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

Abstract

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.