Current Source Density (CSD)

It is the standard way to read which cortical layer is active during a sensory response, and to confirm where a probe sits in the column from the signal rather than from the manipulator.

It is an electrode method before it is an analysis. CSD needs sites in one column at regular pitch along depth — a linear layout at 25 to 100 µm — with every site working, because the derivative is taken across neighbors and a dead site breaks the profile at that depth; and it needs a reference that is not itself in the signal being differentiated. Site area matters less than site spacing: the LFP is present on every site, and what CSD resolves is how it changes from one to the next. Across the cortical layers, a sink in layer IV after a sensory stimulus is the usual landmark. This is the measurement the A-Series linear layouts were drawn for — one column at 50 or 100 µm on 15 µm silicon, with every site read on one clock — and it is why the catalog keeps the linear layout as the default rather than the densest one.