McAlpine D, Jiang D, Palmer A R
MRC Institute of Hearing Research, University of Nottingham, United Kingdom.
J Acoust Soc Am. 1996 Jul;100(1):490-503. doi: 10.1121/1.415862.
The binaural masking level difference (BMLD) is a striking and well-documented psychophysical effect which relates to the ability to use the phase of low-frequency sounds to dissociate them from masking noise. When identical tones and noise are presented to both ears, detectability is improved by up to 15 dB simply by inverting the phase of either the tone or noise in one ear. Measurements of BMLDs were made in single delay-sensitive neurones in the inferior colliculus of the guinea pig. These have confirmed and extended an earlier report [D. M. Caird, A. R. Palmer, and A. Rees, Hear. Res. 57, 91-106 (1991)] by demonstrating that when signals are optimized for the frequency, level, and interaural delay sensitivities of each neurone, BMLDs can be measured which are in a direction, and of a magnitude, consistent with appropriate psychophysical observations in human subjects. In addition, BMLDs were found to be consistent with the delay sensitivities of the neurones to the signal and masker, the major determinant of the masked threshold for optimized signals being the activity evoked in the neurone by the masking noise. Within-channel signal-to-noise (S/N) ratios at masked threshold for single neurones varied from +20 to -7 dB, depending on the binaural configuration and the units' delay sensitivities. In single neurones, the size of the BMLD for optimized signals increased with the level of the noise. The BMLD increased by 5 dB over a 40-dB range of noise, consistent with psychophysical observations. This came about because as noise level increased, masked threshold for optimized tones increased more slowly in Npi noise than in N0 noise. For all binaural comparisons, both positive (pi signals more detectable, as in the psychophysics) and negative BMLDs were observed, often in the same neurone, a result entirely consistent with the sensitivity to the interaural delay of the noise and tone signals. For 500-Hz signals in zero and pi phase masked by identical noise the majority of BMLDs determined with the PEST procedure was negative, a result which is taken to indicate that increases in spike rate may not be an appropriate cue for masked threshold under these conditions.
双耳掩蔽级差（BMLD）是一种显著且有充分文献记载的心理物理学效应，它与利用低频声音的相位将其与掩蔽噪声区分开来的能力有关。当相同的纯音和噪声同时呈现给双耳时，仅通过反转一只耳朵中纯音或噪声的相位，可检测性就能提高多达15分贝。在豚鼠下丘的单个对延迟敏感的神经元中进行了BMLD的测量。这些测量证实并扩展了早期的一份报告[D. M. 凯尔德、A. R. 帕尔默和A. 里斯，《听觉研究》57，91 - 106（1991）]，表明当信号针对每个神经元的频率、强度和双耳延迟敏感性进行优化时，可以测量到的BMLD在方向和大小上与人类受试者适当的心理物理学观察结果一致。此外，发现BMLD与神经元对信号和掩蔽声的延迟敏感性一致，对于优化信号，掩蔽阈值的主要决定因素是掩蔽噪声在神经元中诱发的活动。单个神经元在掩蔽阈值时的通道内信噪比（S/N）在+20到 - 7分贝之间变化，这取决于双耳配置和单元的延迟敏感性。在单个神经元中，优化信号的BMLD大小随噪声强度增加。在40分贝的噪声范围内，BMLD增加了5分贝，这与心理物理学观察结果一致。这是因为随着噪声强度增加，在Npi噪声中优化纯音的掩蔽阈值比在N0噪声中增加得更慢。对于所有双耳比较，都观察到了正的（如心理物理学中pi信号更易检测）和负的BMLD，而且常常在同一个神经元中出现，这一结果与对噪声和纯音信号双耳延迟的敏感性完全一致。对于由相同噪声掩蔽的零相位和pi相位的500赫兹信号，用PEST程序确定的大多数BMLD是负的，这一结果被认为表明在这些条件下，放电率的增加可能不是掩蔽阈值的合适线索。
