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The Interacoustics ASSR is indicated for use in the recording and analysis of human physiological data used for the diagnosis of auditory and hearing-related disorders. This product, Interacoustics ASSR is a diagnostic device intended to be used as part of a set of audiometric test protocols. It is especially indicated for use in defining the configuration of the hearing loss particularly for individuals whose behavioural audiometric results are deemed unreliable. It allows for the estimation of hearing threshold at various frequencies, through the use of ASSR (Auditory Steady-State Response) test protocols. It is designed to be used as a diagnostic test procedure by individuals who are trained in the performance and interpretation of evoked potentials such as audiologists and physicians. The results of the test will be used by trained hearing health care professionals to make recommendations regarding appropriate intervention strategies. The use of the Interacoustics ASSR is to be performed under the prescription and supervision of a physician or other trained health care professional.
An auditory steady-state response (ASSR) is an electrophysiological response that is evoked by a periodically repeated auditory stimulus. The response is stable over time for as long the stimulation is turned on. If the recording time window can be regarded as being infinitely long, then the response will consist of a series of discrete frequency components that are constant in both amplitude and phase over the time window.
Auditory steady-state responses are to be distinguished from auditory transient responses. According to the above, auditory steady-state responses are evoked by a series of sound stimuli that are presented at a high repetition rate, whereas auditory transient responses are evoked by an individual, brief sound stimulus or a series of brief sound stimuli that are presented at a low repetition rate. At medium repetition rates there is a grey zone where the differentiation between the two response types is difficult to make. However, if the stimulus repetition rate is so high that the electrophysiological response to one stimulus overlaps with the response to the next stimulus, then the recorded activity can be meaningfully classified as a steady-state response.
At repetition rates close to 40 stimuli per second, components of the Middle Latency Response (MLR) overlap and the recorded ASSR is named the 40Hz-response. This response is dominated by evoked activity from the higher auditory pathways in the Thalamus (middle brain) and the Cortex and is therefore influenced by test subject conditions like attention, arousal and sleep (anesthesia and sedation).
At repetition rates higher than 70 stimuli per second, components of the Auditory Brainstem Response (ABR) begin to overlap. This ASSR is dominated by early evoked activity from the brain stem and is therefore not influenced by the above mentioned test subject conditions.
The ASSR can be used for hearing screening and for this purpose broad band stimuli are applied. These may for example consist of repeated clicks, chirps and noise bursts or of amplitude and/or frequency modulated noise.
For hearing screening a fixed stimulus level at 35 dBnHL is often used. The hearing screening gives only two possible results: (1) either an ASSR is detected (false or true negative) or (2) an ASSR is not detected (false or true positive).
The ASSR can also be used in the audiological diagnosis. For this purpose narrow band (frequency specific) stimuli must be applied. These can for example be filtered clicks, narrow band chirps, and tone bursts or amplitude and/or frequency modulated narrow band carriers.
The most important audiological application is audiometry, where the observed ASSR thresholds will provide an estimate of the pure tone audiogram. To find the ASSR thresholds different test strategies can be applied based on efficient response detector algorithms.