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What is ALR / Cortical ERA?
Auditory Late Responses (ALRs) are longer latency components that for most part are generated in higher regions of the auditory CNS, including the auditory cortex. Cortical Evoked Response Audiometry (CERA) refers to the technique of measuring ALRs for the purpose of assessing ones hearing abilities. The responses are typically measured using surface electrodes placed on the scalp of an individual.

Why ALR?
ALR/Cortical ERA is traditionally used to help determine the degree of hearing loss in adult populations. Compared to traditional Auditory Brainstem Responses (ABRs), or behavioral audiometry, ALRs demonstrate audibility at the cortex without the need for the listener to play an active role in the procedure. This is a key advantage in various medico-legal scenarios or in cases where the individual is unable or unwilling to provide accurate behavioral responses to a sound.
Other advantages include the relative robustness of the ALRs to myogenic activity, the high frequency specificity of the tonal stimuli due to their longer duration, and is much closer to the behavioral audiometric pure tone compared to traditional ABR octave wide stimuli, and the ability to calibrate the stimuli according to international standards (BS EN ISO 389 as for pure tone audiometers) and not the standard 389-6 used for Tone burst/Tone pips & Chirps .
The key ALR waveforms observed in CERA are the P1, N1, and P2 responses. The ALR latency typically ranges from 50 – 300 ms and N1-P2 amplitude ranges from 0-20 μV. (see Figure 1, showing P1, N1 and P2 responses to a 2 kHz toneburst stimuli, used to provide a typical threshold recording from the left ear. In this case, hearing sensitivity is shown to be within the normal audiometric range.)

Figure 1 ALR responses Right and Left

How to test
Patient Preparation is very important. Patient arousal and attention state has a significant effect on the amplitudes of the ALR. The ALR waveform changes as a person becomes drowsy or falls asleep. When a patient is asleep the N1 amplitude is smaller and the P2 amplitude is larger. However, when the subject is listening for a change or paying close attention to stimuli the N1 increases in amplitude and in subjects who are not attending to the stimuli, the N1 can become difficult to measure at low intensities (Näätänan and Picton, 1987). The response also habituates quickly, an affect which is more apparent nearer to the threshold of the listener, so it is important to limit the number of stimulus presentations within each ‘run’. Typically, between 15 and 20 presentations might be made per run and a number of successive runs are then merged into a grand average in order to reveal the ALR.
The patient is typically instructed to sit quietly during the procedure, maintaining passive attention for example by reading or watching a close-caption movie with the sound muted. It is not advised to perform ALR and under sedation (Crowley & Colrain, 2004).

What is Residual Noise?

•  It is the averaged background noise.
  
  How does it Work?
• It is calculated online during the ABR recording by measuring the stability of the averaged waveform.
• The residual noise calculation used for Fmp is the same used for the calculation of Residual Noise - typically in a time window of 10ms.
• The greater the stability, the less noise in the tracing.
• When the Residual Noise level reaches the set criteria, e.g. 40nV for adults and 20nV for children, the residual noise bar turns green with a checkmark.
  

What is Fmp?

• It is a statistical online analysis of the ABR recording from beginning to end.
• The Response Confidence is a statistical confidence of a true detection of a response, by default 99% (Fmp 3.1).
  
  How does it Work?
• It works on the principle of comparing response amplitude to residual noise to provide confidence level or detection rate.
• The underlying analysis considers the ABR recording typically in a time window of 10ms.
• The Fmp ratio between the response amplitude and the residual noise is calculated.
• In a response situation, lower noise or larger response amplitude will drive the Fmp up (indicated by the red line and bar).
• In a no-response situation, the response amplitude will not rise above the residual noise, thus the Fmp value and the Response Confidence will remain low.
  
  What is the benefit of calculating Fmp?
• The calculation provides statistical documentation and support of the findings.
• Acts as a Quality Meter for the waveform.

Benefits of Fmp and Residual Noise

• Low residual noise levels mean there is less noise in the tracing.
• Less noise in the tracing improves confidence when eyeballing the presence or absence of a response.
• It is unlikely that continued averaging with noise levels below 40nV for adults and 20nV for children will result in a response becoming visible, so residual noise may be used as stop criteria in no-response situations.

Clinical benefits

• Improves confidence in a presence or absence of the response.
• Can reduce test time.
• Relies on statistical data not solely on the experience of the user in determining the presence or absence of the response.

