Chemosensory dysfunction is a frequent postacute sequela of COVID-19. Depending on the type of test used to measure it (self-report vs. direct test), the degree of chemosensory dysfunction in long-term COVID-19 has been found to be highly variable. In this manuscript, we report the cross-sectional data (first assessment) of a longitudinal study (6-month follow-up) examining smell, taste, and chemesthesis in participants affected by long-term COVID-19 (COVID+) and participants without COVID-19 (COVID−) by means of both self-reported and direct psychophysical methods. In total, 208 Italian participants (COVID+ n = 133; COVID− n = 75) completed the Smell and Taste Check developed by the Global Consortium for Chemosensory Research (GCCR), which includes self-reports on smell, taste, and chemesthetic abilities as well as direct intensity ratings of unstandardized smell, taste, and chemesthetic household items. Furthermore, all participants completed SCENTinel, a validated direct smell test. We found a positive association between the self-reported, unstandardized direct test and the validated direct test for smell, indicating moderate to large agreement across measures. Furthermore, the performance on SCENTinel was significantly associated with self-reported smell loss. A positive association between the self-reports and the intensity of household items was also retrieved for taste and chemesthesis. The time relative to COVID-19 onset (267.3 ± 113.9 days) did not modulate the chemosensory performance of self-reported abilities, intensity ratings, and SCENTinel. All in all, we confirm the impairment of three chemical senses (smell, taste, and chemesthesis) in an independent sample of Italian participants affected by long-term COVID-19 by using and comparing self-reported and direct psychophysical methods. We contribute to the discussion on best practices to monitor chemosensory dysfunction in individuals affected by long-term COVID-19.

Olfactory perception can be modulated by the repeated exposure to odours. Olfactory habituation is a reduced behavioural response to repetitive stimulation. Edibility is considered an important top-down feature that can affect olfactory perception, but whether it could modulate olfactory habituation when food or non-food odours are repeatedly smelled remains unclear. Indeed, due to their ecological salience, food odours attract attention automatically which might slow down habituation. This registered report aimed to determine whether olfactory habituation shows a different pattern when participants are presented with food or non-food odours. In a within-subject design, 50 participants were tested under satiated and fasted states in separated experimental sessions. In each session, participants were exposed to the same food and non-food odour in different blocks of 20 trials each. Participants rated the perceived odour intensity and pleasantness after each trial. We used an intermittent odour presentation to reduce olfactory fatigue while capitalising on the effect of cognitive states on habituation. Linear mixed-effects models showed that the perceived odour intensity decreased over time only for non-food odours. Conversely, the perceived odour pleasantness decreased significantly more across trials for food odours. These effects were retrieved regardless of the participant's hunger state. Our results are in line with the olfactory specific satiety theory which posits a specific decrease in the perceived pleasantness of food odours, without changes in the perceived odour intensity. In short, our findings indicate that perceived odour edibility modulates olfactory habituation, extending the previous literature on the impact of top-down factors on olfactory perception.

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Identifying objects in the environment is one of the most important features of our senses. Toward this goal, predictions are made from known environmental regularities. It is unknown whether different sensory systems may rely differently on cues for prediction and evaluation. Olfaction, the sense of smell, provides a means to test this hypothesis. People are often unable to identify odor stimuli based on scent alone, and cues from vision and audition are often needed. We tested whether olfactory (relative to visual) identification is especially dependent on source-based predictive cues. Participants listened to word cues (object-based, e.g. “lilac”, and category-based, e.g. “flower”), and then quickly determined whether a visual or olfactory target stimulus matched or did not match the word cue. As predicted, we found that the greatest response time differences between matching and non-matching trials occurred when the word cue was object-based and the target was olfactory. This interaction effect disappeared when participants were cued with category-based words (“fruit” or “flower”). A follow-up fMRI experiment revealed an increased involvement of the auditory cortex in anticipation of olfactory relative to visual objects. Furthermore, olfactory error signals activated a transmodal cortical network involving primary olfactory and visual cortices, as well as the anterior cingulate cortex. Our results suggest that olfactory identification may be especially reliant on predictive object cues and involve a vast, sensory-specific cortical error detection system. These results provide new insights into the organisation of sensory-cognitive hierarchies in the human brain.