Neuromodulatory influences on propagation of brain waves along the unimodal-transmodal gradient

Brain activity fluctuates over time, and understanding the factors that influence such fluctuations is important to understand the flexible nature of the brain and cognition. Growing evidence suggests that fMRI brain activity shows spatio-temporal patterns of propagation following specific gradients. In particular, activity around global peaks propagates as a travelling wave following a gradient from unimodal to associative areas. Some properties of these travelling waves seem to be related to behavioral and arousal states, however their meaning remains unknown. Here we assess the possibility that travelling waves explain the finding that there are specific time points when the brain presents larger brain integration. We reasoned that a faster speed of propagation would be related to more brain integration as measured with fMRI. Furthermore, we explored whether increased pupil-linked arousal, which has been related to more integration in specific brain regions, would be increased during periods of whole brain propagation. To test these hypotheses, we detected brain travelling waves and characterized them in terms of speed, directionality and ratio. We compared these features between different task conditions, and after a pharmacological challenge affecting neuromodulatory tone. We then studied the relation between travelling wave speed, pupil size and a graph-based measure of brain integration. Our results suggest that neuromodulatory tone affects travelling wave propagation, and that this propagation reflects changes in arousal and integrated functional connectivity features. This study provides a novel view of brain dynamics in terms of the effects of neruomodulatory influences across time scales.