Site Pitch

Site pitch is the centre-to-centre distance between neighbouring electrode sites on a shank, quoted in microns. It is the design parameter that decides whether adjacent sites tend to record from different neurons or the same neuron from different positions. On a linear layout it is the single spacing down the shank; on a tetrode or polytrode layout it is the inter-site spacing inside the cluster, which is deliberately much smaller. In a probe name, it is the site-spacing field — the 100 in A1x32-5mm-100-177.

Pitch is chosen against the detection radius. A spike is visible within about a hundred micrometres of a site, so sites pitched at 100 µm or more each sample their own neurons and recordings along the shank read as a depth profile — the linear layout's purpose. Sites pitched at 20 to 25 µm sit well inside the same 'effective' recording radius, so a neuron near the shank appears on several of them at different amplitudes, and that spatial pattern is a major determinant of what a spike sorter separates units on. This is the primary motivation for tetrode and polytrode layouts. Between the two, 50 µm gives some overlap and finer laminar resolution, which is why it is the common pitch for cortical-layer work in rodents. Denser sites (i.e., more closely spaced, or smaller site pitch) with spacing less than the radius does not reach more neurons. Rather it records the same set of neurons with improved discriminability.

Pitch, site area and site count set the recording span together — 32 sites at 100 µm cover 3.1 mm, at 50 µm 1.55 mm. Shank spacing sets the site pitch across shanks. Shank spacing ranges from 125 to 500 µm on catalog multi-shank designs. 200 µm spacing is the most common, designed so each shank samples its own cylinder (column) of brain. The site layouts associated with these pitches are described in Probe Geometry.