Inhomogeneous degree of connectivity preference and computational efficiency in the olfactory system of the fruit fly*

The mushroom body (MB) is one of the olfactory information processing centers of fruitflies. Receiving information at calyx, different subtypes of Kenyon cells (KC), the intrinsic neurons of MB, respectively translate odorant stimuli into learning or memory behaviors. There were conflicting results reported about the network structure being random or with stereotypical preference. Based on the FlyEM dataset, we quantitatively measured the degree of randomness of each KC subtype by utilizing several exploratory data analyses. The results reveal that the learning-related network is strongly random, while the memory-related circuitry only possesses a mild degree of randomness. In addition, the connectivity preference of calyx corresponds with several innate features, including the development order of KC and the neuronal spatial arrangement of upstream brain regions. While the small-world network coupling, an almost regular network structure but with a few disordered connections, is known to possess high signal-propagation speed, MB displays the opposite topological characteristic: a highly random network but with slight connectivity preference. How a moderate degree of structural preference affects odorant discrimination and computational efficiency would be further discussed.