Calibration of the ASSR equipment is critical to ensure that the estimated threshold for the AEP is as accurate a prediction of behavioral pure tone thresholds as is possible. Although there is not yet a clear standard for calibration of the ASSR stimuli, in the current literature stimulus calibration tends to be in either dB HL64 or in dB SPL57 units. The variation in calibration methods is due to the nature of the stimuli utilized in ASSR testing. The continuous AM or MM tone used for the ASSR has a long duration, similar to a pure-tone, therefore the reference equivalent threshold sound pressure level (RETSPL) used for pure tones should be the same64, 65.
Calibration in dB HL
- Because the AM or MM stimulus is very similar to pure-tones, many researchers and ASSR system manufacturers calibrate the ASSR stimuli in dB HL according to various national or international standards (i.e. ANSI 1996 and/or ITE [need year])65, 66.
- Stapells and colleagues (2005)66 reported that ASSR thresholds obtained with stimuli calibrated in dB HL were elevated when compared to pure tone behavior thresholds66.
Calibration in units used in ABR stimuli calibration
- Other researchers and ASSR system manufacturers calibrate the ASSR stimuli in units of dB peak SPL, dB peak-to-peak equivalent SPL, and dB nHL, such as the ABR stimuli are calibrated66.
- Unlike the dB HL units, there are no national or international standards for these units65.
- Stapells and colleagues (2005)66 reported that ASSR thresholds obtained with stimuli calibrated in dB peak-to-peak equivalent SPL units were similar to ABR thresholds evoked with tones in infants66.
Calibration of the ASSR stimuli is an area that has not been widely discussed in the literature, and is in need of future research.