Noise Levels

Does excessive movement affect the cortical response?

Yes. All muscular activity can create more noise (and therefore more rejections) in the response. Vocalisations can be a significant acoustic noise source (usually when testing young children) and physical movements (voluntary or involuntary) especially neck muscle tension can introduce unwanted electrophysiological noise. The electrical activities of these muscular actions and associated nerve potentials are much closer to the electrodes than the cortical potentials that originate more remotely. These signals are also picked up as noise by the electrodes at higher intensities than cortical potentials.

I am testing infants and young children, and cannot control their movements

HEARLab® ACA will reject epochs that contain large signals originating from muscular activity (moving, talking etc.) This, however, also increases the total assessment time. The number of epochs rejected is displayed on the screen. If a contiguous set of 10 epochs has just been rejected, or the number of rejected epochs exceed the number of accepted epochs, the tester should consider pausing the test and re-establish favourable subject test conditions. The test may then resume. Traffic light indicators show when the residual noise is sufficiently low that the absence of a cortical response can be interpreted reliably. Absence of the cortical response when residual noise is low indicates an inadequate stimulus sensation level (or, in rare cases, abnormal auditory processing).

How can I monitor the subject’s muscular activity through HEARLab®?

By noting changes in the EEG and/or high epoch rejection rate (also the noise indicator) will assist with monitoring the state of the person and therefore the quality of the response. If test conditions are not ideal there is the option of pausing and then resuming once a favourable condition has been restored (or retesting another time all together).

What is the EEG?

EEG is an acronym for electroencephalogram, it shows the ongoing gross brain activity.

What happens if my EEG has gone flat?

Check the electrode cords (which are plugged into the stimulus controller) as they may not be securely plugged in. Check impedance and make sure the electrodes are still attached to the subject. If all else fails, do a reboot of the entire system after having checked all the connections.

What is the residual noise level?

The residual noise level is a calculated value that indicates the quality of the cortical response recording in relation to the noise level of the signal. This indicator provides the clinician with desirable information on the likelihood of a response, if present, being detectable given the number of accepted epochs and electrical activity within the accumulated epochs. A residual noise level value of 3.2 microVolt (represented by a green colour) indicates a good quality recording; a value ≤ 3.6 microVolt (represented by a yellow colour) indicates a mediocre quality recording; and a value > 3.6 microVolt (represented by a red colour) indicates a poor quality recording. Noise in the recorded responses can affect the reliability of the statistical analysis results, thus the higher the residual noise level, the less reliable the resulting statistical analysis.

Does the ambient noise in the room affect the residual noise indicator?

The indicator depends on how the client behaves and moves and not the ambient noise of the room. It measures the activity of the person being tested and indicates the quality of the cortical response in relation to the noise level of the signal. The residual noise indicator tells you how noisy the subject is: if lights turn green quickly, the subject was quiet (also reflected in the low number of rejected epochs).

Can ambient noise affect cortical results?

When performing CTE measurements, a sound treated room is required. When performing ACA measurements, a quiet room may be sufficient. However, when testing at 55 dBSPL with ACA, background noise might become an issue and so a sound treated room is preferable. To obtain a reliable measure of hearing ability in CTE, the ambient acoustic noise levels need to be sufficiently low relative to the levels of acoustic stimuli employed in tests. While electrophysiological test methods are not included in the relevant international standard, ISO 8253-1, the requirements it lays down on maximum permissible ambient sound pressure levels is recommended for CTE when either insert earphones or a bone conductor is employed. HEARLab® ACA includes a tool for automated measurement of ambient noise. See section 5.3.3 of the HEARLab® manual for more information. Preferably the ambient noise level should read less than 35 dB(A). A level of 40 dB(A) is still acceptable however.

Can I check the ambient noise level with HEARLab®?

Yes. You can measure the ambient noise with the Ambient Noise Measurement window. To perform this test, make sure the control microphone is connected to the Stimulus Controller. Position it at the test position (where the patient will be sitting). Open the Tools menu, point at Utilities, and select Ambient Noise Check. See section 5.3.3 of the HEARLab® manual for more detailed information. Preferably the ambient noise level should read less than 35 dB(A). A level of 40 dB(A) is still acceptable however.

Are my results invalid if the traffic lights are still not green after 240 epochs?

If traffic lights are still not green after 240 epochs, it just means the child or person is being noisy, and a retest might be better to test the stimuli that have NOT reached significance. It might for example be a good idea to increase number of epochs to 300 for the retest. If you get more than half of epochs rejected – then it’s a good idea to reschedule another appointment for more reliable results.

Generally speaking it is important to get the noise levels green or at the very least orange, especially when you don’t find responses as cortical responses might be there but obscured by noise. We recommended retesting in the case of red lights once the subject has calmed down (or record more epochs).

On the other hand, if traffic lights are red, but there is a highly significant response present (a low p-value), it is no problem to accept the reading. It means that the noise level is high, but that response is even larger.

What if HEARLab® indicates a response is present but the traffic light is yellow or red?

Generally it doesn’t matter what the noise levels are if you get a significant response (p-value under 0.05). It shows that the CAEP is large enough not to be obscured by the noise. Conversely, when you don’t find anything but noise levels are high, it might be possible the CAEP is there but you can’t see it because it’s hidden in the noise. Yellow lights are borderline, but still acceptable. Green is optimal.

How can I find out what the noise values are once testing is complete, all I can see is the colour of the traffic lights?

To view the residual noise level values for the run:

  • Open the Run Selection window.
  • Right-click on the run in the run list and select View Residual Noise Values – the Residual Noise for Run window will be displayed. See Figure 3.5.7 of the HEARLab® manual. The displayed value is the noise calculated for the last response to the specified stimuli.
Can HEARLab® results be affected by electrical circuits or florescent lighting?

Generally the HEARLab® is quite robust against electrical ‘glitches’ and ‘spikes’, but it is still electrophysiological equipment, so it is indeed sensitive to poor power supply, fluorescent lighting, mobile phones etc. which  can lead to bad test results.

What can I do to reduce electrical interference?

(See Appendix C of HEARLab® manual).

  • Magnetic field related to power supply: Reduce pick-up from transformers and induction motors by minimising areas enclosed by the connector cables. Loosely “braiding” the electrode connector cables together is often practical.
  • Electrostatic field related to power supply: Unshielded fluorescent tubes and lamps and cables carrying power are sources of interference. This can be reduced with distance and shielding
  • Magnetic induction loops: Some facilities are equipped with induction loops to transmit audio signals to hearing aids. They may need to be switched off if the interference is serious.
  • HF interference: Interference from radios, mobile phones, local communication systems and TV broadcasting, if serious, may require shielding. Similarly, medical equipment in proximity, such as short wave diathermy, can be the source of interference. Turn off mobile phones (or put in flight mode).
  • Static and movement: Movement of electrode cables within a static field can induce interference.