The sign is not in the picture: a default sign rule and the olfactory runaway of a whole-brain fly model (research snapshot, 30 September 2026)

Dated snapshot of an independent research project on the fruit-fly connectome. Whole-brain models built on the FlyWire connectome need a sign (excitatory or inhibitory) for every neuron; these signs come from a machine classifier and a convention. In a published whole-brain model (Shiu et al. 2024), one olfactory receptor neuron activates 97% of the antennal-lobe projection neurons (first observed 24 September 2026). The cause is the default given to the 150 antennal-lobe local neurons that receive no transmitter prediction ("no call → excitatory"). The model's sense of smell depends on how much of the local neurons' output is excitatory, not on which cells carry it. Given a real fly's receptor responses (Seki et al. 2017), the published signs reproduce the real projection-neuron responses at r = 0.28, a laboratory-informed prior at r = 0.70. Every test was pre-registered in a hash-frozen file, and every number is linked to the script output that prints it. Work in progress, not peer reviewed.