References 
Don, M. & Elberling, C. (1996). Use of quantitative measures of auditory brain-stem response peak amplitude and residual background noise in the decision to stop averaging. J. Acoust. Soc. Am., 99(1).

Elberling, C. & Don, M. (1984). Quality Estimation of averaged auditory brainstem responses. Scand Audiol., (13) 187-197.

What is AMLR?
Auditory Middle-Latency Responses (AMLR) are related to auditory generators of the subcortical regions Na and Pa components at cortical levels. Na is considered the onset of the AMLR and Pa is considered the most robust component of the AMLR.

Why AMLR?
AMLR has the potential to offer a more complete picture of the status of the auditory system and can be used to help determine the degree of hearing loss. The most common neurological use of the AMLR is for the assessment of the functional integrity of the auditory pathway above the level of the brainstem in cases with suspected lesions and for the assessment of nonorganic hearing loss.

Further, AMLR is used in instances of traumatic brain injury, cortical deafness, multiple sclerosis, and cases of central auditory processing disorders.

Young children and infants may not present AMLR even when their auditory and neurological functions are intact, because of their higher sensitivity to stimulus rate. In general AMLR from children younger than 10 years should be interpreted with caution. It is also important to note that prior to the level of interest the auditory function should be examined and working normally, if not this will affect the AMLR results.

The stimuli used for AMLR is similar to the traditional ABR octave wide stimuli.

How to test
Patient Preparation is very important. The patient is instructed to relax and informed about the test procedure prior to testing. AMLR’s are most reliable when the patient is awake and quiet.

During sedation as with natural sleep the ALMR response is not affected.

Electrode Placement:
It is possible to obtain AMLR from with a standard ABR electrode montage. Due to the latency of the AMLR measurement, it is important to pay attention to the PAM muscle artifact, so it is not misinterpreted as an AMLR. To minimize the influence of the PAM muscle, ensure that the patient is calm and relaxed and place the electrodes on the earlobe rather than on the mastoid.

Setting up the Eclipse
The Eclipse comes with a pre-programmed protocol for AMLR testing (license) and is ready to use immediately. Protocols can be created or modified easily to fit your clinic needs. Consult your Eclipse Additional Information to learn how to create or modify a protocol.

Protocol settings:

• An AMLR should be measured using traditional ABR stimuli, such as Toneburst 250Hz – 4kHz, stimuli from the CE-Chirp® LS family or custom wave files at an moderate intensity level.
• For neuro diagnosis a moderate stimuli intensity below 70dB nHL is appropriate.
• For threshold estimation, present stimuli levels as done with traditional ABR threshold testing.
• A slower rate is indicated for younger children or for patients with cortical pathology. Normal rate for adults is below 7.1 stimuli per second.
• Stimuli rates as low as 1 per second or 0.5 per second are required to consistently record the Pb component.

Interpretation of the AMLR result
The AMLR latency ranges from 15-80 ms and amplitude sizes ranges from 0-2uV.

An AMLR threshold recording here using Tone Burst 1kHz for threshold evaluation.

Cochlear implants
The longer latencies of the AMLR separate them from the cochlear implant stimuli artifacts seen under the traditional eABR. Therefore AMLR’s may be used to assess the efficacy of cochlear implants in activating the auditory pathway.

Reporting
Choose the Report Icon .
When complete, choose Save and Exit.

References 
Atcherson, S.R. & Kennett, S.W. (2013), Applications of middle and late latency responses, ENT & Audiology news (20)4.

Roeser, R.J., Valente, M., Hosford-Dunn, H. (2007). Audiology Diagnosis, Theime 2^nded

Several things can influence the results obtained during ABR / ABRIS / ASSR testing. In this guide, some hints for improved recordings will be described. All the suggestions listed below can be applied to the ABR, ABRIS and ASSR module.

Skin Preparation and Electrodes

1. Preparing the skin
Always use an abrasive preparation gel (e.g., NuPrep) to ensure that the top layer of skin (epidermis) is cleaned and oil is removed. The skin may become a little red after an appropriate preparation, and you should aim to get impedances below 3kOhm.

Note Be careful not to damage the skin.

