Low Noise Neural Recording requires a good connection between the subject’s body and the recording system Ground. A good connection will keep the subject’s body, and thus the average brain potential, at the same voltage as the recording system. Without this connection, the subject’s body voltage can “float” away from the recording system Ground, and as a result, all electrode recordings will show high noise levels and very high AC power line noise. In some cases the difference in the subject and system voltage may exceed the input range of the system (saturation or clip) that you will see as “flat line” (zero signal) waveforms. And in most cases it is very difficult to re-establish the ground connection from the Microdrive/EIB and the subject – which may result in subject sacrifice and a significant loss in the hours for subject preparation and training.Therefore, the ground connection between the EIB and the subject’s skull IS the most important connection of all.
There are many techniques used for making this connection. However, the “worst” technique is to wrap a wire around a skull screw and then assume that a good connection will be maintained to the screw and skull. In fact, when dental cement or other non-conductive adhesive is applied over the screw/wire, it will wick in between the wire and skull screw insulating this key connection.
The EIB Ground wire MUST be bonded to the skull screw for good reliability. The best method is to use a stainless steel skull screw and stainless steel wire (both supplied in Neuralynx’s MicroDrive/Tetrode Tool Kit) and to solder the wire to the slotted screw slot.
Stainless steel is difficult to solder because the metal quickly forms an oxide layer if exposed, and it is this oxide layer that does not accept soldering. (This is what makes it “stainless.”) A very “active” liquid solder flux must be used to both 1) remove the oxide layer during soldering and, 2) be continuously present during the soldering process. Two different fluxes are recommended for this soldering process: Kester 2331-ZX water-soluble (for easy cleanup/removal after) and Kester 817, made specifically for stainless steel.
After soldering, the screw-wire assembly should be placed in alcohol to remove the flux, since it can be acidic.
Installing the threaded screw is easily done by grabbing the head of the screw with forceps and then threading the screw into the skull by turning the forceps.
This soldering process ensures a very reliable, long-term ground connection between the EIB and subject – ultimately saving you time and effort while providing you with low noise neural recording!