L.I.A. Schellekens

Master thesis (1)

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Optimizing the stimulus used to elicit the Acoustic Change Complex: evaluation of the pre-transition duration and stimulus complexity in normal hearing adults

Master thesis (2024) - L.I.A. Schellekens (author), M.P. van der Schroef (graduation committee member), J.L. Vroegop (graduation committee member), R.C. Hendriks (graduation committee member), Richard C. Hendriks (graduation committee member), Richard Hendriks (graduation committee member), R. C. Hendriks (graduation committee member), Richard Christian Hendriks (graduation committee member), M.L. Adank (mentor)

Introduction The Acoustic Change Complex (ACC) is a cortical auditory evoked potential elicited by a change in an ongoing sound that consists of a P1-N1-P2 complex. The ACC holds promise as a non-invasive, passive, and objective measure to monitor speech discrimination. This can improve the treatment of hearing loss for young children and adults that are not able to execute gold standard speech perception tests. However, the reported measurement times of the ACC are lengthy, which might impede the clinical feasibility of measuring young children and adults with behavioural issues that are difficult to test. Measurement time can be reduced by enhancing the signal to noise ratio (SNR). Furthermore, enhancement of the SNR might increase the sensitivity and specificity of the ACC as a measure of speech discrimination. Two stimulus parameters that might increase the SNR are an increase in the pre-transition duration (PTD) and stimulus complexity. The PTD is the duration between the onset of the stimulus and the change in the acoustic characteristics of the stimulus. Besides increasing the SNR, measurement time directly increases with an increase in PTD. Therefore, the optimal PTD regarding efficiency should be studied. Objectives The aims are to study the effect of 1) the PTD and 2) tonal complexity on the N1-P2 peak-to-peak amplitude, baseline noise and SNR of the ACC. To evaluate the optimal PTD, efficiency was included as an outcome measure for the first objective. Methods The ACC was measured in eighteen normal hearing adults to pure tone stimuli with a frequency change from 1kHz to 1.1kHz. The studied PTDs were 0.25, 0.5, 1, 2 and 3s. Furthermore, a complex tone with the same center frequencies and a PTD of 1s was presented. Efficiency was measured as the SNR divided by the total measurement time. Main results An increase in PTD significantly increased the N1-P2 peak-to-peak amplitude up to a PTD of 2s. PTD 0.25s was excluded from all analyses due to overlap with the onset response. The SNR of PTD 0.5s was significantly smaller than all other measured PTDs. The PTD of 1s was significantly more efficient than a PTD of 0.5s, had a measurement time of 6.67min and the ACC was present in 58.8% of the participants. ACC presence increased (to 100% for PTD 3s) with an increase in PTD. There was no significant difference in the N1-P2 peak-to-peak amplitude or SNR between the complex and pure tone. However, ACC presence increased with 22.5 percentage points for the complex tone compared to the pure tone. Conclusion The ACC is affected by the PTD. The PTDs of 2 and 3s generated the significantly largest responses. We recommend utilizing a PTD of 1s instead of 0.5s, as it resulted in a significantly higher N1-P2 peak-to-peak amplitude, SNR and efficiency. Furthermore, increasing tonal complexity or the PTD beyond 1s seems promising to increase presence of the ACC without significantly decreasing the SNR or efficiency.