Functional connectivity, structural connectivity, and inter-individual variability in Drosophila melanogaster

Clarifying the relationship between structure and function is important for understanding the brain. In Drosophila melanogaster, whole-brain and hemibrain electron microscopy connectome data, as well as whole-brain calcium imaging data are available. We applied pre-processing methods from fMRI to whole-brain calcium imaging data and comprehensively investigated the optimal parameters. Then, we found that the FC-SC (functional and structural connectivity) correlation decreased linearly with ROI count, and this trend was the same in flies and humans. We also developed a new, more robust method to quantify the degree of pre- and post-synaptic segregation and investigated this in the fly whole-brain. This revealed that many neurons have unsegregated synapses. We extracted highly unsegregated synapses and compared them with random-extracted null SC matrices. Their FC-SC correlation was significantly higher, indicating that these synapses contribute to FC well. Conversely, highly segregated-synapses showed significantly lower FC-SC correlation and contribute less to FC. Therefore, neurons with unsegregated synapses like non-spiking neurons are spread throughout the whole brain, and they are thought to have a significant influence on FC.