Overview / Cochlea / Auditory pathway / Audiograms
Scientific concept: Jérôme Ruel, Jean-Luc Puel, Nuno Trigueiros Cunha
Drawings: P. Minary, S. Blatrix

Hearing thresholds of an individual may be assessed by both objective and subjective methods. Objective methods are based on recordings of the electrical activity at various stages along the auditory pathway, from the cochlea to the cortex, and do not need the active participation of the individual. On the other hand, subjective methods need the co-operation of the individual being tested, because they rely on the subject's perception of and response to sound.

Objective Methods
Auditory potentials reflect the electrical activity of the various neural pathways involved in the coding of sounds. Individual unitary potentials are recorded directly from a single cell or nerve fibre. Compound potentials, recorded at a distance from their origin, reflect the sum of the unit potentials. The following are three examples of different types of potentials : note the temporal concordance beween the different recordings - at whichever level being measured, the first wave recorded with a latency of 1 ms reflects the auditory nerve action potential.

Unit potentials
Recording of an action potential of a single auditory nerve fibre (experimental technique not used in humans)

A glass micro-electrode (arrow) allows the recording of the electrical activity from a single neurone in the auditory nerve. This auditory fibre responds to sound stimulation (red dot) by sending an action potential (AP) towards the brain. The latency between the sound stimulus (red arrow) and the the AP is about 1ms. The amplitude of individual APs is measured in millivolts (mV).

Recording of the compound cochlear nerve action potential at the Round Window This technique is used in human subjects, and is also known as electrocochleography

A transtympanic electrode is placed on the promontory, near to the cochlear Round Window, and it records the compound action potential (CAP) of the auditory nerve, which is the synchronous activity of all the individual unit action potentials after sound stimulation (red point). This potential is more complex than the single unit AP, but the first wave in this potential (N1) corresponds to the single unit AP in the preceding example. The amplitude of the CAP measured between N1-P1 is in the order of several dozen microvolts (µV). Successive responses are added and then averaged to eliminate background electrical noise.

Evoked Potentials
Brainstem Auditory Evoked Potentials (BAEPs)
Surface electrodes placed on the skull allow recording of 5 main waves of electrical activity through the brainstem. Wave I, with a latency of 1 ms, reflects the auditory nerve potential ; waves II to V reflect processing centres in the auditory system. These potentials are of very weak amplitude (< µV) and therefore averaging of many repeat stimulations (1000 to 2000) is required, in order to be able to differentiate them from background noise. After these early evoked potentials, the same skull electrodes can record later potentials from activity in the thalamocortex (middle latency and cortical auditory evoked responses).

Subjective Methods

Pure tone audiograms recorded in a normally-hearing subject over the course of their life. Each curve represents the average hearing loss as a function of age.
(see. presbycusis)

x-axis : pure tone frequency (kHz); y-axis : hearing loss in dB.Age 20 (green curve) : normal audiogram, with a non-significant loss in the very high frequencies (8 kHz). Age 40 (yellow) : this high frequency loss is more obvious, but is not yet a handicap.Age 60 (orange) : the hearing loss is becoming significant (>40dB) at 4 kHz ; une gêne apparaît dans la compréhension : par ex. pour la perception des "sifflantes" et de leurs harmoniques.Age 90 (red), losses of > 40 dB are obvious in the middle frequencies (2 kHz), and hearing has significantly deteriorated.


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