Abstract:
In this study, rs-fMRI data were used to investigate BOLD functional connectivities between multiple auditory-correlated, cortical and subcortical brain regions of interest (ROIs). The data were extracted from the dataset of McMahon K., Queensland University, Australia, stored in the databank of the “1000 Functional Connectome Project” (http://fcon_1000.projects.nitrc.org/fcpClassic/FcpTable.html). The dataset includes recordings from 19 healthy subjects with normal hearing, encompassing both females and males, ranging in age from 20 to 34 years. Due to issues with the data quality, information from one female participant was deemed unsuitable for analysis and consequently excluded. This resulted in a final sample of 18 subjects, whose ages ranged from 21 to 34 years, being included in the study.
The analysis included brain ROIs that are considered to play a significant role in the genesis and maintenance of hearing disorders such as tinnitus, a phantom sound whose neurophysiological mechanisms remain open to scientific debate (Hofmeier et al., 2018).
High synchronicity was observed between all homologous regions as well as within subcortical auditory brain nuclei. Additionally, all ROIs within the aforementioned (auditory, limbic/emotional distress, temporofrontal/attentional) brain networks demonstrated a significant degree of synchronicity with one another.
The posterior insula region BA13P exhibited wide interconnectivity with auditory cortical areas. This extensive network of functional correlations suggests a robust level of synchronicity, which may reflect effective auditory processing capabilities and potentially contribute to the optimization of sound detection thresholds.
In contrast, regions of the limbic/emotional distress network showed comparatively lower levels of involvement, indicating minimal auditory-related distress. This suggests a state of physiological equilibrium in auditory processing, reflective of a balanced emotional response to auditory stimuli. Remarkably, the current study revealed the absence of connectivity between auditory limbic regions, such as the amygdala and the anterior insula, and subcortical auditory nuclei. This finding is noteworthy because previous research (Hofmeier et al., 2018) had identified such connections as a possible underlying cause for auditory distress.
Similarly, in regard to the temporofrontal attentional network, both the stress-enhancing medial prefrontal cortex BA9M and the stress-inhibiting dorsolateral prefrontal cortex BA9DL exhibit a sparse yet uniformly distributed pattern of functional connectivity. Notably, when compared to tinnitus participants from prior studies (Hofmeier et al., 2018), no correlations were observed between regions of the attentional network and regions of the auditory brainstem in this research. This absence of connectivity not only lends support to the notion of HPA axis balance but also supports the hypothesis that synchronicity between temporofrontal areas (such as BA9M and BA45) and auditory brainstem regions might be implicated in, or contribute to certain sensorineural disorders, including tinnitus.
In this study, a higher prevalence of positive correlations was found in male and older participants compared to female and younger ones across cortical and subcortical ROIs in the aforementioned brain networks. This contradicts earlier studies suggesting stronger connectivity in females. The observed discrepancy might be due to the uneven sex distribution in the sample (11 males vs. 7 females). Additionally, older participants showed higher BOLD-signal synchronicity than younger ones, which could reflect individual variability rather than age-related changes, potentially influenced by the small group sizes. The similarities between male and older participants, as well as between female and younger participants, suggest that differences in average ages within the sex subgroups may have affected the results. Consequently, it was challenging to discern significant sex differences due to this age disparities. Future research should aim for larger, age-balanced subgroups to better distinguish sex differences from age effects and clarify the neural mechanisms involved.
These findings highlight potential neural mechanisms that could differentiate between normal auditory processes and pathological conditions like tinnitus. These findings provide an overview of physiological functional connectivity patterns between auditory regions and auditory-correlated areas of interest in the brain, as measured by BOLD activity, reflecting the inherent neural processes associated with normal auditory functions. The results presented here can serve as a comparative basis for future research aiming to identify and characterise alterations in functional connectivity that may arise following the onset of auditory disorders as hearing loss and tinnitus.
prefrontal attentional Network