Cerebral plasticity and peripheral auditory fatigue

Today, the reference screening tool for assessing permanent or temporary auditory threshold shifts is the pure-tone air-conduction audiometry (PTA). The technique allows the tone perception from background noise to be measured. Therefore, it performs subjective measurements as a function of individuals’ ability to determine thresholds of hearing sensations to frequency-specific acoustic stimuli within an acoustic booth. In this cognitive process, the auditory central system analyzes each piece of information coming from the peripheral auditory receptor, and might counterbalance subtle ear dysfunctions or metabolic fatigue to assure the highest level of performance. Since more than ten years, the understanding of the mechanisms involved in the central auditory plasticity (CAP) has made progresses. Two major nucleus can illustrate the fast modulation of the CAP mechanisms on the peripheral performances: (1) the cochlear dorsal nucleus can up-regulate the cochlear inputs in case of peripheral hidden dysfunction, (2) and the inferior colliculus can also down regulate the inputs to assure the highest level of performance. As a result, a normal audiogram can be obtained in spite of hidden cochlear dysfunctions. Unfortunately, the use of these compensatory mechanisms during youth anticipates the first signs of presbycusis. Thus, cochlear dysfuntions may remain underestimated due to potential retro-cochlear compensation mechanisms, which may postpone the awareness of changes in hearing. Penalty comes later on and it is difficult to establish a relation between exposure and deafness. It is therefore important to provide tools allowing the peripheral receptor function to be measured without any CNS control. Cubic distortion product oto-acoustic emissions (DPOAEs) allow that. The DPOAEs measurement at 2f1-f2 in response to stimulations by two primaries at f1 and f2 requires the presence of healthy and motile OHCs, and can be easily measured in the external auditory canal. DPOAEs are therefore good candidates to assess cochlear dysfunctions. By adding a contralateral acoustic stimulation to the ipsilateral DPOAE measurements (CAS DPOAEs), we have the possibility to measure a reduction in the transmission of the sound energy through the middle-ear into the cochlea and, in the meantime, to evaluate the efferent reflex strength. The difference of thresholds of the CAS DPOAE trigger is a good indicator of the peripheral auditory fatigue; it gathers (1) the lowering of the cochlear performance, (2) the metabolic fatigue of the afferent, efferent pathways, (3) the metabolic fatigue of the central (cochlear, olivocochlear complex, facial) nucleus, and finally (4) the fatigue of the middle-ear muscles. All these modifications obtained with negligible influence of the CAP makes the peripheral auditory fatigue measured with CAS DPOAES more sensible than that measured with ATL.
The measurements of the auditory fatigue carried out with workers from noisy environments showed a 9-dB temporary auditory threshold shift with Echoscan compared to a 3-dB with a PTA.