A Connectome of the Adult Drosophila Central Brain

Key results

1. ShownThe hemibrain is described as the largest dense reconstruction to date, and its 26-teravoxel image volume reduces to a connectivity graph about a million times smaller.

   Around 25,000 neurons and about 20 million chemical synapses; the graph has 25,000 nodes and 3 million edges and occupies 26 MB even when split by brain region, roughly a million-fold reduction from the raw data. Imaging produced 26 teravoxels at 8 bits per voxel; over 50 person-years of proofreading over 2 calendar years went into it. · 1 Introduction; 2 Connectome Reconstruction

2. ShownNeurons were typed by a pipeline that starts from cell-body fibre bundles and combines morphological with connectivity clustering, yielding a type for nearly every reconstructed cell, most of them previously unnamed.

   18,478 cell bodies found, 15,084 of them in the right central brain, falling into 192 cell body fibre bundles of which 154 matched 102 known clonal units. 4,768 cell types were assigned to 20,607 neurons, over a thousand of them represented by a single cell. Clustering used NBLAST followed by iterative connectivity clustering and manual review. · 2.7 Methodology for assigning cell types and…; 2.8 Results of cell typing; 2.9 Assessing Morphologies and Cell Types

3. ShownAutomated flood-filling segmentation was deliberately run in an over-merging regime, trading a high false-merge rate for long run lengths that human proofreaders then repaired.

   Flood-filling networks were trained on roughly five billion voxels of ground truth and applied coarse-to-fine at 32, 16 and 8 nm after CycleGAN normalization. Base segmentation: expected run length (ERL) 163 microns at a 0.25% merge rate; after agglomeration, ERL 585 microns at a 27.56% merge rate. On 500 olfactory projection and Kenyon cells, ERL was 825 microns at 15.92%. · 2.2 Automated Segmentation

4. SuggestedConnections are deliberately sampled rather than complete, so the authors give explicit guidance on reading synapse counts: strong connections are trustworthy, weak ones often are not.

   More than 90% of T-bars sit in identified neurons, but post-synaptic densities are traced at completeness typically between 20% and 85% per region. A connection of strength 10 is argued to be recovered with over 99.9% probability if half its synapses are traced. The authors advise ignoring connections of 3 or fewer synapses, and report single-connection pathways correct over 76% of the time. · 2.11 Assessing connection completeness; 2.10 Assessing Synapse Accuracy; 2.12 Interpreting the connection counts

5. ShownBecause every compartment comes from the same brain, their wiring statistics can be compared directly, and they differ widely on most measures except average graph distance.

   Across compartments fully inside the volume with at least 100 neurons: neuron count varies 74-fold, mean partners per neuron 36-fold, network diameter 4-fold, mean connection strength 5-fold and the fraction of reciprocal connections 5-fold, while average graph distance between neurons varies only 2-fold. · 5.1 Compartment statistics