Peripheral auditory lateralization assessment using TEOAEs
Introduction
The asymmetrical anatomy of the human brain is well known (Geschwind and Levitsky, 1968). The auditory lateralization in speech processing (Kimura, 1961) probably is not dependent on a single mechanism (Jäncke et al., 1992), but involves both cognitive and perceptual components. For example, a perceptual auditory asymmetry has been demonstrated (Brown and Nicholls, 1997) in an experiment carried out with 24 normal adults whose reaction times, response biases and response errors revealed a right ear (i.e. left hemisphere) advantage in gap detection. Absolute auditory thresholds also seem to be better, i.e. lower, in the right ear (see McFadden, 1993), especially at 3–4 kHz (Eagles et al., 1963). Recently, further evidence for the existence of a left/right asymmetry in the functioning of the human peripheral auditory system has been provided by the demonstration of stronger auditory efferent activity in the right than in the left ear (Khalfa and Collet, 1996).
In this study, following a method defined by Collet et al. (1990), medial olivocochlear (MOC) activity was assessed through the suppression of transient evoked otoacoustic emission (TEOAE) amplitude elicited by contralateral acoustic stimulation (CAS). To assess this peripheral suppression, an equivalent attenuation (EA) calculation procedure has been used (Collet et al., 1992; Chéry-Croze et al., 1994). EA corresponds to the decrease in the ipsilateral stimulus intensity (eliciting TEOAEs) which would be necessary to obtain, in the absence of the contralateral broad-band noise (BBN) stimulus (eliciting TEOAE suppression), the same response than with CAS, i.e. activating the MOC system. Five different EAs calculated with five different ipsilateral stimulus intensities were averaged to obtain a more representative EA. Besides the interaural asymmetry in olivocochlear system functioning, an asymmetry in the cochlear functioning itself is indicated by the finding of larger TEOAE amplitudes in the right than in the left ear (Khalfa et al., 1997).
The first aim of this study was to test whether these two features of peripheral auditory lateralization were linked. Then, in order to complete the study of interaural TEOAE asymmetries, a second aim was to test for the existence of an interaural asymmetry in TEOAE growth function slopes. It has been shown in a previous study (Veuillet et al., 1991) that the CAS activating MOC system increases TEOAE input/output (i/o) function slopes, i.e. the slopes of ipsilateral stimulus intensities as a function of TEOAE amplitude, corresponding to regression lines computed from the five experimental points compared to the slopes obtained without CAS. These slopes represent physiological mechanisms reflecting TEOAE dynamics and show to a certain extent the degree of TEOAE amplitude enhancement when stimulus intensity is increased. The effect of CAS on the TEOAE slopes has been studied in our present large population to compare with previous results. MOC system functional asymmetry can act on either ear of each subject, with varying strength and also vary according to ipsilateral acoustic stimulation intensity. The influence of the MOC system on the increase in TEOAE i/o function slope was therefore studied in both ears, considering that this system tends to be more effective in right ears.
Section snippets
Subjects
Sixty-seven normal-hearing subjects (<20 dB loss between 250 and 8000 Hz per octave on pure tone audiogram), aged from 18 to 31 years (mean=22.735 years; S.D.=2.773) were included in this study. Their hand preference laterality quotient was calculated using the Edinburgh Handedness Inventory (Oldfield, 1971). Only right-handed subjects (with positive handedness laterality quotient) as assessed by this test were included in this experiment, because hemispheric specialization is less marked in
Relationship between EA and TEOAE amplitudes
No significant correlation was found between EA and TEOAE amplitude in the right ear (Fig. 3), or in the left ear (Fig. 4). Whatever the ipsilateral stimulation intensity used (80, 71, 67, 65, 63, 59±3 dB peSPL) to evoke OAEs, no significant correlation was observed between TEOAE amplitude and EA.
Relationship between EA asymmetry and TEOAE amplitude asymmetry
Right ear EA minus left ear EA was plotted against right ear minus left ear TEOAE amplitude for 0 dB gain, in Fig. 5. No correlation between the two asymmetries was found, whichever the ipsilateral
Two independent signs of peripheral auditory lateralization
It has been established by previous studies that EA is lower in the right than in the left ear, and also that the medial efferent system is more effective in the right ear (Khalfa and Collet, 1996). Moreover, TEOAE amplitude reflecting active cochlear mechanisms has been found to be greater in the right ear (Khalfa et al., 1997).
EA does not seem to be linked to TEOAE amplitude in either ear. Indeed, in both the right (Fig. 3) and the left ear (Fig. 4), the absence of correlation shows that
Conclusion
The results of the present study indicate that the asymmetry of the MOC system reflected in the difference between right and left ear EA is not the result of the asymmetry of the TEOAE amplitudes involved in the EA calculation. These results provide further support to the notion that auditory efferent activity is asymmetrical between the two ears and attenuates right ear TEOAE amplitude more effectively than left ear TEOAE levels. However, the changes in TEOAE i/o function slope observed upon
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