Frequency-specific objective audiometry: tone-evoked brainstem responses and steady-state responses to 40 Hz and 90 Hz amplitude modulated stimuli.

Tone-evoked Auditory Brainstem Responses (tone-burst ABRs) and Auditory Steady-State Responses (ASSRs) with 40 or 90 Hz amplitude modulation (AM) were compared, using the same equipment and recording parameters, to determine which of these three methods most accurately approached the behavioural hearing thresholds in response to 500 Hz and 2000 Hz stimuli in eleven awake adults with normal hearing. Estimates of the thresholds obtained with the three methods were 10, 18, and 26 dB SL at 500 Hz; and 10, 12, and 22 dB SL at 2000 Hz; using 40 Hz ASSR, 90 Hz ASSR, and tone-burst ABR, respectively. ASSRs with 40 Hz AM stimuli produced significantly better results (lowest thresholds with SD=0), whereas the wave-V analysis on the tone-burst ABR produced the poorest results. In the averaged ABRs, a robust steady-state potential was also visible. Analysis of those steady-state responses showed estimated thresholds of 13 and 14 dB SL (at 500 and 2000 Hz, respectively), thus considerably better than the estimated thresholds from the wave-V analysis. It is concluded that the 40 Hz ASSR showed superior results, especially at 500 Hz.

EEG derivations providing auditory steady-state responses with high signal-to-noise ratios in infants.

OBJECTIVE: To identify EEG derivations that yield high signal-to-noise ratios (SNRs) of the auditory steady-state response (ASSR) in infants aged 0 to 5 months. DESIGN: The ASSR was recorded simultaneously from 10 EEG derivations in a monopolar montage in 20 sleeping infants. Stimuli were tones of 0.5 or 2 kHz that were 100% amplitude modulated and 20% frequency modulated, presented at 65 dB SPL for 4.4 minutes in either the right or the left ear. An amplitude modulation frequency of 90 Hz (left ear) or 94 Hz (right ear) was used. From the 10 measured monopolar derivations, all 45 bipolar derivations were calculated mounting up to 55 EEG derivations. EEG derivations were selected in the preferred set if they had the largest SNRs within subjects and if they were obtained significantly more frequently across subjects than was expected by chance (Monte Carlo simulation and Wilcoxon signed ranks test). RESULTS: The preferred derivations are both mastoids ipsilateral to the stimulated ear with Cz as common reference. These derivations improved SNRs compared with each of several conventional EEG derivations (excluding the preferred derivations) between 16 and 69% (500 Hz, left ear), 9 and 132% (500 Hz, right ear), 31 and 193% (2 kHz, left ear), and 3 and 105% (2 kHz, right ear). In contrast to results reported earlier for adults, high SNRs were not found at the inion-Cz derivation in these infants. CONCLUSIONS: High SNRs were obtained in infants aged younger than 6 months if the ASSR was recorded from the mastoids ipsilateral to the ear of stimulation referenced to Cz.

Signal-to-noise ratios of the auditory steady-state response from fifty-five EEG derivations in adults.

The Auditory Steady-State Response (ASSR) was recorded in 20 awake adults with normal hearing on ten EEG channels simultaneously to find derivations with the best signal-to-noise ratios (SNRs). Stimuli were 20% frequency modulated tones of 0.5 and 2 kHz at 20 dB SL, 100% amplitude modulated at 90 or 94 Hz, and presented one at a time to one ear. ASSR recordings using a set of at least three channels improved SNRs significantly by an average of between 6% (500 Hz right ear) to 118% (2 kHz right ear) above the SNRs from the conventional channels. Assuming that the recording time was proportional to 1/(SNR)2, this translates into a recording time of 89% (500 Hz right ear) to 21% (2 kHz right ear) of that for conventional single-channel recording. The three channels comprised the electrode positions inion, right mastoid, and left mastoid. All three electrode positions were referenced to Cz. Adding a fourth channel (Pz-Cz) increases the number of participants with significant responses from the 500 Hz right ear stimulus from 13 to 17. Electrode position F4 and other commonly used positions such as the forehead and right earlobe made significantly less contribution to test efficiency.