Extended Data Fig. 8. Direct MBONs to modulatory neuron connectivity is very sparse, in contrast to the very dense connectivity via one-and two-step feedback pathways.

a Connectivity matrix showing normalized synaptic input (expressed as % input) each modulatory neuron (columns) receives from each MBON (rows). Only reliable connections for which the postsynaptic neuron receives at least 1% of input from the presynaptic neuron are shown. When the neurotransmitter of the MBON is known, the circle is color-coded to represent type of connection: excitatory (ChAT) or probably disinhibitory (GluT). Color shades represent the valence of the memory formed in a given compartment (red: aversive memory, blue: appetitive memory). True within-compartment feedback connections from an MBON that receives direct synaptic input from that modulatory neuron are boxed in bold. Very few modulatory neurons receive direct input from MBONs, in contrast to the dense connectivity between MBONs and modulatory neurons via the indirect one- and two-step feedback pathways (b). b Connectivity matrix showing indirect connections between MBONs and modulatory neurons via one-step and/or two-step feedback pathways. The matrix was obtained by summing the matrices from Fig. 3b and Fig. 5e. The color indicates the type of indirect connection existing between a given MBON and a given DAN. Bubble size represents a connectivity index computed as in Fig. 3b and Fig. 5e. A connectivity index of 1 or 10 means that for all connections comprising that indirect feedback pathway the presynaptic neuron accounts for 1% and 10% of input onto that postsynaptic neuron, respectively. One- and two-step feedback drastically increases the connectivity between MBONs and modulatory neurons, compared to direct connections (a).