Subjects were tested twice—before and after OT intervention. Before inclusion in the study, a standardized medical interview was pursued to collect information about the factors that could potentially undermine olfactory abilities such as diabetes, smoking, or current infections [29]. All patients received nasal endoscopy to exclude sinonasal causes of olfactory dysfunction, e.g., chronic rhinosinusitis with or without polyposis.

Subjects were asked to refrain from smoking or eating 1 h prior to the testing session and to avoid wearing strong perfumes on the day of testing. Subjects were tested individually in a well-ventilated room. Based on the interview and the results of the Sniffin’ Sticks test for olfactory function, subjects were categorized as hyposmic or anosmic patients (scores =  < 30.5 points) or healthy controls (scores > 30.5 points) [28]. Baseline measurements were taken during the first meeting, including tests for (1) olfactory function: the Sniffin’ Sticks test battery with three subtests for olfactory threshold, discrimination, and identification [30]; (2) retronasal olfaction using 20 selected grocery-available products [30] (3) individual significance of olfaction measured with eighteen statements relating to the use of odors in everyday life [31]; (4) Montreal Cognitive Assessment (MoCA) test for screening mild cognitive impairment (MCI) with the maximum score of 30 points and 26 points being a cut-off for MCI [32]; (5) Controlled Oral Word Association Test (COWAT) measuring spontaneous production of words [33]; (6) verbal semantic fluency task wherein subjects were asked to name as many supermarket-available products as possible within 60 s; (7) Beck Depression Inventory (BDI) comprising 21 items scored from 0 to 3 points, the higher the result the more intense depressive symptoms [34] and (7) Positive and Negative Affect Schedule (PANAS) comprising 10 items to estimate experience of positive and negative effects [35].

Each participant was equipped with an electrical odor dispenser (cylindrical shape, height 8 cm, diameter 2.3 cm [Aromastic; Sony, Tokyo, Japan]) which allowed to distribute 5 odors (initially subjects used: grapefruit, lavender, lemon grass, ylang-ylang, peppermint). Odors were changed for each subject after approximately 3 months OT period (odors were changed to: menthol, thyme, tangerine, green tea, and bergamot). The change of odors was also used to re-instruct the participants in terms of the OT and to reinforce the motivation to perform the procedures.

The choice of odors was guided by (1) pleasantness of the odors, (2) presence of slight trigeminal activation in some of the odors, e.g., peppermint, (3) evaporation characteristics, so that the odors would last for the duration of the experiment, (4) technical issues mostly in terms of compatibility with the odor cartridge, (5) availability, and (6) inspiration from the previous studies [2, 3, 5]. When pushing a button, a quantum (approximately 4 ml odorized air) of the selected odor were released over approximately 1 s using a silent piezo-based air pump; different odors were selected manually by turning a wheel on top of the odor dispenser. Subjects were instructed to sniff each odor for approximately 30 s, by pushing the button repeatedly. They were randomly assigned to one of the two experimental conditions according to the generated numbers they received when entering the study. They either trained twice (standard OT regimen) or four times a day (intense OT regimen). Additionally, they were asked to perform training before or at least 30 min after the meal at intervals of 12 or 6 h, respectively. Subjects were also told that OT should be performed in quiet, odorless places. The study was concluded after reaching the estimated sample size. All subjects who completed the training and made post-training appointment were included. For two patients (1 with post-viral olfactory loss and 1 post-traumatic olfactory loss), we were not able to acquire olfactory performance measurements, so these two subjects were excluded from the models concerning the Sniffin’ Sticks scores.

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