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LESSON 128

Clicks, tone bursts and chirps

We explain the structure and frequency content of the click, tone burst and chirp stimuli used in the auditory brainstem response (ABR), and when each one is chosen. We also cover details that shape the recording, such as stimulus polarity (rarefaction, condensation, alternating), stimulus rate and insert earphone delay.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn the three stimuli used for the auditory brainstem response, or ABR: the click, the tone burst and the chirp.
We will also see when each is preferred, and how polarity and stimulus rate affect the recording.
The ABR is an onset response produced by thousands of auditory nerve fibers firing at the same time.
To achieve this synchrony, the stimulus has to start very rapidly.
However, there is an unavoidable trade-off between time and frequency.
The shorter a sound is, the wider the frequency region its energy spreads over.
The longer a sound is, the clearer its frequency, but as its onset slows, the ABR becomes weaker.
The three stimulus types sit at different points along this trade-off.
A click is a brief, rectangular electrical pulse sent to the transducer.
The pulse usually lasts 100 microseconds, or 0.1 milliseconds.
The earphone turns this pulse into a very short sound with an abrupt onset.
Because of the abrupt onset, a wide part of the basilar membrane is stimulated almost at the same time.
The result is a strong, repeatable ABR; waves I, III and V can often be clearly identified.
This is why the click is the standard stimulus for neurodiagnostic ABR.
In this example graph, we see that the energy of the click spreads over a broad frequency band.
Because of the earphone's own properties, the energy drops quickly at high frequencies, usually above a few kilohertz.
Even so, the click-evoked ABR mainly reflects hearing in the region of about 2000 to 4000 Hz.
This is because regions at the base of the cochlea respond faster and more synchronously.
So the click is broadband, but the source of the response is not spread evenly across all frequencies.
The most important limit of the click is that it gives no frequency-specific information.
This example audiogram shows a moderate loss in the low frequencies and normal hearing in the high frequencies.
In such an ear, the click ABR threshold may come out normal or near normal.
So a normal click ABR result does not mean that the audiogram is normal at every frequency.
The click suits neurodiagnostic ABR, which assesses brainstem conduction; for threshold estimation, it is not enough on its own.
A tone burst is a brief piece of pure tone lasting a few cycles.
The example on screen is a 2000 Hz tone burst.
In a common design, the sound rises over 2 cycles, stays steady for 1 cycle and falls over 2 cycles; this is called 2-1-2.
This way the sound starts abruptly, but its energy is concentrated around the chosen frequency.
Because duration is defined by the number of cycles, tone bursts are longer at low frequencies and shorter at high frequencies.
This approach aims to give similar frequency specificity at every frequency.
When the number of cycles is fixed, the total duration is easy to calculate.
At 500 Hz, one cycle is 2 milliseconds; a five-cycle tone burst lasts 10 milliseconds.
At 1000 Hz, one cycle is 1 millisecond, and the total duration is 5 milliseconds.
At 2000 and 4000 Hz, the total duration drops to 2.5 and 1.25 milliseconds.
The longer, slower onset of low-frequency tone bursts is one reason the 500 Hz response is broader and later.
The sharper the start and end of a sound, the more its energy spreads to both sides of the main frequency.
A linear ramp makes the sound rise and fall along a straight line; it is a simple and common option.
Smoothly curved gates, such as the Blackman window, clearly reduce the side lobes outside the main frequency.
The curves in the graph are only examples; the goal is to keep the energy around the chosen frequency.
Also remember: at high intensity, excitation spreads along the basilar membrane, so frequency specificity decreases.
That is why the most valuable tone burst ABR information comes from low intensities near threshold.
The click has a problem too: the traveling wave moves from the base of the cochlea toward the apex.
The high-frequency region at the base of the cochlea responds to the stimulus first.
The wave reaches the low-frequency region at the apex a few milliseconds later, and this reduces synchrony.
The chirp stimulus is designed to compensate for this delay; low-frequency components come first, high-frequency ones later.
This way, responses from different frequency regions combine more synchronously in the auditory nerve.
The result is a larger wave V than with a click, especially at low and moderate intensities.
