Henning G Bruce, Richards Virginia M, Lentz Jennifer J
Hearing Research Center, Biomedical Engineering Department, Boston University, Boston, Massachusetts 02215, USA.
J Acoust Soc Am. 2005 Nov;118(5):3229-40. doi: 10.1121/1.2047167.
Detection thresholds for tones in narrow-band noise were measured for two binaural configurations: N(o)S(o) and N(o)S(pi). The 30-Hz noise band had a mean overall level of 65 dB SPL and was centered on 250, 500, or 5000 Hz. Signals and noise were simultaneously gated for 500, 110, or 20 ms. Three conditions of level randomization were tested: (1) no randomization; (2) diotic randomization--the stimulus level (common to both ears) was randomly chosen from an uniformly distributed 40-dB range every presentation interval; and (3) dichotic randomization--the stimulus levels for each ear were each independently and randomly chosen from the 40-dB range. Regardless of binaural configuration, level randomization had small effects on thresholds at 500 and 110 ms, implying that binaural masking-level differences (BMLDs) do not depend on interaural level differences for individual stimuli. For 20-ms stimuli, both diotic and dichotic randomization led to markedly poorer performance than at 500- and 110-ms durations; BMLDs diminished with no randomization and dichotic randomization but not with diotic randomization. The loss of BMLDs at 20 ms, with degrees-of-freedom (2WT) approximately 1, implies that changes in intracranial parameters occurring during the course of the observation interval are necessary for BMLDs when mean-level and mean-intracranial-position cues have been made unhelpful.
N(o)S(o)和N(o)S(π)，测量了窄带噪声中音调的检测阈值。30赫兹的噪声带平均总声压级为65分贝声压级，以250、500或5000赫兹为中心。信号和噪声同时门控500、110或20毫秒。测试了三种电平随机化条件：(1)无随机化；(2)双耳随机化——每次呈现间隔时，刺激电平（双耳共同）从均匀分布的40分贝范围内随机选择；(3)双耳分听随机化——每只耳朵的刺激电平各自独立地从40分贝范围内随机选择。无论双耳配置如何，电平随机化对500和110毫秒时的阈值影响较小，这意味着双耳掩蔽级差（BMLD）不依赖于单个刺激的双耳电平差。对于20毫秒的刺激，双耳随机化和双耳分听随机化导致的表现均明显比500毫秒和110毫秒时长时差；无随机化和双耳分听随机化时BMLD减小，但双耳随机化时没有。20毫秒时BMLD的丧失，自由度（2WT）约为1，这意味着当平均电平和平均颅内位置线索变得无用时，观察间隔期间颅内参数的变化对于BMLD是必要的。
