otometrics Madsen AccuScreen PRO, User Manual

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GN Otometrics A/S 17
AccuScreen PRO User Manual Test Methods
The statistical method used by the AccuScreen PRO makes it possi-
ble to evaluate the spectrum specifically at the frequency of the ex-
pected DP. Thus, no comparison with adjacent spectral lines is
necessary.
During a specified time the acoustic activity at the expected DP fre-
quency F
DP
=2F
1
-F
2
is analyzed by reducing the spectral component
to a unit vector. Its phase is determined mathematically. By repeat-
ing this procedure, a set of unit vectors with different phases is ob-
tained. These vectors can be added by using the laws of vector
addition in the polar coordinate system.
The evaluation is based upon the laws of statistical distributions: it
can be shown mathematically that the vector sum of
n randomly dis-
tributed unit vectors will not exceed a certain value with a probabil-
ity 99.7%. Spectral components of random noise with phases that are
independent of the primary tones behave like that. A DP with fixed
phase relative to the phases of the primaries will add a constant val-
ue to the vector sum. Therefore, if the vector sum exceeds this limit,
the presence of an emission can be claimed on a 99.7% confidence
level.
This criterion is set up for each of four frequencies applied: 2 kHz,
2.5 kHz, 3.2 kHz and 4 kHz.
An overall PASS result requires 3 passes out of 4 frequencies.
The Diagnostic DP facility performs DP testing at two stimulation
levels, which results in 8 measurements (4 frequencies at 2 levels: 1.5
kHz, 2 kHz, 3 kHz and 4 kHz). The statistical analysis described
above is also used for Diagnostic DP. An overall
PASS result requires
6 passes out of the 8 measurements.
2.2 About Auditory Brainstem Response (ABR)
Sounds are processed by the different parts of the ear and trans-
formed by the auditory sensory cells to a series of action potentials,
which are transmitted to the brain by neural conduction. On their
way to the auditory cortex the action potentials pass a number of re-
gions called nuclei, where the coded acoustic information is filtered,
processed, compared to other information, and distributed to differ-
ent pathways. These nuclei are the origin of „bursts" of synchronous
multiple cell discharges, which cause electro-magnetic „far-field po-
tentials", which can be tested via scalp electrodes. The potentials
picked up by the electrodes are called Auditory Evoked Potentials
(AEP) or auditory responses.
There are different AEP-producing regions between the cochlea and
the primary auditory cortex. However, the responses of the brain-