A broadband chirp can be used instead of a click; one commercial example is known as the CE-Chirp.
Because cochlear delay changes with intensity, the best chirp shape also depends on intensity.
A level-specific chirp takes this change into account and adapts its shape at each intensity.
A narrowband chirp focuses on a specific octave band and, like a tone burst, is used for frequency-specific threshold estimation.
These stimuli have become widespread, especially in newborn screening and threshold estimation in infants.
In studies, wave V amplitude with chirps has usually been clearly larger than with clicks at low and moderate intensities.
Because the response separates from noise earlier, threshold searches can be shorter, a valuable time saving in a sleeping baby.
However, latencies measured with chirps differ from click values, depending on the timing of the stimulus.
So chirp recordings should be interpreted with norms obtained with the same stimulus and device.
At high intensities, excitation in the cochlea spreads, so the difference between chirps and clicks becomes smaller.
Stimulus polarity describes whether the sound first lowers or raises the pressure in the ear canal.
With rarefaction polarity, the earphone diaphragm first pulls back and the pressure in the canal drops first.
With condensation polarity, the diaphragm first pushes forward and the pressure rises first.
With alternating polarity, these two forms take turns on successive stimuli.
Because rarefaction stimulates the hair cells slightly earlier, in many people it can make the waves slightly earlier and clearer.
But this difference varies from person to person; in some people the best recording is obtained with condensation polarity.
With alternating polarity, the electrical stimulus artifact has opposite signs in the two polarities, so it largely cancels in the average.
For the same reason, the cochlear microphonic, or CM, which follows the stimulus waveform, also cancels in an alternating recording.
However, in some people, especially with high-frequency hearing loss, wave latencies differ clearly between the two polarities.
In that case, the two responses overlap in the alternating recording, widening and blurring the waves.
When auditory neuropathy is suspected, rarefaction and condensation recordings are obtained separately and overlaid for comparison.
Stimulus rate is the number of stimuli given per second, and it directly sets the length of the test.
As the rate increases, nerve fibers have less time to recover, and the latency of wave V gets slightly longer.
At the same time, the amplitude of the early waves, especially wave I, decreases; the graph shows this trend as an example.
So for neurodiagnostic ABR, about 11 to 21 stimuli per second are usually used, so that the waves are clearly visible.
In threshold searches, the main target is wave V, so the rate is often raised to about 30 to 40 per second.
A non-integer rate such as 21.1 is chosen so that 50 Hz mains interference does not lock to the stimulus and cancels in the average.
A clinical tip: with insert earphones, sound travels through a tube from the transducer to the ear.
This tube adds a delay of about 0.8 to 0.9 milliseconds.
Some devices automatically subtract this delay from the latencies, and some do not.
Make sure your norms were obtained with the same earphone and the same correction setting; otherwise all latencies may be misread.
The intensity of ABR stimuli is usually given in dB nHL, that is, normalized hearing level.
For device calibration, peak-equivalent sound pressure level, or peak-equivalent SPL, is used for brief sounds.
Insert earphones are the most widely used transducer because their interaural attenuation is high and they prevent canal collapse.
ABR can also be recorded with a bone vibrator, but its maximum intensity is limited; we will cover this in detail in a later lesson.
The click produces a strong, clear ABR, but it mainly reflects the 2000 to 4000 Hz region and is not frequency-specific.
The tone burst is a brief piece of tone defined by its number of cycles and is used for frequency-specific threshold estimation.
The chirp compensates for the traveling wave delay in the cochlea and usually produces a larger wave V.
Polarity and stimulus rate affect waveform and latency; they are chosen according to the purpose of the test.
Test yourself: does a normal click ABR threshold rule out a hearing loss in the low frequencies?
The correct answer is B: no, a low-frequency loss may be missed.
The click response mainly reflects high frequencies, so low frequencies are tested separately with tone bursts or narrowband chirps.
In the next lesson, we will see how we use the click ABR to investigate retrocochlear problems.
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Prerequisites

LESSON 127 ABR waves I to V and latencies

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