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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>) :</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: 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">Cell types considered</h2><div> <cite class="ltx_cite raw v1">\citep[see][]{Jenett_2012}</cite></div><div><cite class="ltx_cite raw v1">\citep[see][]{Jenett_2012}</cite>, we picked driver lines for central complex&nbsp;cell types based on the strength of expression and the sparseness of the pattern in the central complex. At this stage, the 37 driver lines (for 24 cell types) cover the main columnar neuron types and PB interneurons&nbsp;&nbsp;<cite class="ltx_cite raw v1">\cite{wolff_neuroarchitecture_2015}</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>. The drivers used and the nomenclature are listed in Table&nbsp;<span class="au-ref raw v1">\ref{786393}</span>. Throughout&nbsp;this paper we follow the naming convention set out in&nbsp;<cite class="ltx_cite raw v1">\cite{wolff_neuroarchitecture_2015}</cite> for full names, and abbreviated following the scheme described in&nbsp;<cite class="ltx_cite raw v1">\cite{Kakaria2017}</cite> and used in&nbsp;&nbsp;<cite class="ltx_cite raw v1">\cite{Green2017}</cite> and&nbsp;<cite class="ltx_cite raw v1">\cite{Turner_Evans_2017}</cite>. At the time of writing this manuscript, the dataset explores 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. Connectivity of different cell types within the FB have not been explored.</div><div></div><h2 data-label="856444" class="ltx_title_subsection">A functional connectivity screen</h2><div>For any given pair of neurons, the overlap between pre- and post-synaptic looking regions was assessed based on publicly available expression patterns (<cite class="ltx_cite raw v1">\citealp{Tirian_2017,Jenett2012}</cite>, see Figure&nbsp;<span class="au-ref raw v1">\ref{831433}</span>B) digitally aligned on a common reference brain (as described in&nbsp;<cite class="ltx_cite raw v1">\citealt{Aso_2014}</cite>). For each combination selected, one or several genotypes were prepared to express CsChrimson and GCaMP6m in the potential pre- and post-synaptic partners, respectively (Figure&nbsp;<span class="au-ref raw v1">\ref{831433}</span>C, see Materials and Methods). Brains and VNCs were dissected out  and tested using a standardized protocol (see Figure&nbsp;<span class="au-ref raw v1">\ref{831433}</span>D, and  Materials and Methods). The light stimulation intensities were kept at low levels (50uW/mm<sup>2</sup>, delivered in trains of 2 ms pulses) to reduce the chances of observing indirect connections, while at the same time  allowing us to see near-saturating responses in strongly connected pairs (see Supplementary Figure&nbsp;<span class="au-ref raw v1">\ref{723169}</span>). Whenever large responses were observed  pharmacology was used to 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 <span class="au-ref raw v1">\ref{841071}</span>).&nbsp;</div>