Sharpening

Why sharpen?

Out of the box, Neuropixels are sharp in the dimension you see when looking at the face of the probe with the sites.

However, they are not sharp in the dimension that you see when looking at the edge, and in this dimension have a square-shaped tip that is the thickness of the probe (~25 µm). The probe tip out of the box is therefore a line of length ~25 µm and width ~1 µm.

The edge dimension can be sharpened using procedures described below, which can result in the probe tip being a point ~1 µm in size.

It is evident from unpublished tests (by this author, Nick Steinmetz, and others), as well as from logic, that a probe sharpened in this way can more easily enter the brain, e.g. by puncturing dura. If you have difficulty inserting the probe into the brain in your preparation, sharpening can be a way to achieve this.

More speculatively, we infer that a sharp probe will pull/tear the brain tissue less after entering. Moreover, tissue compression, as when pushing on the surface of the dura before the probe passes through it, may negatively impact recording quality - so we additionally infer that near-surface recording quality (e.g. in cortex) may be improved by compression-less insertions achieved with sharpening. So far as I know these observations have not been quantified (let me know if I'm wrong).

How to sharpen?

A comparison of methods