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Synonyms: Brain stem evoked response audiometry, Auditory brain stem response, ABR audiometry, BAER (Brainstem auditory evoked response audiometry). Definition: Bera is an objective way of eliciting brain stem potentials in response to audiological click stimuli. Even though BERA provides information regarding auditory function and sensitivity, it is not a substitute for other methods of audiological evaluation. Procedure: The stimulus either in the form of click or tone pip is transmitted to the ear via a transducer placed in the insert ear phone or head phone. Electrode placement: Since the electrodes should be placed over the head, the hair must be oil free.
Since the potentials recorded are in far field, well displaced from the site of impulse generation, the wave forms recorded are very weak and they need to be amplified. How to improve signal to noise ratio: Three parallel approaches are designed to achieve this goal.


Filtering: This is employed to reduce the recording bandwidth so that only the important components of the siganal generated are recorded.
Repeated stimulation: This is done with synchronous time domain averaging to increase the amplitude of the components of the signal. Polarity alteration: By altering the polarity of impuses recorded, the artifacts are cancelled making the brain stem waves stand out. In auditory brain stem evoked response audiometry, the impulses are generated by the brain stem.
These peaks occur in most readable form in response to click stimuli over a period of 1 - 10 milliseconds after the stimulus in normal hearing adults. Auditory cortical response: Records the impulses generated by brain in response to tone stimuli.
The wave froms of impulses generated at the level of brain stem are recorded by the placement of electrodes over the scalp.
In real time situations these two can be achieved by connecting the recording electrodes to a preamplifier, with appropriate filter settings.


It is an effective screening tool for evaluating cases of deafness due to retrocochlear pathology i.e.
The standard electrode configuration for BERA involves placing a non inverting electrode over the vertex of the head, and inverting electrodes placed over the ear lobe or mastoid prominence. CERA is very useful for threshold estimation of hearing, where as BERA is highly useful for objective threshold estimation of hearing as well as differential diagnostic purposes. These reponses are more generalised and originate form the brain cortex occurring between 50 - 300 milliseconds after the onset of stimulation.
Since these responses are generally elicited with a tone burst lasting approximately for about 200 milliseconds, its responses are highly frequency specific. This is in contrast to BERA because brain stem responses are evoked by click stimuli and are not frequency specific.



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