Site Pitch

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. An s after the number means the columns are staggered. The name's number is the spacing between neighboring sites; the datasheet and the design space also give the pitch along the shank, which on a staggered layout is smaller because the columns are offset: A1x32-Poly2-5mm-50s-177 has 50 µm between neighbors and a site every 25 µm along the shank. Read both numbers from the record; the ratio between them is not the same on every design. A1x32-Poly3-6mm-50-177, with no s, is three columns at 50 µm with no offset. Among single-shank 32-site polytrodes with 177 µm² sites, the tightest catalog designs are the Poly3 25s (12.5 µm along the shank); the Poly5 designs are tighter again at other site areas (A1x32-Poly5-6mm-35s-100, 17.5 µm along the shank at 98 µm²; A1x32-Poly5-6mm-40s-225, 20 µm at 220 µm²). At 64 sites, V1x64-Poly3-10mm-25s-177 (12.5 µm along the shank, on a Vector support body) and A1x64-Poly2-6mm-23s-160 (23 µm) are the catalog options.

Pitch is chosen against the detection radius. A spike is visible within about a hundred micrometers 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. Sites spaced closer than the radius do not reach more neurons; they record 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 and 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.