Improved detection method for auditory steady state responses in cochlear-implant users
Michael H Hofmann, Jan M Wouters · Lirias · 2011
Electrically Auditory Steady State Responses (EASSRs) are EEG potentials in response to periodic electrical stimuli presented through a cochlear implant (CI). For slow rate pulse trains in the 40 Hz range, electrophysiological thresholds derived from response amplitude growth functions correlate well with behavioral thresholds at these rates. The aims of this study were: 1) to show that auditory steady state potentials can be reliably evoked by modulated high-rate pulse trains with clinical carrier rates and modulation frequencies in the 40 Hz range, 2) to demonstrate that stimulus artifacts for such stimuli can be completely removed from the recording, 3) to analyze the properties of the resulting responses with regards to amplitude, phase and apparent latency, and 4) to examine the predictive value of electrophysiological thresholds derived from such responses for behavioral thresholds at these high rates. For six users of a Nucleus cochlear implant, EASSRs to symmetric biphasic bipolar pulse trains were recorded with seven scalp electrodes. Responses to six different stimuli were analyzed: two slow rate pulse trains with pulse rates in the 40 Hz range as well as two amplitude-modulated (AM) and two pulse-width-modulated (PWM) pulse trains with a carrier rate of 900 pps and modulation frequencies in the 40 Hz range. Measurements were done at eight stimulus intensities. Artifacts due to the electrical stimulation were removed from the recordings. To determine the presence of a neural response, improved robust statistics based on Hotelling’s T-square test were used that could cope with the presence of limited remaining artifact components. Measurements from different recording electrodes and adjacent stimulus intensities were combined to increase statistical power. The results show that EASSRs can be evoked by AM and PWM high-rate pulse trains in CI users. In the recorded EEG, these responses can be completely separated from the artifacts generated by the electrical stimulation. At lower intensities, the obtained response amplitudes for AM and PWM stimuli are higher than for slow-rate stimuli, while apparent latencies are similar for all three stimuli. Electrophysiological thresholds derived from responses to AM and PWM high-rate stimuli are significantly lower than for slow-rate stimuli and correlate well with behavioral thresholds.