Heil Peter
Department of Auditory Learning and Speech, Leibniz Institute for Neurobiology, Magdeburg, Germany,
J Assoc Res Otolaryngol. 2014 Apr;15(2):219-34. doi: 10.1007/s10162-013-0432-x. Epub 2014 Jan 3.
Absolute auditory threshold decreases with increasing sound duration, a phenomenon explainable by the assumptions that the sound evokes neural events whose probabilities of occurrence are proportional to the sound's amplitude raised to an exponent of about 3 and that a constant number of events are required for threshold (Heil and Neubauer, Proc Natl Acad Sci USA 100:6151-6156, 2003). Based on this probabilistic model and on the assumption of perfect binaural summation, an equation is derived here that provides an explicit expression of the binaural threshold as a function of the two monaural thresholds, irrespective of whether they are equal or unequal, and of the exponent in the model. For exponents >0, the predicted binaural advantage is largest when the two monaural thresholds are equal and decreases towards zero as the monaural threshold difference increases. This equation is tested and the exponent derived by comparing binaural thresholds with those predicted on the basis of the two monaural thresholds for different values of the exponent. The thresholds, measured in a large sample of human subjects with equal and unequal monaural thresholds and for stimuli with different temporal envelopes, are compatible only with an exponent close to 3. An exponent of 3 predicts a binaural advantage of 2 dB when the two ears are equally sensitive. Thus, listening with two (equally sensitive) ears rather than one has the same effect on absolute threshold as doubling duration. The data suggest that perfect binaural summation occurs at threshold and that peripheral neural signals are governed by an exponent close to 3. They might also shed new light on mechanisms underlying binaural summation of loudness.
绝对听觉阈值会随着声音持续时间的增加而降低，这一现象可以通过以下假设来解释：声音引发神经活动，这些神经活动发生的概率与声音幅度的约3次方成正比，并且阈值需要恒定数量的神经活动（海尔和诺伊鲍尔，《美国国家科学院院刊》100:6151 - 6156，2003年）。基于这个概率模型以及双耳完美总和的假设，本文推导了一个方程，该方程明确表达了双耳阈值是两个单耳阈值的函数，无论它们是否相等，还与模型中的指数有关。对于指数>0的情况，当两个单耳阈值相等时，预测的双耳优势最大，并且随着单耳阈值差异的增加，双耳优势向零减小。通过将不同指数值下的双耳阈值与基于两个单耳阈值预测的阈值进行比较，对该方程进行了测试并得出了指数。在大量具有相等和不相等单耳阈值的人类受试者样本中，以及针对具有不同时间包络的刺激所测量的阈值，仅与接近3的指数兼容。当两只耳朵同样敏感时，指数为3预测双耳优势为2分贝。因此，用两只（同样敏感）耳朵聆听而不是一只耳朵，对绝对阈值的影响与将声音持续时间加倍相同。数据表明在阈值处发生了完美的双耳总和，并且外周神经信号受接近3的指数支配。它们也可能为响度双耳总和的潜在机制提供新的线索。
