Tetrodes are a specific type of electrode, made up of four wires that are twisted together and heated to seal. This is the part of the acquisition system that actually interacts with the brain, passing tiny changes in the strength of the electric field on to the headstage. Electrode design focuses on reducing the amount of signal lost before it reaches the headstage, and on increase our chances of separating out single unit (single cell) activity. 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. This allows us to triangulate and get a better separation of individual cells, increasing the number of 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.