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.
The main aim of this work was to determine the influence of contralateral stimulation (CS) on the psychophysical tuning curves (PTCs) and distortion product otoacoustic emissions (DPOAEs). PTCs and DPOAEs were measured in two modes: in the presence or absence of CS. The contralateral signal was a wideband noise (bandwidth 0.2-10 kHz) at a level of 50 dB sound pressure level (SPL). The primary tones (F_1 and F_2) were presented at levels of L_1=60 dB SPL, and L_2=50 dB SPL. The signal frequency used in the measurements of the PTC was 1 kHz or 2 kHz. For both PTC signal frequencies the CS significantly reduces the sharpness of the PTCs. The average change in level of DPOAE under the influence of CS throughout the whole range of frequencies takes an effect of suppression. The CS has a significant effect on decreasing the value of the quality factor (Q_{10Roex}) of PTCs (F(1,9)= 19.36, p=0.002). The CS caused a decrease in the level of DPOAE in 88% of cases. The maximum suppression of the DPOAE level occurs for the F_2 frequency from 1 kHz to 2 kHz.
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