The New Generation of ASSR

Evoked potentials are now a routine part of the audiological world. They are objective and reliable and enable us to estimate the audiogram where it would be impossible by any other means. Testing is also straightforward. One electrode behind each ear and another on top of the head connect the patient to the test equipment. During the test, the audiologist adjusts the dials and after a while we have an estimated audiogram.

Simple? Well, not quite. The electrical signals from the cochlea and auditory nerve are very small compared to other electrical signals in the head and body and around the room. Electrode placement, patient state and several other factors play a role in obtaining accurate results. But given suitable conditions, the test equipment decides the outcome.

So how do we separate the hearing signals from the electric noise that surrounds them? There are a variety of techniques, each with their own acronym. The two that matter most in this article are ABR and ASSR (also known as SSEP).

In ABR (auditory brainstem response) we stimulate the ear with a toneburst and measure the response plus noise at the electrodes. We repeat this thousands of times and add the results together. The noise, being random, cancels out and the response appears as notches on the electrical waveform. The notches get smaller as we get closer to the hearing threshold.

In ASSR (auditory steady state response) we do something similar, but now the stimuli are very rapid. We know that responses to the tonebursts will occur at the same repetition rate as the stimuli, so we do amplitude and frequency analysis to find them. By using four frequencies in the stimulus, each at a different presentation rate, we can test four frequencies in each ear at the same time. Electronics and mathematics take care of the calculations, but the result is a good estimate of the audiogram.

Recently, Interacoustics® has patented two advances that take ASSR to a new level. We have redesigned the stimulus to adjust for cochlear delay. This means that more hair cells fire at the same time for a given stimulus level, producing greater electrical activity and a better response. This is especially useful near hearing threshold. And when looking for target frequencies in the response, we also examine their harmonics to further improve the distinction between signal and noise.

Put these advances together and you have a system that can approximate the audiogram in about half the time required to do the same thing using traditional ASSR or ABR. At present ASSR is used to perform diagnostic audiometry on babies referred from screening procedures, but it isn’t difficult to see it finding other applications in future.