The Effect of Amplitude Modulated Contralateral Signals on Distortion Product Otoacoustic Emissions

Contralateral stimulation of the auditory system causes changes in levels of otoacoustic emissions. The main objective of this study was to determine the changes in the level of distortion product otoacoustic emissions for contralaterally presented unmodulated broad-band noise, or amplitude modulated broad-band noise. Two types of modulating signal were used in the investigations (sine or rectangular wave form). The modulation depth was 100% for both the sinusoidal and the rectangular modulation. The modulation rate was 4 or 100 Hz. The generation and acquisition of the distortion product otoacoustic emissions signal lasted 2 s. The values of the distortion product otoacoustic emissions level changes were defined as the difference between the mean distortion product otoacoustic emissions levels with and without contralateral stimulation. Ten normal-hearing subjects participated in this study. The results showed that reduction in the level of the distortion product otoacoustic emissions (suppression effect) was highest for the low $F_2$ frequencies and decreased along with the increase in the $F_2$ frequency. The modulation type of the contralateral stimulation did not influence the mean suppression effect significantly. However, the distortion product otoacoustic emissions level reduction reached higher values for the unmodulated contralateral stimulation than for the modulated, and these differences were statistically significant.