An auditory steady-state response (ASSR) is an electrophysiological response that follows the envelope of a periodically repeated narrow band stimulus. The stimuli may consist of pure tones modulated in amplitude and/or frequency, repeated filtered clicks or repeated band-limited chirp signals.
The portion of the basilar membrane being stimulated is restricted to the stimulus frequency range, thus a frequency specific assessment of hearing is possible. ASSR can be recorded over a wide range of stimulus rates. Different stimulus rates result in stimulation at different levels of the auditory pathway. At fast stimulus rates (> 70 Hz, so called 80-Hz-response) the response is dominated by early evoked activity from the brain stem and is therefore not affected by subject state (sleep, sedation, attention, level of arousal). So, the 80-Hz-response is mainly utilized in sedated or lightly anesthetized children/babies. At stimulus rates around 40 Hz, named as 40-Hz-response, components of the Middle Latency Response (MLR) contribute to the ASSR. Therefore the response includes activity from the higher auditory pathways in the Thalamus and the Cortex. They are mainly used in awake adults.
The narrow band stimuli allow either single frequency or multiple frequency stimulation to one or both ears simultaneously. When multiple frequencies are tested, different stimulus rates are used so that the ASSR to each frequency can be detected separately .
Recordings of ASSR are obtained differentially from electrodes placed on the scalp at locations typically used for the recording of other auditory evoked potentials (vertex, mastoid). The ASSR consists of neural activity that follows the rate of the narrow band stimulus. So the frequency of interest in the brainwaves is that corresponding to the stimulus rate.
Averaging is commonly used to extract the ASSR from other electrical activity (e.g. EEG) to increase the signal-to-noise ratio and with that enabling response detection. Unlike ABR, ASSR analysis occurs in the frequency domain rather than the time domain. Objective response detection algorithms use amplitude and/or phase of the response and its variability to determine whether a response is present or not.
Case example – comparison of PTA and ASSR of a cochlear hearing loss patient.
- Stimuli : Click, chirp, narroband chirp
- Polarity : Condensation, Rerefaction, alternating
- Level : 5 dB HL to 90 dB HL in 5 dB steps
- Stimulus rate : 10Hz to 90 Hz, optional jitter
- Recording window : 0 to 30ms
- Recording bandwidth : 15Hz to 2kHz
- Sampling rate : 16kHz, 16 bit
- CMRR : >90 dB @ 50Hz
- ASSR : Modulation frequencies : 40 and 80 hz, stimulus level : up to 100 dB HL, up to 4 simultaneous test frequencies per ear – adjustable maximum averaging time, artefact threshold, and significance level