Probe Geometry and Site Layouts

From a high-level neural interfacing perspective, single-shank probes provide a 1D neural interface. Single-shank probes are most comparable with micro-wire electrodes, with the important attribute of multi-site sampling along the electrode insertion trajectory. Multi-shank probes provide a 2D neural interface. Matrix arrays, which consist of stacked 2D probes, provide a 3D neural interface.

Site Layouts

Over the years, NeuroNexus developed several types of canonical site layouts that help to tune a probe for particular use cases and/or target regions.

Linear Layout

Sites distributed evenly in one column along a shank. This design evenly samples along the electrode tract. This allows, for example, one placement to simultaneously sample (record) across multiple layers rather than at just the tip position (for a microwire). This can augment or replace the need for passage-type experiments, where the microelectrode is advanced until cells are found. The inter-site distance (i.e., site pitch) determines the degree of overlap in the sites' respective recording volumes.

Edge Layout

A linear placement of sites, but moved to the edge of the substrate. Edge sites can capture more of the circumferential recording volume to improve unit isolation and yield.

The Buzsaki layout is a variant of the Edge layout with staggered closely-spaced sites near the edges of the tip segment of a shank. For the site count, this trades site span for unit isolation in the tip region, which has made the Buzsaki layout the de facto standard for densely packed structures, such as hippocampus.

Tetrode Layout

A closely-spaced cluster of four sites. The tetrode layout is designed for unit isolation. Because the site cluster is small relative to the reach of one neuron's field, a neuron in the composite recording volume tends to be recorded on all four sites with different amplitudes and shapes, leading to a spatio-temporal waveform that is beneficial for spike sorting. A tetrode probe design may have several site clusters spaced along the shank, each resolving units at its own depth with gaps between them.

Polytrode Layout

Poly2 is two columns of sites; Poly3 is three. The Poly2/3 layout mixes the benefits of closely spaced sites with uniform sampling along the shank: sites are close enough that one neuron tends to be 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. The polytrode layout is functionally similar to active CMOS probes, such as the SiNAPS and Neuropixels probes.