Neonates Some clinicians use only alcohol wipes/pads to remove vernix prior to ABR recording on neonates (age 0-3 months). A disinfectant agent such as alcohol can also be used for preparing the skin of neonates.

Preparation instructions: Remove any oil/lotion/vernix from the contact point on patient’s head. Wipe all preparation gel off with an alcohol wipe/pad or a soft dry non-stick cloth (e.g., gauze).

• For sensitive/allergic skin, it may be best to use only a soft dry cloth since alcohol can dry out the skin. If the patient suffers from any known allergies e.g., perfume, pay extra attention to the use of disinfectant agents.
• Since alcohol may take time to dry, impedances may be slightly higher when using alcohol. Make sure that the alcohol is completely dry before applying conductive gel and electrodes.
• Some clinicians prefer not to use alcohol pads/disinfectant agents, but remove the preparation gel with a dry non-stick cloth (e.g., gauze).
• If the patient’s skin is dry or you are obtaining high impedances, apply a small amount of conductive gel/paste to the skin prior to connecting the surface electrode. This will help to rehydrate the skin and to lower impedances.

2. Mounting the electrodes
Always prepare the skin prior to mounting either disposable or reusable electrodes.
Some disposable electrodes are pre-gelled (e.g., PEG15), and no further gel is required.

Note When mounting pre-gelled disposable electrodes (e.g., PEG15), do not press in the middle of the electrode as gel to be dispersed to the adhesive outer edge causing the electrode to loosen from the skin, causing very high impedances during testing.

• Mount the disposable electrode by pressing down around the adhesive outer edge.
• If you gently pull on the electrode a few seconds after application, the electrode should remain tightly adhered to the skin. This should ensure very low impedances (≤ 1kOhm).
• Recommended impedance are below or equal to 3kOhms and balanced (e.g., all electrodes should have similar impedance values within 2kOhms).

Reusable electrodes are expected to have higher impedance than the disposable electrodes.

• For reusable electrodes, it should be possible to achieve impedances in the range of 1-5kOhms.
• Always apply a conductive electrode gel/paste (e.g., 10-20 paste) to all reusable electrodes before mounting.
• Use medical tape (e.g., micropore) to hold the reusable electrode cups in place on the skin.

Lead is sometimes soldered onto the reusable cup electrodes by a hospital to improve their conductivity; producing lower impedances. Reusable electrodes containing lead are not supplied, because of the hazardous nature of lead

Reducing Noise

1. Test Room Parameters
The test room and location of the room can greatly effect ABR recordings.

The following test room parameters should be aimed for:

• A magnetically shielded booth/room (if possible).
• A quiet or soundproofed booth or sound treated room. This ensures the patient can relax without disturbances and the stimulus noise is not masked by background noise.
• Use of a dedicated ground (isolated socket) for the ABR equipment.
• Lights and other equipment not being used should be turned off or unplugged as the patient can work as an antenna and picks up electrical interference from these sources.
• Separate equipment cables to reduce interference (e.g., braid electrode cables, keep electrode cables and transducer cables away from each other).

In some cases, it may be necessary to find another test location if there is too much ambient or electrical noise in the current test room. 
Try moving the test bed within the room. It may be placed unknowingly, adjacent to a wall that has hidden cables and electrical sources.

2. Patient Instructions
The quality of the ABR recordings depends highly on the state of the patient. If the patient is not physically/mentally relaxed, more unstable, noisy recordings will be seen.

Instruct the patient so that:

• They are relaxed and calm.
• Their eyes are closed and that their face and jaw muscles are relaxed.
• They are lying down on a comfortable bed or reclining in a comfortable chair. The duration of an ABR test can be long, so ensure that the patient is as comfortable as possible (use pillows and blankets as needed).
  
  Note An upright sitting position should be avoided due to contracted neck/head muscles.
  
  
• Turn off lights in the test room to avoid electrical interference and to help the patient to relax/go to sleep. Sleeping during recording will provide the lowest noise from the patient, so a sleeping state is preferred if possible.
• Test infants and children during sleep is recommended as they cannot be instructed to or find it difficult to relax for the entire test duration.

It is important to try and use the same test conditions and parameters for each test when comparing results.

3. Set an appropriate rejection level

• A recording cannot be collected if the system rejects the signal.
• Adjust the rejection level to an appropriate level according to the patient and test type. Typically, ABR recordings can be made using a rejection level of 40μV or less when the patient is relaxed and/or sleeping.
• A lower rejection level means a lower amount of noise is recorded during each average. This equates to faster and more accurate recordings.

