Our study investigated whether higher-order olfactory abilities, namely odour discrimination, can predict cognitive functioning, particularly impulsive behaviour. Taken together, the findings of this study confirm our hypothesis that olfactory functioning relates to both subjective (trait) impulsivity and objective measures of behavioural impulsivity. Therefore, our findings extend earlier demonstrations of the relationship between human olfactory capacity and executive functioning, specifically disadvantageous decision-making and cognitive flexibility37. Here, we further characterise the utility of olfaction as a marker for behavioural control function by showing that olfactory discrimination ability predicts the capacity to inhibit pre-potent motor responses (motor impulsivity) and is associated with lower subjective ratings of trait impulsivity in particular ratings reflecting the ability to plan (BIS Non-Planning impulsivity levels). Equally importantly, olfactory abilities were not related to other impulsivity measures.

Our finding that odour discrimination is a significant predictor of low self-reported non-planning impulsivity extends the relationship between olfactory dysfunction and trait impulsivity established in clinical populations17. One possible mechanism might be the common brain structures involved. It has been previously shown that the volume of right OFC negatively correlates with BIS non-planning impulsivity42,43,44 and is reduced in hyposmic individuals45.

Previous literature has associated olfactory function with inhibitory control; however, this work centres almost exclusively on clinical populations15,16. We provide important new evidence within a non-clinical population that broadens and reinforces this association. Moreover, our findings occur in the context of increasing understanding from neuroimaging studies of the neural mechanisms underlying impulsivity. For example, the same SST paradigm used in the current study is known to engage dorsolateral PFC, the inferior frontal gyrus, pre-supplementary motor area, basal ganglia, and insular cortex in the inhibition of pre-potent motor responses46.

Importantly, lesion and neuroimaging studies link odour discrimination to the functional integrity of a human brain network encompassing cerebellum, thalamus, caudate, insula, OFC and inferior frontal gyrus12,47,48. These latter three regions also underpin the control of motor impulsivity, likely through complementary neurocomputational processes: The inferior frontal gyrus is sensitive to stimulus salience and violations of expectation, responding to the key stimuli in both tasks (stop cues in SST, or ‘the odd one out’ in odour discrimination)49.

In addition, our results indicate that higher odour sensitivity (lower detection threshold) is a predictor of higher SSRT, a marker of poorer response inhibition. Thus, in contrast to odour discrimination performance, better odour detection threshold seems to be related to higher motor impulsivity. Olfactory sensitivity is not associated with function of the prefrontal cortex, but instead peripheral olfactory areas. Importantly our findings are in line with the literature on ADHD. Unmedicated patients with ADHD show increased odour detection sesnitivity compared to healthy controls19,50 together with hightened motor impulsivity levels, evidenced by poor performance on the SST51,52. Interestingly, medication with methylphenidate, known to influence dopaminergic transmission, normalizes odour sensitivity19 and simultaneously, improves response inhibition in children with ADHD53.

The olfactory bulb is rich in dopaminergic interneurons which inhibit presynaptic olfactory receptos via action on the D2 receptors. D2 receptor agonist quinpirole causes a decrease in odour detection performance, while pretretment with spiperone, D2 receptor antagonist, eliminates those results54. It is possible, therefore, that increased odour sensitivity derives from a decrease in dopaminergic activity in the olfactory bulb, and that increase in dopaminergic synaptic availability, mitigates this effect (causes a decrease in detection sensitivity). Dopaminergic system is also implicated in motor impulsivity55,56,57. Together, this evidence indirectly suggests that the relationship between enhanced odour sensitivity and increased motor impulsivity may be mediated by dopaminergic (dys)functioning, which underlies both processes.

Together, our results are consistent with the proposal that odour discrimination has a specific value in predicting the inhibitory control capacity of an individual through indexing the functional integrity of shared neural substrates. Moreover, the findings also suggest a relationship between increased olfactory sensitivity (decreased detection threshold) and poorer response inhibition. Diminished inhibitory control is frequently associated with maladaptive eating behaviour58,59, which could also relate more directly to olfactory dysfunction. Thus, the assessment of olfactory function has broad importance in the clinical setting.

Olfactory functioning was not predictive of the ability to delay gratification (temporal impulsivity) or reflection impulsivity. Such findings further suggest that impulsivity is a heterogeneous concept with separate mechanisms60,61 and distinct underlying neural networks.

Noteworthy, in our study with normative sample, we found an association between impulsivity measures and odour discrimination but not odour identification abilities. This is in contrast to some previous studies in clinical populations which pointed to the importance of odour identification15,16,17. Possibly, odour identification plays a greater role as a marker of impulsive tendencies in clinical populations, while odour discrimination may be more significant in normative samples. It is important to note, however, that odour discrimination abilities in those aforementioned studies was not assessed at all, suggesting a need for more comprehensive olfactory assessment in clinical samples.

The current study carries several strengths, including a relatively large sample of males and females tested. As our participants were screened for medical history, they were healthy, not suffering from any mental or neurological disorders and not currently taking any medication (apart from hormonal contraceptive pills). Thus, our study shows that olfactory ability is a predictor of cognitive functioning, particularly motor inhibitory control, in a normative population of healthy adults. Most of the previous studies looking at the relationship between olfaction and cognition have tested clinical populations15,16,17,26,27,62 else focus predominantly on memory35.

However, there are some limitations to our research. Our sample was selected from the population of university faculty and students. As a result, only young adults (up to 35 years old) were tested, all of whom were high functioning individuals. Additionally, our study did not involve collecting neuroimaging data to confirm the overlap between brain regions involved in the performance of the tasks and olfactory abilities. Therefore, we depended on previously published data for inference about the underlying neuronal circuitry, although the SST used in this study were the same as used in previous neuroimaging studies46. Future studies may usefully employ neuroimaging to confirm directly the sharing of neural substrates by olfactory and executive functions. Additionally, our regression models explained only a small proportion of the variance in the dependent variables. This may be due to individuals differences not explored in the current study, such as socioeconomic status or general heath aspects. Finally, only one odorant was used to assess odour detection threshold. Using various odorants with distinct odour qualities (e.g. characteristic unpleasant odours) might be advantageous63.

In conclusion, the relationship was established in a group of healthy young adults free of any medications, which makes olfactory abilities an especially good measure of impulsivity in general population and not only in clinical populations. Specifically, our data indicate that good odour discrimination is an especially good predictor of low impulsivity, particularly trait non-planning impulsiveness and pre-potent response inhibition (motor impulsivity), and that odour detection threshold might be an additional predictor of high motor impulsivity.