Electrode Setup and Connections

The iA4 has four differential inputs plus a ground input. This means that you can configure up to four channels to acquire low impedance signals of different kinds. The electrode leads for the (+) input and (-) input will depend on the signal type and how you want the signal to be referenced.

What is Differential Amplification?

A differential amplifier takes two inputs and amplifies the difference between their signals.

Any operational amplifier takes input from a recording channel (+) and an individual reference (-) and amplifies the difference. The iA4 has four differential amplifiers as channel inputs, so each channel can have its own reference (-) as needed for multi-modal recordings. You can jumper (-) inputs together with a TDT-supplied touch-proof jumper kit if a common reference is required for multiple channels.

Electrodes and Surgical Placement for EEG and EMG

Example EEG and EMG connection to a subject. The screw placements are typical, but not required. Not all wires are shown in the right-hand image and the screws and dental cement are anterior compared to how they would be in a real surgery to show the setup without the ear getting in the way. The colors used are consistent with subsequent figures.

TDT recommends consulting your Vet and several literature sources prior to writing your protocol for the most relevant surgical and electrode placement protocols.

In rodent sleep and epilepsy studies, EEG is normally recorded with a skull screw where each active channel is amplified with respect to a dedicated reference above the cerebral aqueduct. The electrode over the cerebral aqueduct is considered 'quiet' so you can record activity closer to ground truth over your active electrode channel. However, you can also do multiple differential EEG screw pairs and still get a good global representation of the subject's state of vigilance. The ground can be anywhere, but it is usually a skull screw that is placed posterior around lambda. The screws should be placed inside a craniotomy where the screw is in contact with cerebral spinal fluid (CSF) and touching the dura mater. The screw should not penetrate the dura mater.

Screw electrodes can vary in size for rats vs mice. Rats usually use 0-80 thread sized screws or smaller and mice usually have 000-120 screws. Companies like Pinnacle or Protech make great spot welded screws.

You can make your own screws by purchasing them from places like McMaster-Carr (note the screw size) and soldering stainless steel wires to the screw heads. You must use flux for the soldering, though, because stainless steel has an insulating layer of oxide the prevents good electrical contact otherwise.

Do not wrap a loose wire around a screw neck and seal it with dental cement. The wire will loosen and increase the impedance of your electrodes, which will lead to more common mode noise and motion artifacts.

EMG is normally a true differential signal acquired from adjacent leads in the neck (trapezius) muscle. This can be done with silver wire, hook electrodes, or other means.

Wiring to the iA4

iA4 module with colored wires representing connections to the touch proof inputs.

The above image shows wiring of electrode leads to the iA4 for our 2 EEG, 1 EMG, 1 GND example. The EEG channels will be amplified using the black dedicated reference electrode on the animal. The EMG channels will be amplified in a differential fashion against one another. The green ground is common to the entire circuit but is still a screw on the subject's head. All of the inputs on the iA4 are male 1.5 mm DIN ('touch-proof') connectors which mate to female touch-proof connectors. TDT supplies a jumper kit for shorting (-) positions together, like the green REF for CHAN-1 and CHAN-2 EEG inputs in this example.

Chronic Recording with a Commutator

Mouse connected to a 6-lead commutator positioned on an abstracted cage lid. The leads then connect to the iA4 via female touch proof connectors.

The above image is an example mouse connected to a six-wire commutator with leads on the stationary side (above the cage lid) then connecting to the appropriate spots on the iA4. The electrode wires are braided together to reduce susceptibility to electrical noise.

Protech has bare wire six-lead and twelve-lead commutators if you want to solder yourself or if you have a specific connector on the subject's head. The twelve-lead setup would be good for full differential or more than 6 leads in total. Other vendors like Moflon and Moog also sell bare wire commutators.

For a 'complete system' setup you can get double-brush commutators with 363 connectors, wherein the animal side would use a 363 pedestal plus electrode sockets with a cable mate and the stationary iA4 side would connect with a 363 cable to touch-proof connectors. You can also find complete ecosystems through vendors like Pinnacle with convenient Mil-Max connector electrode interface boards.

TDT is happy to help with wiring of commutators to EIBs or other connectors at a cost.