Changing the rejection level
The rejection level should be increased until the real time EEG signal (top of the screen) is no longer red (indicating rejection). The rejection level used will depend on the patient and the electrical interference in the test room. A black EEG curve indicates that the system is ready to measure.

The higher the rejection level value, the more noise is recorded during each average. Therefore, always use the lowest possible rejection level value, without rejection. The level can be adjusted during a recording, by double clicking on the EEG window (EP15/25 only) and adjusting the input level by dragging the horizontal bars.

If a high rejection setting is needed, check that electrode impedances are sufficiently low and that the patient is relaxed before starting the test. Muscle tension of the face, back or neck due an uncomfortable or incorrect position will disturb the ABR recordings as these muscles are close to the recording site.

4. Use of a Ground
Grounding is crucial for good ABR waves and safe operation. A separate ground dedicated for the ABR equipment should be used. A true ground uses a minimum of three earth rods.

The Eclipse’s power cord contains a ground lead (typically indicated by yellow and green colors), but often the ground at the test site may not be sufficient.

• Always check the wall outlet for a proper ground when establishing an ABR test room. Sometimes the ground lead is found inside the wall outlet, but is not connected to the ground. In these cases where the ground is not connected or even missing, the ABR recordings will be dramatically distorted.
• Electrical interference may appear through the ground lead if the wall outlet used is connected to a shared ground (e.g., other wall outlets with electrical equipment connected share the ground). In this case, a dedicated ground wall outlet for the ABR recording equipment should be established.
• The ABR system cabinet is connected to the ground lead via an internal capacitor. If the ground lead is not connected, the ABR system will pick up electrical noise/interference. This will be seen on the screen as very large harmonic distortion curves completely overlaying/destroying the ABR curves.
• Ground the patient bed if it is made of metal. On the backside of the Eclipse, there is a special ground plug which can be connected to the patient bed.

Check the ground for proper and correct function
Due to High Voltage, only experienced technicians/ properly trained staff must check and change the ground.

To check and verify the ground, various methods can be used.

1. Adedicatedgroundtester.
2. Avoltage/impedancecomparisonfromthewalloutletgroundleadtoatriangleofearthrods.
3. Ground should have max 8ohm, and 0.5V deviation compared to the true ground.
4. A more simple check is to use a Voltage Meter and measure directly from the wall outlet. Please verify these specifications:
   1. The Voltage between Phase (Hot) socket and Zero (Neutral) socket must be a stable 230V for Europe /110V for US (country specific).
   2. Check the Voltage between phase (Hot) socket and Ground socket, 230V for Europe / 110V for US (country specific). The voltage value found in step a, should match the voltage value found in this step (within 5V). If the recorded voltage is much less than the expected voltage (e.g., a voltage reading of 50V for US), the ground is not connected to a true ground, even though you may be able to see the lead in the wall.
   3. Check the Voltage between Zero (Neutral) and Ground. It must be 0V. If the recorded voltage is much less than the expected voltage, the ground is not connected to a true ground, even though you may be able to see the lead in the wall.

Reducing noise in the EP15/25 module

1. Optimizing Settings
Changing the filter settings can reduce excessive environmental electrical interference.

With the EP software, go to File - System setup – Auto Protocols tab.
For 15ms tests (ABR-15), change the high pass filter to “100Hz 12/oct”.

Note Using a filter setting like this may reduce the amplitude in the ABR waveforms. However it may be needed if it is impossible to obtain ABR curves without excessive electrical interference.

Change the stimulus rate so that it isn’t time locked to other electrical interference (e.g., 50/60Hz mains) can reduce noise.

For periodic interferences, use the Minimize Interference option. Small random pauses are inserted between stimulus presentations, minimizing the synchronization with electrical interference. These pauses do not influence latency times or in any other manner, change the behavior of ABRs.

Please refer to the Eclipse Additional Information for more information.

2. Use Insert Headphones
The recording below was carried out using reusable electrodes and headphones. Note the very large spikes before 1ms, especially on the curves at high intensities.

This is an electrical artifact caused by electrical coupling from the headphones to the input circuit when high sound stimuli intensities are used.

