The overlapping and differing brain mechanisms underlying itch and affective touch: An Activation Likelihood Estimation metaanalysi

Abstract

Itch has a seriously negative influence on the quality of daily life for individuals suffering from a chronic form of itch, such as in skin diseases. A common treatment for itch is the administration of antihistamine, however not all patients respond to this form of treatment. In order to find a new form of treatment, the current study focussed on a sensation that interacts with itch: affective touch. Little research is done on the interaction between affective touch and itch. In order to advance our understanding of the possible inhibitory influence of affective touch on itch, it is of importance to have a clear view of the mechanisms of both sensations and how these overlap. In this study, the brain mechanisms underlying affective touch and itch were compared. This was researched by conducting an Activation Likelihood Estimation (ALE) metaanalysis of experimentally induced itch from the published fMRI literature (n = 160, 12 experiments), and generating a comparison ALE map of affective touch (n = 363, 22 experiments). Finally, a meta-analytic contrast analysis was conducted between the two sensations (total n = 523, 34 experiments). Based on the results of previous meta-analyses, the expectation was that the overlapping clusters of itch and affective touch would be found within the thalamus and the operculum/insula. These meta-analyses yielded one clusters of activation more likely to be activated by affective touch: 1) the insula. The results showed five clusters of activation more likely to be activated by itch: 1) insula, claustrum, lentiform nucleus and extra-nuclear; 2) thalamus, lentiform nucleus; 3) precentral gyrus, superior temporal gyrus and insula; 4) precentral gyrus and superior temporal lobe; and 5) insula and inferior frontal gyrus. Unfortunately, the current research yielded no significant clusters for the overlapping activation of itch and affective touch. This indicates that there were no areas that made a statistically comparable contribution to both itch and affective touch. However, as the fields of research into induced itch and affective touch are still expanding and the number of research is growing, the hope of finding significant clusters for the overlapping activation of itch and affective touch grows. With this, the hope of finding a form of treatment for patients that do not respond to conventional treatments increases as well.s