Why is CE-Chirp So Beneficial for Threshold Estimation?

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by Tony Lombardo, MS

For over 30 years, threshold estimation in very young or difficult to test patients has been accomplished with Auditory Brainstem Response (ABR). The ABR is an onset response, a large number of neurons must fire at the same time to elicit the response. To ensure this synchronous firing, a short duration stimulus is used. The two most common short duration stimuli are the click and the tone pip.

The traditional click stimulus is a 100 µs electrical pulse that has a frequency range of approximately 100 - 10,000 Hz. The broadband nature of the click provides stimulation of a large portion of the cochlea, which causes a large number of neurons to fire simultaneously. The resulting AEP provides information on the neural synchrony of the auditory nerve. The tone pip (also called tone burst) stimulus assists in the evaluation of frequency-specific performance of the auditory system. The frequency-specific stimulus is achieved by presenting a sine wave for a brief duration. The tone pip stimulus is based on the number of cycles presented. Typically, the rise and fall times of the stimuli are 2 cycles and the plateau is either 1 or 0 cycles. With this approach, the duration of the stimulus varies with frequency, but the energy content of stimulus is consistent for each frequency.

The ABR response to click and tone pip stimuli is highly efficient and results in a clear, repeatable waveform. However, the ABR is limited by the cochlea’s traveling wave. It takes time for a signal to travel from the high to low frequency regions of the cochlea.

The goal of overcoming the traveling wave delay in ABR is not a new concept. Research on this subject dates back over 20 years. An ideal approach to solve the traveling wave delay in the ABR was introduced by Claus Eberling and others. The CE-Chirp is a broad-band stimulus designed to enhance Wave V of the ABR through adjustment of the stimulus frequency composition. This adjustment counteracts the temporal dispersion of the travelling wave inherent in the cochlea by presenting lower frequency energy before higher frequency energy. The CE-Chirp stimulus adjustment maintains the same frequency content of the click. The frequency timing, however, maximizes the response of the cochlea, increasing the synchronous neural firings of the auditory pathway. CE-Chirp Octave Bands are frequency specific short duration stimuli derived from the CE-Chirp stimulus providing more robust frequency specific threshold estimation. The ABR response to the CE-Chirp stimulus has been demonstrated to be 1.5 to 2 times greater in amplitude than the ABR amplitude to click stimuli in normal hearing subjects.

CE-Chirp stimuli are ideal stimuli for electrophysiological threshold estimation. Threshold estimation can be difficult to achieve in a single appointment such as when testing infants and young children. The CE-Chirp and the CE-Chirp Octave Band stimuli have been demonstrated to generate a repeatable and reliable Wave V response that is larger in amplitude than the Wave V elicited by traditional click and tone-pip stimuli. The robust responses are often generated with fewer averages which shorten the time of the evaluation. Additionally, the use of CE-Chirp Octave Band stimuli provides robust and fast frequency-specific threshold estimation for a more thorough evaluation. Although clinical studies are not yet available for neurophysiologic diagnostic evaluation, threshold estimation is an immediate and effective use for the CE-Chirp stimuli.