14.01.2011
The human ear is not a passive sensory organ: If a healthy ear is stimulated by two pure tones that are a musical minor third apart, it responds by producing the fundamental of the major triad. This inaudible tone which originates from the inner ear is called a distortion product otoacoustic emission, or simply a "DPOAE", and can be both elicited and recorded by a small probe inserted into the ear canal. Conventionally, the two miniature loudspeakers in the ear canal probe are used to elicit DPOAEs. In an innovative study, we investigated a combined stimulation of otoacoustic emissions, with one tone presented as is usual by a miniature loudspeaker and the other applied by a bone vibrator placed on the cranial bone of the test person. This combined stimulation yielded detectable DPOAEs.
The early detection of hearing deficiencies is indispensable to minimize their negative impacts on the development of communication and learning skills of human beings. A commonly applied objective test to assess inner-ear functionality is a test by which otoacoustic emissions are detected. Such a test can be carried out without the cooperation of the patient and is, therefore, suitable for analyzing the hearing ability of newborn babies or small children.
Distortion product otoacoustic emissions (DPOAEs) represent a response of the inner ear to two pure tones having a frequency ratio of a musical minor third (i.e. f2/f1 = 1.2). These pure tones are presented to the ear via two miniature loudspeakers embedded in an ear canal probe. The presence of DPOAEs indicates a well-functioning inner ear. However, the validity of these tests can be affected by an inaccurate adjustment (calibration) of the stimuli, resulting in no measurable DPOAEs in spite of an intact inner ear.
In our study [1], DPOAEs were stimulated by air and bone conduction in a ”combined” procedure, which has not been performed before. The f1 tone was presented by one of the probe loudspeakers and the f2 tone by means of a bone vibrator placed behind the same ear of the test person (see Fig. 1). The first tone (f1 via air conduction) travels through the ear canal and the middle-ear chain towards the inner ear. On the other hand, the second tone (f2 via bone conduction) reaches the inner ear directly and is not affected by the calibration errors which arise in the ear canal.
Figure 1: Procedure for recording otoacoustic emissions elicited by the combined stimulation by two pure tones (f1, f2). The f1 tone is presented via air conduction (by a probe loudspeaker) and the f2 tone via bone conduction (by a bone vibrator placed on the mastoid). The otoacoustic distortion products (amplitude: LDP) are recorded by the probe microphone and depicted over the f2 frequency in a so-called “DPgram”.
With the combined stimulation of DPOAEs, it is possible to present the pure tones (f1 and f2 in Fig. 1) via the alternative acoustic pathway (bone and air conduction, respectively) and to repeat the measurement with the same position of the probe. This provides a simple way to assess the effective stimulus levels in the inner ear. The presence of a DPOAE in one configuration and its absence in the other (after swapping the signal delivery pathways) does not indicate a pathologic inner ear but is rather evidence of an unfavourable adjustment between the stimulus levels. This caused the suppression of the DPOAE despite a healthy inner ear.
References:
[1] M. Zebian, J. Hensel and T. Fedtke, "Estimulación combinada de otoemisiones acústicas por vía aérea y vía ósea", 41º Congreso Nacional de Acústica | 6º Congreso Ibérico de Acústica, vol. León, 2010. Download (in Spanish): 
http://www.sea-acustica.es/Leon10/AFP%20002.pdf
Contact person:
Makram Zebian, Dept. 1.6, WG 1.61, E-mail: makram.zebian@ptb.de