TEOAE contralateral suppression in Neonates and Adults

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By This email address is being protected from spambots. You need JavaScript enabled to view it. , Ph.D.
Louisiana State University Health Sciences Center
Department of Otorhinolaryngology and Biocommunication and the Kresge Laboratory

Introduction

Two of the most exciting findings in hearing research of the past years have certainly been the discovery of active mechanisms in the cochlea (Davis, 1983) and of two distinct efferent auditory pathways between the brain and the cochlea (Rasmussen, 1946; Warr and Guinan, 1978), which imply that auditory input can be modified before it reaches the brain. The first population of efferent neurons, thin and unmyelinated, arises from the lateral superior olivary complex and synapses with cochlear afferent neuron dendrites, close to the inner hair cells (IHCs) which are the primary, sensory receptors of the auditory system. The second population of neurons, known as the medial olivocochlear system (MOCS), is composed of large myelinated neurons originating from the medial nuclei of the superior olivary complex. These neurons project mainly contralaterally to innervate the outer hair cells (OHCs) which are presumably the source of cochlear active mechanisms (CAMs).

The MOCS’ role in hearing is still a matter of research and debate. The MOCS is thought to protect the cochlea against acoustic injury (Cody and Johnstone, 1982; Reiter and Liberman, 1995). This system also seems to be involved in the detection of signals in noise (Winslow and Sachs, 1987; Micheyl et al., 1995; Micheyl and Collet, 1996), such as speech sounds (Giraud et al., 1997), by modulating CAMs. Evidence of this modulation comes mainly from numerous studies on otoacoustic emissions (OAEs). OAEs are thought to be the by-products of CAMs, i.e., the activity of OHCs (Kemp, 1978; Brownell et al., 1985).

Since Buno (1978) and Murata’s work (1980) showing that acoustic stimulation of one cochlea may modify afferent fiber responses in the contralateral cochlea, other experiments have shown a method of studying the MOCS’ activity non-invasively in adults by coupling contralateral stimulation with OAE recording. The result is a frequency specific decrease of OAE amplitude (Collet et al., 1990; Ryan et al., 1991; Veuillet et al., 1991; Berlin et al., 1993). This technique is objective and non-invasive and may be applied to neonates and infants.

How to Record Efferent Suppression

In neonates, suppression is usually explored by stimulating the MOCS with a white noise presented to the ear contralateral to TEOAE recording. A common way to record TEOAEs is to elicit them with 80 ųsec linear clicks at 60-65 db SPL. The continuous white noise in the contralateral ear is 5 dB above the click stimulus with the level being monitored throughout testing. An average of two hundred responses repeated twice for each recording is sufficient to explore MOCS function.

The contralateral noise can be delivered through an earphone embedded in a foam cushion on which infants lie on their bellies, -- one ear down for noise, one ear up for TEOAE testing.

It is important to record suppression in a very quiet environment. Larger suppression can be found in infants tested post-isolette (2.177 dB) over pre-isolette (1.455 dB) (Goforth et al.,2000).

In infants it is of the up-most importance to be sure that the TEOAE probe does not move during the 2 consecutive recordings without and with noise recordings.

MOCS function or suppression of TEOAEs is determined by subtracting the "with noise" average from the "without noise" average.

MOCS activity can also be investigated with binaural and ipsilateral stimulation. In this case, the noise used to activate the MOCS precedes the click used to elicit TEOAEs (forward masking paradigm).

Characteristics of Suppression in Adults

With a classical 20 ms TEOAE recording, the majority of suppression takes place between 8 and 18 ms.

Binaural stimulation with white noise appears to be the most powerful stimulus for eliciting efferent suppression of TEOAEs in humans.

MOCS activity is frequency specific: the efferent system appears to be more functional at low and middle frequencies than at high frequencies (Veuillet et al. 1991).

The MOCS appears to be more efficient in right ear than in left ear (Khalfa and Collet, 1996).

Auditory Neuropathy patients have no efferent suppression of TEOAEs with binaural, contralateral or ipsilateral noise.

Some hyperacousic patients show abnormally large efferent suppression.

Efferent Suppression in Neonates

Efferent suppression is not present at an early age whereas cochlear active mechanism asymmetries are already present (i.e., in young pre-term neonates) (Morlet et al., 1993; Goforth et al., 2000).

Suppression progressively appears (Figure 1).

The MOCS does not appear to be fully mature at full-term birth.

There is no relation in infants between TEOAE amplitude and amount of suppression as in adults (Figure 2).

Its development was found to be asymmetrical (Figures 3-4).

Because it is not clear as to when in development the efferent system begins to function, it is not a useful diagnostic tool at birth as are OAE. However, if no efferent suppression is noticed 2-3 months after birth, other tests of the auditory function are recommended.

Figure 1:Efferent Suppression Increases with Conceptional Age

Figure 2: Efferent Suppression Increases with Conceptional Age

Figure 3A:Asymmetrical Development of Efferent Suppression for conceptional age < 36 weeks

Figure 3B:Asymmetrical Development of Efferent Suppression for coceptional age > 36 weeks

Figure 4: Efferent Suppression in Normal Children