Site Area

A 177 µm² site is a circle about 15 µm across; 703 µm² is about 30 µm; 1,250 µm² is 40 µm. Those three cover most of the silicon probe catalog. The surface grids extend the range from 25 µm to 1 mm in diameter.

Site area trades spatial selectivity against noise. A smaller site has a higher impedance and a higher thermal noise floor, but it has a corresponding smaller recording region of interest. It is dominated by the nearest few neurons and separates their spikes more sharply. A larger site has lower impedance and less noise, and it averages biopotentials from more tissue. Larger sites record less unit (discriminated neuron) activity, and generally are indicated for field potential recordings, especially when the goal is to capture population-level activity. The catalog's modal 177 µm² sits where a site still isolates single units on an iridium surface without an impedance that fights the amplifier; the 703 µm² sites are chosen where the field potential or stimulation matters more, and the grid sites are large because a surface electrode records a population signal through the pia or dura, not individual cells.

Area also sets what a site can deliver. Charge capacity scales with surface, so a site that must stimulate is either larger or coated: Z-Coat increases the effective surface inside the same footprint, lowering impedance and raising charge capacity without changing the layout. Area and pitch are chosen together, because a site cannot be wider than the spacing that the detection radius argues for; the layouts are under probe geometry.