Tetrodes are a specific type of electrode, made up of four wires that are twisted and heated to seal them together.
This is the part of the acquisition system that actually interacts with the brain. The tips of the tetrode pick up changes in the strength of the electric field at that point, caused by movement of charges in the brain. These tiny signals are sent to headstage before they are amplified and digitized. Only one tetrode is shown below for clarity, but the mouse shuttleDrive can hold 16 tetrodes (64 channels).
Because each tetrode has four non-insulated recording tips, each cell that is picked up by the tetrode is actually recorded by multiple channels, each at a slightly different location. Having the same spike recorded on 4 channels gives us a 4-dimensional ‘description’ of a spike, making it easier to distinguish spikes from different individual cells, and increasing the number of single units we can record from in one go.
Tetrode tips are electroplated, often with gold, which stacks on to the tips and greatly increases their surface area (see insert on right, from S.A. Desai et al. Front. Neuroeng., 06 May 2010). This plating reduces tip resistance and results in less voltage attenuation (and precious signal lost) across the electrode, before it reaches the amplifier in the headstage.
At the other end of the tetrode, the four wires are separated out into different holes (vias) on the EIB (electrode interface board), and held in place using gold pins (and a layer of epoxy once you’ve gold-plated!). The signal from each via is routed to a different pin in the connector, which links to the headstage.
When the assembly of the shuttledrive is complete, you’ve gold-plated and you’re sure you won’t have to replace tetrodes, you can attach the EIB to the shuttleDrive body. Gently twist the EIB when you place it on so that all the tetrode wires are safely inside the drive body. Make sure the EIB is glued on well to the drive body.