## 4 Discussion  
The primary aim of this study was to investigate and describe any differences in the attentional and salience network connectivity in the four MDD subtypes as defined by the scores on groups of items from the SDS obtained by the participants regardless of whether or not they met the criteria for clinical depression. Theta coherence increased along with the overall SDS scores in a short three-node system typically involved in orienting visual attention across all the networks used in this study [43; 44]. As increases in theta activity are typically associated with drowsiness or similar low-energy states, this indicates that people with MDD are increasingly likely to have less interest in monitoring their surroundings, leading to a sense of withdrawal from the environment given that it is less likely that attentional networks will identify anything considered worth engaging with. This finding is congruent with the 'depression as behavioural withdrawal' hypothesis mentioned in the Introduction. This situation is increasingly the case with the Cognitive and particularly the Anhedonic depression subtypes. The theta band coherence system associated with the Cognitive depression subtype includes all the nodes found in those listed in the broader SDS depression and two more nodes from the DAN (the right frontal eye field and right posterior IPS). Given that activation in the DAN is indicative of increased top-down processing effects, this indicates that Cognitive depression may also be affected by the top-down control of attention [45; 46]. The theta results relating to Anhedonic depression are even broader in scope but mostly involve the DAN and VAN, indicating that Anhedonic depression may be the result of a more chronic cause. This view is backed by the inclusion of the dorsal ACC (as part of the SN) in the theta results, given the broad range of mental processes that the dACC is associated with [47; 48]. As distinct from theta coherence, beta coherence decreased as the overall SDS scores increased in a short three-node system primarily in the SN. Similar results were also found with the Cognitive subtypes with the addition of the middle frontal gyrus node from the VAN. Each node here plays an important role in identifying relevant stimuli based on episodic memory, particularly the supramarginal gyrus and the lateral parietal lobule [49; 50]. This function would still be active during resting states such as those used in this study because monitoring the environment is an ongoing process rather than a task-positive one. Since beta and gamma activation is typically associated with active cognitive processing, this would indicate that greater degrees of MDD are associated with a fall in cognitive resources available for engaging with the environment (perhaps contributing to the behavioural withdrawal noted above). Similar results were found in the gamma band as those in the beta band where gamma coherence between the left supramarginal gyrus and left insula decreased as Anhedonia, Cognitive, Mood, and overall SDS scores increased. Figure 3: Significant negative correlations (\(p<0.1\)) between gamma coherence and the Anhedonic, Cognitive, Mood, and overall MDD depression scores. Views are from the left, top, and front, respectively. The overlap between the beta and gamma bands is likely the result of the same processes discussed above in the beta band results. The Mood and Somatic depression subtypes are conspicuous in their absence from these results; apart from a single connectivity result in the gamma band, there was no relationship between either depression subtype and any measure of connectivity in the attentional networks.