Institute of Hearing Research, MRC, University Park, NG7 2RD, Nottingham, UK.
Adv Exp Med Biol. 2013;787:65-72. doi: 10.1007/978-1-4614-1590-9_8.
Under certain conditions, detection of a masked tone is improved by a preceding sound ("precursor"). This phenomenon is referred to as the "temporal effect" or "overshoot". A prevalent model of overshoot, referred to as the "gain reduction model", posits that overshoot is caused by efferent reduction in cochlear gain mediated by the medial olivocochlear (MOC) bundle. The model predicts that reduction in cochlear gain will reduce masking when masking is suppressive or when masking is excitatory and the signal-to-masker ratio is high. This study was aimed at testing the validity of these predictions. It consisted of two experiments. The first experiment investigated the relative contributions of suppressive versus excitatory masking to overshoot. The signal was a short 4-kHz tone pip, and the masker and precursor were limited to contain energy either only within (-on-frequency) or only outside (off-frequency) the cochlear filter around the signal frequency. The on-frequency masker would be expected to cause mainly excitatory masking, whereas masking by the off-frequency masker would be expected to be mainly suppressive. Only the off-frequency masker and precursor yielded -significant overshoot. This suggests that measurable overshoot requires suppressive masking. The second experiment sought to quantify the effect of a precursor on cochlear -suppression more directly by measuring the amount of suppression caused by a 4.75-kHz suppressor on a lower-frequency (4-kHz) suppressee with and without a precursor present. Suppression was measured using a forward-masking paradigm. While we found large suppression and large overshoot, we found no reduction in suppression by the precursor. This is contrary to the gain reduction model. Taken together, our results indicate that measurable overshoot requires off-frequency masking and that off-frequency overshoot must be caused by a mechanism other than MOC-mediated reduction in cochlear suppression.
在某些条件下，先前的声音（“前导音”）会提高对掩蔽音的检测。这种现象被称为“时间效应”或“过冲”。一种流行的过冲模型，称为“增益降低模型”，假定过冲是由内侧橄榄耳蜗束（MOC）介导的耳蜗增益的传出减少引起的。该模型预测，当掩蔽是抑制性的或掩蔽是兴奋性的且信号与掩蔽器的比率较高时，耳蜗增益的降低将减少掩蔽。本研究旨在检验这些预测的有效性。它由两个实验组成。第一个实验研究了抑制性掩蔽和兴奋性掩蔽对过冲的相对贡献。信号是一个短的 4kHz 音峰，掩蔽器和前导音仅限于在信号频率周围的耳蜗滤波器内（同频）或仅在外部（异频）包含能量。同频掩蔽器预计会引起主要的兴奋性掩蔽，而异频掩蔽器的掩蔽预计会主要是抑制性的。只有异频掩蔽器和前导音产生了显著的过冲。这表明可测量的过冲需要抑制性掩蔽。第二个实验试图通过测量带有和不带有前导音的情况下，4.75kHz 抑制剂对低频（4kHz）抑制剂引起的抑制量，更直接地量化前导音对耳蜗抑制的影响。使用前向掩蔽范式测量抑制。虽然我们发现了大量的抑制和大量的过冲，但我们没有发现前导音对抑制的减少。这与增益降低模型相反。总之，我们的结果表明，可测量的过冲需要异频掩蔽，而异频过冲必须由 MOC 介导的耳蜗抑制减少以外的机制引起。