To solve this:

• Always use insert earphones.
• If using headphones they must be shielded. A shielded headset will still produce a small artefact using intensities above 90 dB SPL.
• Start the recording window after the artifact.

Prepare the equipment

1. Turn on Titan by pressing the R or L button.
2. Open the OtoAccess or Noah database and enter new patient details.
3. Double click on the Titan Suite icon to launch the software and click on the ABRIS module tab.
4. Select the desired test protocol from the dropdown list.
5. Select the ear for testing.

Test environment 
The ideal test environment is a quiet room where lights and other electronic equipment are turned off.

Prepare the Infant 

Check impedance
At the top of the screen, the impedance is indicated by the green/amber dots on the infant image.

When the dot is amber, this means the impedance is poor (> 40kΩ). In this instance , it may be necessary to clean the skin and/or use some conductive gel and replace the surface electrode.

NB. Testing is possible when impedances are poor. This may, however, affect test time and measurements

Place transducer/s
Place the probe or insert earphones in the infant’s ear/s or place the headset over the infant’s ears.

The probe light will turn green when a good seal is obtained.

Run test 
Click on START in the software, press the spacebar or press the preamplifier button.

Patient noise (EEG)
After starting the test, patient noise or EEG is displayed at the top of the screen (depending on setup).

The dark green bars should not reach the black bar

The EEG indicator should remain green

If the patient noise goes above the black line/EEG indicator turns red, try to calm the infant to reduce movement, crying, sucking etc.
If the infant appears calm and the patient noise/EEG is not ideal, stop the test and increase the rejection level using the arrow buttons.
If the patient noise/EEG signal is still not ideal, refer to the EEG troubleshooting section below.

Results

PASS result

REFER result

PASS –  when the result reaches 100% for the pass criteria, PASS is displayed in green above the measurement.
REFER –  when the result does not reach 100% for the pass criteria within the measurement time, REFER is displayed in amber above the measurement.
INCOMPLETE –  if the test is stopped before a PASS or REFER is generated by the system, INCOMPLETE is displayed above the measurement indicating that the full test was not completed.

EEG Troubleshooting 
When the patient noise/EEG signal is poor and the patient is calm, try the following:

• Turn off (if possible) all electronic devices/sources, such as lights, computers, mobile phones etc.
• Ensure that the Titan battery is not being charged (via the cradle) during testing.
• If impedances were initially poor, clean the skin/use conductive gel and replace the electrodes.

What is Bayesian Weighting?
Bayesian Weighting is a tool designed to assist the clinician when testing in less than optimal conditions. While giving sweeps with less noise a higher “score”, sweeps with more noise are not given the same level of importance in the overall recording. Each sweep is analysed and not simply accepted ot rejected as with traditional averaging but is given a unique signficance based on its level of noise. Baysian Weighting can be used in all typical ABR testing situations and will be effective when EEG levels vary during recording.

When and why to use Bayesian Weighting?
When

• As Bayesian Weighting does not change the response of a waveform, you may use Bayesian in all typical ABR recording situations

Why

• The optimum use of all data reduces test time.
• More stable recordings, as residual noise will never suddenly rise (and a good waveform deteriorate) during recording, even if patient starts to be uneasy.
• The difficulty of selecting the optimum rejection rate is reduced, as a softer rejection rate can be used without the noisier sweeps contaminating the more quite sweeps.

When is Bayesian Weighting less Relevant?
If the patient does not have a fluctuating EEG during the session: Bayesian weights noisy sweeps less and quite sweeps more. In a situation where all sweeps are the same, they will be weighted equally. This is identical to normal averaging. Hence, there is no difference between recordings with and without Bayesian.

If a very tough rejection level is set: In this case, all noisy sweeps are simply rejected for both Bayesian and non-Bayesian recordings. Only the most quite sweeps are accepted, and you have a situation almost similar to above. With softer rejection criteria (e.g. 80μV) you would benefit from the contributions of the more noisy sweeps in your averaging thus arriving at your desired residual noise level in less time.

If you run long enough to get a low residual noise level
If two waveforms each have 40nV residual noise, then they will look equally clean. It does not matter whether you have used 1.000 or 10.000 sweeps to get there or what type of weighting is used. Weighting will simply get you there faster if the patient has fluctuating EEG during the session. It should be noted that two waveforms each with 40nV residual noise may exhibit minor variations in wave morphology due to e.g. the frequency distribution of the residual noise. This may differ with different test situations, and quite patients tend to provide residual noise with more high frequency content and less low frequency content, which may look nicer to the eye. So keeping the patient as relaxed as possible is still recommended.

