with the dominant nostril alternating from one nasal passage to the other over a period of hours (Eccles, 2000).The physical reason for such a change is the asymmetric congestion of the nasal swelling bodies which leads to partial blockage of one side of the nose (Proctor DF & Andersen IHP, 1982). The functional role of the nasal cycle is still unclear. Among many things, air conditioning and filtering of the air has been discussed as one of its possible functions (Abolmaali et al., 2013; Hanif et al., 2000; Letzel et al., 2022)
Sobel and colleagues showed that the nasal cycle may help in more differentiated olfactory perception by mediating lateralized nasal airflow (Sobel et al., 1999). Results from their study showed that the majority of the study population (17 of 20) exhibited a nasal cycle though no dominant nostril was established. Another study by the same group, using a portable rhino flowmeter or “Nasal Holter” device, examined 33 subjects and found that the left side was patent for a longer time than the right side (left = 2.63 ± 0.89 hours, right = 2.17 ± 0.89 hours, p < 0.05) (Kahana-Zweig et al., 2016)
The nasal cycle changes with body posture (Hasegawa, 1982), age (Williams & Eccles, 2015), handedness (Searleman et al., 2005), ultradian rhythms of cerebral activity (Werntz et al., 1983) and is also evident in various non-olfactory related brain measures as seen using EEG (Shannahoff-Khalsa, 1993) and cognitive task performance. Another EEG study revealed nasal cycle as an important parameter for left and right cerebral differences when awake and during sleep. This study showed a correlation between hemispheric dominance and the nasal cycle (Shannahoff-Khalsa, 1993). There is evidence suggesting both ipsilateral (Block et al., 1989) and contralateral (Klein et al., 1986) effects of nasal cycle between gender, where males and females express more coherence with spatial task with left side breathing. Results of its effect on the cerebral hemisphere still are unclear. Cerebral hemispheres exhibit functional and structural asymmetry in terms of performance based on spatial processing and logical processing which might be due to handedness differences (Sun & Walsh, 2006).
Previous studies showed a correlation between nasal airflow and handedness (Searleman et al., 2005). For the majority of the population, the dominant nostril was found to be positively correlated with the dominant hand. Evidence from a study of 19 subjects discovered greater EEG activity in the hemisphere contralateral to the dominant nostril (Werntz et al., 1983). This was confirmed in another, lager study (n = 126, right-handed) which also observed a link of nostril dominance with the contralateral hemisphere. Enhanced verbal task performance was found at times of right nostril dominance whereas greater spatial task performance was found at times of left nostril dominance (Samantaray et al. 2008).
The literature suggests most linguistic processing by left hemisphere, whereas odor perception is more lateralized to the right side (Herz et al., 1999). Hence, we assume that if the nasal cycle has a dominant side, then one side should receive more odorous stimuli than the other side, leading to more olfactory activations in the dominant hemisphere. We hypothesize that the nasal cycle could affect the anatomy of central brain structures such as OB volume (Buonviso et al., 2006) or the functionality of higher order olfactory processing areas of the brain in humans. The OB is a specialized structure and is the first odor decoding center (Scott et al., 1993). It is the most important relay station in the odor pathway, integrating peripheral and central olfactory information. OB volume is associated with olfactory dysfunction (Hummel et al., 2011; Mazal et al., 2016). Reduced OB volumes have been related to a decrease in olfactory sensitivity (Negoias et al., 2010). OB volume increases significantly after recovery from olfactory loss or with olfactory training in healthy subjects (Negoias et al., 2017).
The aim of the present study was firstly, to examine effects of a lateralized nasal cycle on OB volumes. In other words, we expected subjects with right dominant nasal cycle to have larger right OB volume and larger left OB volume for individuals with left dominant nasal cycle. Secondly, we aimed to investigate the lateralized effect of nasal cycle on brain activations when perceiving a selective olfactory stimulant.