Probe Geometry and Site Layouts

Linear Layout

Sites in one evenly spaced column down the shank. The laminar design covers a longer stretch of tissue than a single tip site, so one placement samples a whole trajectory rather than one depth. It either supports or removes the need for passage-type experiments, where the probe is advanced until cells are found. It fits the widest range of applications, and is the default a design takes unless another layout is specified.

The trade is isolation. Sites spaced far enough apart to cover a track are usually too far apart for one neuron to reach several at once — and that shared view is what a spike sorter separates units on. Choose linear when the question is how activity varies with depth; choose a clustered layout when the question is which neuron.

In a model number the site layout is the field after the shank and site counts — A4x8-tet-5mm-100-200-177. A linear design leaves it empty.

Edge Layout

A linear layout, moved to the boundary of the substrate. Site spacing and the reasoning behind it are unchanged — sites remain evenly spaced along the shank. What differs is placement across the shank's width: the column runs along the edge instead of the centre line.

What it buys is an incremental gain in spike amplitude. Moving the sites to the edge of the substrate puts them at the boundary of the shank rather than on its face. The gain is incremental rather than transformative — it does not change what the layout is for, and the spacing argument is still the linear one — but on a design where unit amplitude is marginal it is the reason to choose the edge variant over Linear.

The Buzsaki layout is a variant of Edge. It staggers sites near the edges of the tip segment of a shank — closely spaced, and concentrated where the shank sits rather than spread along the trajectory. That trades reach for unit isolation, which is the configuration that made the layout standard for densely packed structures.

Tetrode Layout

A cluster of four sites. Because the group is small relative to the reach of one neuron's field, a unit near the shank appears on all four at different amplitudes. That ratio is close to unique to the neuron's position, which is what gives the layout its high-quality cell discrimination.

A design may carry several groups spaced along the shank, each resolving units at its own depth with gaps between them. The Q-Trode line is the dedicated single-group product — one four-site cluster on one shank.

In a model number the layout field reads tet.

Polytrode Layout

Poly2 is two columns of sites; Poly3 is three. The layout mixes the benefits of the other two: sites are close enough that one neuron is represented on several at once, as in a tetrode, while the columns still run the length of the shank and sample a larger space than a discrete cluster.

Read the number as the column count. The catalog documents two variations; the corpus carries four, and all four are orderable:

Poly2 and Poly3 are the common cases and are what the catalog illustrates. Poly4 and Poly5 are narrow but real — five catalog designs between them, on the densest shanks in the A-Series. More columns across a fixed shank width means more sites seeing the same neuron, which is what a sorter separates on; it also means more channels spent per millimetre of trajectory.

In a model number the layout field reads poly2, poly3 and so on.

Multi-Shank Layout

Two-dimensional coverage. Shank spacing and site spacing are both specified, so the array covers an area of tissue instead of a single track — a more detailed picture of a larger space than one shank can give.

The layouts compose: a multi-shank design may carry tetrode or polytrode groups on each shank, putting unit isolation and 2D coverage in the same probe.

In a model number the leading field is shanks by groups per shank — the 4x8 in A4x8-tet-5mm-100-200-177. What a "group" is depends on the layout token that follows, so the figure does not resolve to a site count on its own: a2x2_tet carries 4 sites and a2x2_tet_3mm_150_150_121 carries 16, on the same leading field. Read the site count from the specification, never from the name — across the A-Series tetrode designs, "sites per shank" is the correct reading on 16 records and the wrong one on 44.

Site Density

Site density describes how closely spaced recording sites are along a probe, usually expressed as sites per millimeter or micrometers between adjacent sites. This determines the spatial resolution of your recordings.

Why it matters: Higher density provides finer spatial sampling, allowing better discrimination of signals from different neurons or tissue layers. However, very high density may record redundant information from the same neurons on multiple sites.

Density classifications:

• Low density (50-100 µm spacing): Distinct sampling, minimal redundancy
• Medium density (25-50 µm spacing): Good spatial resolution, some overlap
• High density (10-25 µm spacing): Very fine sampling, significant overlap

Single neuron considerations: A single neuron's electrical field typically extends 50-100 µm, so sites closer than this will detect the same cell. This redundancy can improve spike sorting accuracy but increases data processing requirements.

Depth profiling: For studies mapping activity across cortical layers, higher density provides better laminar resolution. For counting unique neurons, spacing should match or exceed the detection radius.

Recording Span

Recording span (also called shank length or site span) is the total vertical distance covered by recording sites along a probe, measured in micrometers or millimeters. This determines how much tissue depth you can sample simultaneously.

Why it matters: Recording span defines your spatial coverage across tissue layers or brain regions. Larger spans let you record from multiple cortical layers or across different anatomical structures in a single penetration.

Common spans:

• Small (1-3 mm): Single cortical layer, focused recordings
• Medium (3-6 mm): Multi-layer cortical recordings, hippocampal studies
• Large (6-10 mm): Deep brain structures, cross-region connectivity

Selection guidance: Choose span based on your target brain region(s) and whether you need simultaneous recording across multiple layers or areas. Longer spans are more rigid and may cause more tissue damage during insertion.

Maximum Depth

Maximum depth refers to how deep into the brain a probe can be implanted, determined by the probe's total length. This is distinct from recording span - a probe might extend 10mm deep but only have recording sites across the final 3mm.

Why it matters: Maximum depth determines which brain structures you can access. Deep brain recordings (hippocampus, thalamus, basal ganglia) require longer probes than cortical surface recordings.

Depth considerations:

• Shallow (0-2 mm): Cortical surface, superficial layers
• Medium (2-5 mm): Full cortical depth, rodent hippocampus
• Deep (5-10 mm): Deep brain structures, primate recordings
• Ultra-deep (10+ mm): Large animal deep brain structures

Practical factors: Longer probes are more rigid (to prevent buckling) and require more careful surgical technique. Consider the mechanical strength needed to reach your target without the probe bending or breaking.

Substrate Thickness

Substrate thickness is the width of the silicon or wire shank that forms the probe body, typically measured in micrometers. This physical dimension affects both the mechanical properties of the probe and the amount of tissue damage during insertion.

Trade-offs:

• Thin substrates (15-50 µm): Minimal tissue damage, more flexible, may bend during insertion
• Medium substrates (50-100 µm): Balance between rigidity and damage, most common
• Thick substrates (100-200 µm): Maximum rigidity for deep insertions, more tissue disruption

Practical implications: Thinner probes cause less tissue damage and gliosis but may not reach deep structures without buckling. Thicker probes can penetrate deeper but disrupt more tissue and may cause more scarring.

Selection guidance: For cortical recordings, thinner is often better. For deep brain structures or chronic implants requiring stability, thicker substrates provide necessary mechanical strength.