References
Elberling, C. & Wahlgreen (1985). Estimation of auditory brainstem response, ABR, by means of Bayesian interference. Scand. Audiol (14) 89-96.

What is Neuro Latency Protocol?
The default Neuro Latency protocol is designed to evaluate the integrity of the neurologic system. The changes in response latency between the right and left ear at slow stimulation rates are recorded and compared. The Eclipse offers both markers and performs the calculation between the inter-peak Wave I, III & V latencies (between Left and Right). The purpose of the test is to look for retro cochlear pathology.

How does it Work?
The test is run like a traditional ABR. The clinician tests at a high intensity (ex. 80 or even 90dBHL) with a slow rate (i.e. 11.1Hz). The Wave I, III & V for the tracings is marked with the appropriate markers. After they are marked, interaural wave I, III & V and the intra-aural change is calculated.

An interaural difference of maximum 0.3ms is often used as critical value in clinical practice (Stürzebecher et al., 1985; Olsen et al., 1997). 

References
Stürzebecher, E. Kevanishvili Z., Webrs, M., Meyer, E., Schmidt, D. (1985). Interpeak intervals of auditory brainstem response, interaural differences in normal-hearing subjects and patients with sensorineural hearing loss. Scand Audiol (14)2 83.

Olsen, W.O., Pratt, T.L., Bauch C.D. (1997). Consistency in latency measurements and interpretation of ABR tracings.American Journal of Audiology, (6)57-62.

Prepare the equipment 

1. Turn on Titan by pressing the R or L button.
2. Choose the test protocol from the list using the up & down arrow keys and then press Select.
3. To use a different protocol than the one selected, press Protocol and then repeat step 2 above.
4. Select the transducer (only if more than one connected to the preamplifier).
5. Select the ear for testing.

Test environment 
The ideal test environment is a quiet room where lights and other electronic equipment are turned off.

Prepare the Infant 

Check impedance
Impedances are indicated by the green/amber dots on the infant as well as in numerical format on screen.

When the dot is amber, this means the impedance is poor (> 40kΩ). In this instance, it may be necessary to clean the skin and replace the surface electrode.

NB. Testing is possible when impedances are poor. This may, however, affect test time and measurements.

Place transducer/s
Place the probe or insert earphones in the infant’s ear/s or place the headset over the infant’s ears.

The probe light will turn green when a good seal is obtained.

Run test
Press the START button on the Titan or press the preamplifier button.

Patient noise (EEG)
After starting the test, the patient noise/EEG bar is displayed at the top of the screen.

If the patient noise goes above the black line, try to calm the infant to reduce movement, crying, sucking etc.

If the infant appears calm and the patient noise is not ideal, stop the test and select a protocol with a higher rejection level (if available).

If the patient noise/EEG signal is still not ideal, refer to the EEG troubleshooting section below.

Results

PASS result

REFER result

PASS – when the result reaches 100% for the pass criteria, PASS is displayed in green above the measurement.
REFER – when the result does not reach 100% for the pass criteria within the measurement time, REFER is displayed in amber above the measurement.
INCOMPLETE – if the test is stopped before a PASS or REFER is generated by the system, INCOMPLETE is displayed above the measurement indicating that the full test was not completed.

EEG troubleshooting 
When the patient noise/EEG signal is poor and the patient is calm, try the following:

• Turn off (if possible) all electronic devices/sources, such as lights, computers, mobile phones etc.
• Ensure that the Titan battery is not being charged (via the cradle) during testing.
• If impedances were initially poor, clean the skin/use conductive gel and replace the electrodes.

1. Prepare the infant’s skin before placing electrodes (if required). Clean oily skin using alcohol wipes/skin preparation gel. Use a small amount of conductive electrode gel to improve impedances.
   

   Forehead	Right mastoid	Left mastoid

2. Place the surface electrodes (on the prepared skin).
   

   Forehead	Right mastoid	Left mastoid

3. Attach the electrode cables to the surface electrodes.
   
   
   NB: Ensure the electrode cables are correctly attached to the preamplifier.