Multisensory interaction: different pain perception through smell and taste

Pain is a fundamental perception for our survival, acting as an alarm sys-tem. Nevertheless, patients may complain about the presence of pain even without actual physical harm. When neuropathic pain becomes chronic, those patients’ quality of life decreases dramatically, and more or less severe conse-quences on several life aspects become evident. Nowadays, alternative pain-relief therapies are increasingly necessary in order to avoid drugs shortcom-ings, as well as to reduce the costs for healthcare systems. Pain can be influenced by many factors, and the combined use of other senses in a multisensory interaction represents one of the most effective strategies to alter its perception. Smell and taste are strongly linked to emo-tion and cognition, and they might be useful in pain modulation, providing al-ternative approaches for pain treatment and management. Here, we provide evidence of the useful tools that smell and taste could represent when interacting with pain. Firstly, we report the results of a litera-ture review on adults stating the effects of olfactory and gustatory substances on both experimental and clinical pain. Smell of different types influences pain unpleasantness and intensity (the so-called qualitative measures of pain), while taste has an effect on pain threshold and tolerance (the quantitative measures) with more contradictory results. Scarce literature is reported in clinical pain and with the use of neuroimaging measures. Secondly, we performed an experimental study on healthy adults (n = 60) where we induced pain with capsaicin cream applied on the back of their hand. Such methodology has been chosen to resemble a sort of tonic neuro-pathic pain. We found evidence that pain unpleasantness is reduced after the administration of the pleasant smell condition. No effect was found on pain intensity, nor with other substances. Thirdly, we performed a very similar experimental design on a clinical population that suffers from chronic oral burning pain (n = 22), revealing that pain unpleasantness was increased by the unpleasant smell and taste sub-stances. With reference to the smell substances, such effect was related to the subjective perceived pleasantness of the patients. Moreover, a stronger effect was found in patients with longer disease duration. No effect was found on pain intensity, nor with other substances. Several clinical and psychological variables were also collected. To sum up, smell and taste could temporarily alter pain unpleasantness perception in tonic pain, both experimentally induced and in a clinical condi-tion. Future experiments that select custom-designed substances for each pa-tient could reveal a stronger effect in chronic pain populations and explore long-term efficacy. Within this frame, it is important not to forget the key role that attention and emotions play in the relation between pain and chemical senses, when exploring the effectiveness of those alternative options. The three works here described have been published in international peer reviewed journals.