Recent frontiers on cerebral functional neuro-connectivity in relation to tinnitus

Abstract

We make preliminary remarks about concepts and data on CNS neuro-plasticity processes and on cross-modal and multisensory networks, both in general and in relation to tinnitus. We report on multiple and interesting studies on cerebral functional connectivity in general and within or between individual areas. We also report on the “connectome”, seen as a general and highly “dynamic” map of cerebral, especially functional, connection networks, which interactively operate and co-operate in order to adapt and change depending on different functions. In this dynamic environment, we dwell on early observations of changes in cerebral functional connectivity in tinnitus subjects. In the last 10-12 years, almost all connectome investigations were mainly, though not exclusively, carried out by means of fMRI (functional magnetic resonance imaging) neuroimaging equipment and methods, in order to achieve volumes (voxels) of a few millimetres and the highest possible resolution in teslas. For general purposes, but even more in audiology and tinnitus studies, investigations reported in the literature tend to study changes in functional connectivity in one of the most widely known and vast networks, the default mode network (DMN). As for test conditions, the so-called resting state is generally, though not exclusively, adopted; in this rest condition, achieved by means of well-established strategies to reach the desired parameters, any quantitative and qualitative changes in DMN connectivity within it or to other networks in subjects with known diseases or symptoms, in our case tinnitus, can be observed and assessed. We briefly report on several studies by selecting, among the multiple connectivity changes, those concerning DMN connections to attention networks, especially the one relating to the DAN (dorsal attention network), as well as various types of functional disconnection-hyperconnection to complex limbic networks and especially to the neural network that is controlled by the amygdala. This result would, on the one hand, confirm the central role of this vast network in the processes of cerebral functional neuro-plasticity, and, on the other hand, partially explain why symptoms in tinnitus patients often include anxiety, depression, fear, distress and sleep disorders, which either follows the simple sound perception or are an integral part of the clinical picture from the beginning. Many changes in functional connectivity in tinnitus subjects typically involve the precuneus, a formation that is anatomically well identifiable and that is functionally part of the DMN, though its functional significance is still unclear. The results of the studies carried out so far, though fascinating, are still found lacking, both because identification and systematisation of the most widely involved and significant brain networks and areas are not homogeneous, and because the suggested correlation with clinical data on tinnitus is incomplete and fragmentary, but as a whole they represent a strong incentive to future and by now unstoppable research on the connectome and tinnitus.