Stationary auditory evoked potentials

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Stationary acoustic evoked potentials (StAEP) are nerve potentials which are caused by a continuous stimulus in the form of a sound signal. In contrast to the click-evoked potentials , the sound signal consists of several pure tones , which can be modulated in different ways .

The StAEP represent an important group of procedures with which the function of the hearing can be examined and, above all, researched without injurious and destructive interventions .

StAEP is measured by attaching electrodes to the head of a test person and then stimulating the hearing with sound from headphones. The electrical voltages between the electrodes are recorded and evaluated by a measuring computer.

Classification

The StAEP can be classified on the basis of the sound stimulus. Tests were carried out with:

• Pure tone
• Combinations of pure tones
• Amplitude modulation of pure tones
• Frequency modulation of pure tones
• Amplitude modulation of narrowband noise

Additional variations result from the placement of the electrodes. The different origins of the potentials in the brain stem and in hearing processing have different effects here.

Amplitude-modulated pure tones

The StAEP with amplitude-modulated pure tones are of particular importance. Here, a sine tone in the audible range (carrier frequency) is amplitude-modulated with a relatively low-frequency sine signal, i.e. simply multiplied. The measured potentials show maxima that are synchronous with the low-frequency modulation frequency. The occurrence of this modulation frequency in the potentials is now assessed as an indication of a "functioning" of the hearing organs at the carrier frequency. This is based on the well-established assumption that the neurons of the auditory pathway cannot fire as quickly as the carrier frequency oscillates (e.g. 1500 Hz) but significantly faster than the modulation frequency (e.g. 80 Hz). On the basilar membrane, the actual sensor of the ear, however, only the carrier frequency and the side bands - in our example 1420 Hz and 1580 Hz - are stimulated, i.e. a small part of the spectrum.

If a maximum that occurs regularly with a frequency of 80 Hz is now visible in the measured potential, then the stimulus must have been processed in the ear, which suggests that hearing is at least partially intact.

Research goals

In particular, the measurement methods with amplitude-modulated signals should be refined to such an extent that it is possible to determine the hearing threshold at different frequencies.