Lecture program Psychoacoustics and instrumentation

1. Instrumentation
Used In The
Measurement of
Acoustic Signals
And Auditory
Function
Dr. Ghulam Saqulain
M.B.B.S., D.L.O., F.C.P.S
Head of Department of ENT
Capital Hospital, Islamabad

2. Electrocochleography
The term electrocochleography refers to the measurement of
electrical events generated either within the cochlea or by
primary afferent neurons.
Thus it includes measures of summating (SP) and cochlear
microphonic (CM) potentials from the cochlea, and whole nerve
or compound action potentials (AP) from the auditory nerve

3. Ways to Measure:
Trans tympanic Electrode:
In the most invasive recording technique, a needle electrode is
passed through the tympanic membrane and placed against the
promontory.
This electrode placement results in very robust and easily
measured responses; however, it is not without risk,
It requires that the tympanic membrane be anesthetized and a
Physician is required to place the needle.

4. Ear Canal Needle Electrode:
In one variation on this procedure, needle electrodes are
placed in the external ear canal wall.
This placement results in smaller responses than those
observed with transtympanic electrodes, but has
virtually identical restrictions, such as
requiring local anesthetic

5. Ear Canal Electrode (tip-trode):
Recently, an ear canal electrode has become available that is
easy to place,
comfortable,
results in fairly low impedances,
 and increases the amplitude of the AP in relation to the
values typically observed with earlobe and/or mastoid
electrodes

6. These "tip-trode" electrodes are used in conjunction
with insert earphones (Etymotic ER-3A).
Unfortunately, the responses measured with these
earphones typically do not result in clear SP or CM, and
may not increase AP amplitude sufficiently to make it
measurable in cases of moderate or severe high-
frequency hearing loss.

7. Applications of Electrocochleographic
Measurements.
There are two applications of electrocochleographic
measurements.
1. In one application, measurements of SP and AP amplitudes are
compared to determine if there is an abnormal ratio between
these two potentials. In normal ears or ears with cochlear
hearing loss unrelated to a pressure imbalance in the cochlea,
the ratio of SP/AP amplitude typically is less than 0.30.
2. In patients in acute symptomatic stages of endolymphatic
hydrops, the amplitude of the SP increases in relation to the
amplitude of AP, thus resulting in abnormally large SP/AP
amplitude ratios.