Ensemble learning and ground-truth validation of synaptic connectivity inferred from spike trains
Fig 4
Validating synaptic connectivity inference with parallel HD-MEA/patch-clamp recordings.
A A single patched neuron on the HD-MEA, including the patch pipette. The neuron was labelled with Alexa Fluor 594 through the patch pipette; HD-MEA electrodes can be seen in the background. B Example recording of a patched neuron (lower panel, intracellular signal in blue), and some of its extracellular signals on the HD-MEA (upper panel, spikes in black); HD-MEA and patch-clamp recordings were performed in parallel and later synchronized. C The temporally aligned HD-MEA/patch-clamp signals allowed inference of the exact location of patched neurons on the HD-MEA and their electrical footprints (EFs) D. To infer the putative pre-synaptic connectivity of individual patched neurons, we first performed long-term HD-MEA baseline recordings. Next, we applied a post-processing step to match the EF of the patched cell with the EF templates obtained from the spike-sorted HD-MEA baseline recording. Panel E depicts the overlap of the EF generated during the patch session (in black), and the matched EF obtained from the spike-sorted baseline recording (in red). F HD-MEA network recording (upper panel) and simultaneous whole-cell voltage-clamp (VC) recording (lower panel). VC recordings were used to measure the excitatory/inhibitory postsynaptic currents (ePSCs/iPSCs) in the patched cell, and to link their occurrence to the extracellular activity of neurons recorded on the HD-MEA. Panel G depicts three exemplary connections of the patched cell (EF in black) to three presynaptic neurons; the EFs of these cells are depicted in light blue, orange, and purple. H IC model fit to patch-clamp recording. Spikes of identified presynaptic neurons are shown on top, aligned with the recorded currents of the patched neuron. PSCs of three units are shown as insets, with the average PSC signal depicted in black and the fitted model PSC displayed in different colors. The shaded area shows the 5–95% quantile range. I PSCs of four cells, three connected presynaptic neurons, and one unconnected neuron, are shown along with their corresponding CCGs. Note, that a high-chloride internal patch-clamp solution was used to simultaneously measure the synaptic activity of excitatory and inhibitory presynaptic cells [58]. As a result, as depicted, inhibitory input currents also have a negative polarity. On the right side of the CCG, a colored circle indicates whether the respective connectivity method found a connection or not (cross). J Panel depicts the performance of the different connectivity methods averaged over three patched neurons.