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<div>The central complex consists of three main neuropiles — the protocerebral bridge (PB), the ellipsoid body (EB, Central Body Lower in other insects) and the fan-shaped body (FB, Central Body Upper in other insects) — and at least three accessory neuropiles — the noduli (NO),  gall (GA) and  lateral accessory lobe (LAL)  (Figure&nbsp;<span class="au-ref raw v1">\ref{288889}</span>A and&nbsp;<cite class="ltx_cite raw v1">\citealt{Wolff2015,Lin2013,Hanesch1989}</cite>). One of the most striking neural elements of the central complex are the&nbsp;<i>columnar neurons</i>, which innervate one of the eighteen (in Drosophila) glomeruli of the PB, one vertical section of either the FB or EB, and one accessory neuropile — a&nbsp;<i>column</i> being constituted by the PB glomeruli and FB/EB section. A total of 12 different columnar cell types have been described, with stereotypical  correspondences between  the PB glomerulus  and the EB/FB section. In addition to these "principal cells", there are a number of neurons innervating multiple columns of one neuropile. These neurons often innervate subdivisions orthogonal to the columns. Moreover they sometimes also project to neuropiles outside the central complex . This set of neurons includes the ring neurons, which innervate a ring within the EB and an accessory neuropile, and a collection of inputs and interneurons with processes in the FB and PB. From this light level anatomy and putative synaptic polarity, one can derive a hypothetical general information flow diagram of the central complex (Figure&nbsp;<span class="au-ref raw v1">\ref{288889}</span>Bi) :</div><ul><li>Ring neurons provide input to the EB columnar neurons.&nbsp;</li><li>Recurrent connections between EB columnar neurons form and sustain a ring attractor for heading direction</li><li>Information is transferred from the EB columnar system to the FB columnar system via the PB (interestingly, only one columnar neuron type displays presynaptic terminals in the PB, the E-PGs)</li><li>FB columnar neurons also receive inputs in the FB</li><li>All columnar neurons but the E-PGs also receive inputs in the PB</li><li>Interneurons in the PB and FB further interconnect the columns</li><li>All accessory structures are potential outputs&nbsp;</li></ul><div>We show that this overall flow of information is generally supported functionally for the parts we have tested so far, but with a few potentially important differences (Figure&nbsp;<span class="au-ref raw v1">\ref{288889}</span>Bii): the observed connectivity in the PB is  sparse, rendering the function of PB interneurons possibly critical; accessory structures are usually input rather than output areas; and, consequently, output channels of the CX are scarce.</div><div></div><h1 class="ltx_title_section">Results</h1><h2 data-label="268389" class="ltx_title_subsection">A functional connectivity screen</h2><div>We picked driver lines for functional connectivity mapping by visually inspecting the Janelia Gal4-driver collection&nbsp;<cite class="ltx_cite raw v1">\citep[see][]{Jenett2012}</cite>&nbsp;for strength of expression in the cell types of interest, and sparseness of the expression pattern in the central complex. The 37 driver lines (for 24 cell types) cover the main columnar neuron types and PB interneurons&nbsp;<cite class="ltx_cite raw v1">\cite{Wolff2015}</cite>, three types of&nbsp;ring neurons,&nbsp;a LAL-FB neuron and neurons innervating accessory structures, namely a LAL interneuron and three types of neurons connecting the LAL to the noduli. Neuron types are schematized in Figure&nbsp;<span class="au-ref raw v1">\ref{831433}</span>A and Supplementary Figure&nbsp;<span class="au-ref raw v1">\ref{147797}</span>. At the time of writing,  the dataset includes inputs to the EB system, connections between EB columnar neurons, connections in the PB as well as potential inputs and outputs in the LAL, Gall and noduli. The connectivity of the multitude of cell types within the FB has not been explored.</div><div></div><div>Cell-type pairs to be tested were chosen based on  overlaps between their expression patterns in light-level images.&nbsp; For each combination selected, we expressed CsChrimson and GCaMP6m in potential pre- and post-synaptic partners, respectively (Figure&nbsp;<span class="au-ref raw v1">\ref{831433}</span>B,C), and probed their connection  in an ex-vivo preparation using a standardized protocol (see Figure&nbsp;<span class="au-ref raw v1">\ref{831433}</span>D, and  Materials and Methods).   Whenever large responses were observed, we used pharmacology to both check that observed transients were synaptically mediated, and to narrow down the neurotransmitters involved (Supplementary Figures&nbsp;<span class="au-ref raw v1">\ref{488013}</span> and&nbsp;<span class="au-ref raw v1">\ref{841071}</span>).&nbsp;</div>