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Running the Experiment

After you feel comfortable with the system, you can begin to collect data. We recommend reading through the entire guide before using a live subject.

Electrode Placement

TDT can provide an electrode kit with suitable needle electrodes for ABR testing. One electrode is required for each channel, plus one for Ground and one for Reference. The electrodes should be inserted just under the skin and connected to the appropriate input on the Medusa4Z. Correct placement is essential for acquiring good data.

For this single channel configuration, position electrodes as follows:

Channel 1 - Vertex (base of skull)

Reference - Ipsilateral ear (same side as stimulus delivery)

Ground - Contralateral ear (or hind hip or base of tail)

Check Impedance

The Medusa4Z includes built-in impedance checking. Impedance is a measure of resistance from electrode to ground. High impedance can result in lower response signals or no signal at all. After inserting electrodes, run an impedance check for the subject to verify a good insertion and working electrodes.

To check the impedance level:

  1. Press the button next to the channel indicator (e.g. CHAN-1). The impedance between channel and ground will be shown on the screen.

  2. Press the REF impedance button to test the reference electrode impedance to ground.


The impedance frequency was set in the Medusa4Z Setup options, see The Test Equipment.


The impedance of the input channel (CHAN-1) and the reference (REF) must match, ideally at 1K - 3K for each.


If the impedance is very high (>20 kOhm), it is likely that one of the electrodes is not properly connected. If both the output channel and reference show high impedance, it is likely the ground electrode is not making good contact.

Check Sedation

If the subject is not fully sedated throughout testing, subject responses, such as muscle movements or distress, can interfere with ABR recordings. To avoid this, always check the subject before beginning your experiment and monitor the subject for signs of wakefulness during testing.

Toe Pinch For small lab subjects, a toe pinch is the most effective method to check for full sedation. Extend the rear leg and use your fingernails to firmly pinch toe or foot webbing. There should be no recoil. If there is any response, such as leg movement, the subject is not adequately anesthetized.


Startling the subject with a loud clap or loud noise is not a rigorous enough way to check sedation. A subject may not respond to clapping even when only partially sedated. Also, a test subject with hearing loss may not respond regardless of sedation level.

Begin Experiment

BioSigRZ is the user interface for ABR testing. It has three modes of operation: Idle, Running, and Averaging. Before you begin, it is in Idle mode. You can begin stimulus presentation or run the experiment using buttons on the left side of the BioSigRZ software main window. The options change depending on the mode. See below for more information about the operating modes.

To begin stimulus presentation:

  1. Click the Start button. You may be prompted to specify a response record file where data will be stored.

  2. If prompted, type a name in the File Name field and click Open. You are now in Running Mode.

The stimulus signal is presented using the first SigGen Index (SGI). No data is collected.

To begin data averaging:

  1. Click the Begin button.

  2. A Subject Information window opens. Information entered here is stored with the data in the response record file.

  3. Enter any subject information and click OK. You are now in Averaging Mode.

The first stimulus is repeatedly presented and the ABR response data is acquired and included in the running average until the desired number of responses has been collected at that level. As averages are completed, the averaged ABR records will be appended to the History plot and the .arf data record file.

Modifying the Stimulus Schedule

BioSigRZ provides several functions that allow you to override the current stimulus schedule and control data averaging in real-time.

Advance Current SGI is halted and running average is saved. Stimulus presentation advances to the next SGI in the stimulus schedule.

Repeat Current SGI is halted and running average is saved. Stimulus is presented again at the current SGI.

Skip Current SGI is halted and running average is discarded. Stimulus presentation is advanced to the next SGI in the stimulus schedule.

Redo Current SGI is halted and running average is discarded. Stimulus is presented again at the current SGI.

Multi-Purpose Plot

During averaging, the Multi-Purpose Plot may be used to display a variety of signals. The plot is controlled through the use of the Plot Control Toolbar, available from the right-click menu. You can also control scaling using the toolbar. Consider using the options described below.

Running Avg This view displays the current running average.

EEG Plot This view lets you see the last incoming EEG waveform.

The signal displayed is not averaged, so sudden/ temporary anomalies will be visible. If the waveform jumps, for example, it may indicate that the subject is awake. Other anomalies, such as heartbeat artifact, can be seen here as well.

Zooming Click and drag from left to right over the area of interest to zoom in and change the scale of the time axis. Click and drag from right to left to zoom out.


In BioSgRZ, control of termination of the signal presentation and acquisition is called boundary control. When a boundary is reached and the SGI stops incrementing, signal generation will continue for the current SGI, but no more response data will be collected.


You can manually terminate data averaging at any time.

To manually terminate data acquisition:

  • Click the End button.

This returns the system to Running mode. You can halt stimulus presentation and return the system to Idle mode or run the experiment again.

To halt stimulus presentation:

  • Click Stop.

To run the experiment again:

  1. Click Begin.

  2. Enter the new subject information.