Neuronal wiring diagram of an adult brain

Our reconstruction includes both left and right brain hemispheres. This is important for tracing sensorimotor pathways that cross from one side to the other, and more generally for understanding interactions between the two hemispheres. The projectome (Fig. 4a) reveals that most projections are ipsilateral or between neuropils on the same side of the brain.

The low fraction of non-ipsilateral neurons is primarily due to their scarceness in the optic lobes. Only 139 neurons (0.2%) in the optic lobes cross hemispheres and cross the central brain without making synapses there (Supplementary Fig. 6)—these neurons are considered to be ‘fully contained’ in the optic lobes because our definition depends only on synapse locations. These neurons mediate direct interactions between the two optic lobes, and their rarity suggests that these interactions represent a smaller fraction of the computations that occur within the optic lobes. Integration of information from both eyes may rely more on the abundant crossing connections between the central brain targets (AOTU, PLP and PVLP) of VPNs.

A higher proportion (40%) of central brain neurons are non-ipsilateral, largely owing to central neuropils, similar to those of the central complex and SEZ. To classify non-ipsilateral neurons, we began by examining the spatial distributions of their postsynapses (inputs). We divided the neuropils into three categories. Left and right categories included the neuropils that come in mirror-symmetric pairs. Centre included the seven remaining neuropils that are located on the midline. For each neuron, we computed the proportions of its postsynapses in left, right and centre neuropils (Extended Data Fig. 7). Each neuron was assigned to the dominant category, and near-ties were rare. The exceptions are symmetric neurons with cell bodies at the midline of the brain (Supplementary Fig. 7, n = 89).

Next, we explored how many neurons of left and right categories have presynapses (outputs) in the other hemisphere. Similar to the analysis of the 1st instar larval connectome, we found that neurons projecting to the other hemisphere can be grouped into bilateral neurons, those with outputs in both hemispheres, and contralateral neurons, which almost exclusively had presynapses in the other hemisphere (Fig. 4g–i). Notably, a much larger fraction of VCNs projected to the contralateral hemisphere than VPNs, and both VCNs and neurons of the central brain contain a large fraction of bilateral neurons (Fig. 4h). As stated above, this analysis again revealed the dominance of ipsilateral connections in the brain. Whereas mixing between the hemispheres is more rare, mixing between sensory modalities within a hemisphere is common (Fig. 6).

Many types of fly neurons are known to exhibit striking stereotypy across individuals, as well as across both hemispheres of the same individual. Schlegel et al. show quantitatively using FlyWire brain and hemibrain data that these two types of stereotypy are similar in degree.