Current-source density analysis

To reveal more precisely the spatial pattern of the major sites of synaptic activity, we calculated the current-source density (CSD) profile of visual and mechanosensory inputs along the neuropil as described in Hamodi and Pratt (2015) and Mitzdorf and Singer (1977) using a spatial differentiation grid of 20 µm. CSDs reveal current sinks, generated by positive current moving into the neuron, and sources, which are generated by positive current moving out of the neuron or negative current moving in through, for example, chloride channels. To generate CSDs, the second spatial derivative of FP traces was calculated by subtracting the trace obtained at a given position [V(x)] from the sum of the two flanking traces (i.e [V(x+n*∆x)]+ [V(x-n*∆x)]) and dividing by the square of the distance between the sites of recording (∆x). All calculations were performed using IGOR Pro and R software. Image plots were generated using Plotly software.