Checking mastoids and ground impedance with the handheld meter.

Given that Neuroscan does not provide an explicit impedance value for ground or the reference (in our lab- the left mastoid), we use the PREP-CHECK hand held impedance meter to do this. Use of the handheld meter requires a separate adapter that is provided by ECI (called an electrode board adapter). The adapter connects directly to the cap, and each lead wire is separated so that each cap electrode has its own lead wire, thus enabling us to check the impedance of each cap channel. Each lead wire is labeled accordingly, however for our purposes, we will be using the AFz (the ground), M1 (the online reference in our lab) and M2 (used to re-reference to average mastoid offline) lead wires. For a more thorough understanding of the PREP-CHECK meter, refer to instructions on its use.

The PREP-CHECK is used to test electrode contact impedance as follows:

1. Prepare electrode site using recommended procedures. Prepare ground (AFz) and both left and right mastoid (labeled M1 and M2 respectively). Following preparation of these sites, plug the EEG cap directly into the adaptar.

2. Identify appropriate lead wires on the adapter and place lead wires in the appropriate jacks of the PREP-CHECK. In order to check the mastoid channels to ground, place the ground lead wire (AFz) in the “C” jack. Place the left and right mastoid connectors into the + and - jacks (see figure on the following page). In our lab, we leave these three leads permenantly plugged into the meter to reduce wear and tear on the adapter.

3. After all lead wires are connected to the appropriate jacks, measure impedance. To do so, simply press the ON/Auto-off button.

a. To check the impedance of the difference leads, you must move the switch from C (ground) to the + and – Positions. Record impedance levels of all three channels. Each selected lead is tested against the average of the other two leads.

4. Good contact is indicated by the green “GOOD” light. The digital readout should read less than 5,000 (5K) ohms. Specifically, the meter will indicate a green “good” light when the digital display reads at or below 5.0. Poor contact impedance is indicated by the red “POOR” light indicating that impedance is GREATER that 10,000 (10K) ohms (Digital display reads at or above 10.0).

5. Electrodes indicating poor may have to be replaced or the site prepped again. A defective lead wire will also indicate poor.

a. Lead wires are tested by stretching the lead wire with moderate force. A defective lead wire will cause the red “Poor” light to flash or stay on continuously.

b. Lead wire snaps may be tested by moving them around on the electrode. A bad snap will cause the RED light to flash briefly or stay on continuously.

6. Disconnect the adapter from the EEG cap, and plug the EEG cap directly into the Neuroscan head-box. Continue to test the impedance of the rest of the channels using the Scan 4.3 program.

WARNING: When removing wires, you must be very careful as to not pull the connectors away from the lead wires. It seems that the connector of each lead wire is somewhat fragile and being rough with them, could damage the connector, and thus our ability to test impedance.

On/Auto-Off button

M2 Lead wire of EEG cap adapter

AFz ground lead wire of EEG cap adapter

M1 lead wire of EEG cap adapter

Connector Switch:

C = Ground

+,- = Left/Right mastoid