Nelson Paul C, Smith Zachary M, Young Eric D
Center for Hearing and Balance, Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21205, USA.
J Neurosci. 2009 Feb 25;29(8):2553-62. doi: 10.1523/JNEUROSCI.5359-08.2009.
An organism's ability to detect and discriminate sensory inputs depends on the recent stimulus history. For example, perceptual detection thresholds for a brief tone can be elevated by as much as 50 dB when following a masking stimulus. Previous work suggests that such forward masking is not a direct result of peripheral neural adaptation; the central pathway apparently modifies the representation in a way that further attenuates the input's response to short probe signals. Here, we show that much of this transformation is complete by the level of the inferior colliculus (IC). Single-neuron extracellular responses were recorded in the central nucleus of the awake marmoset IC. The threshold for a 20 ms probe tone presented at best frequency was determined for various masker-probe delays, over a range of masker sound pressure levels (SPLs) and frequencies. The most striking aspect of the data was the increased potency of forward maskers as their SPL was increased, despite the fact that the excitatory response to the masker was often saturating or nonmonotonic over the same range of levels. This led to probe thresholds at high masker levels that were almost always higher than those observed in the auditory nerve. Probe threshold shifts were not usually caused by a persistent excitatory response to the masker; instead we propose a wide-dynamic-range inhibitory mechanism locked to sound offset as an explanation for several key aspects of the data. These findings further delineate the role of subcortical auditory processing in the generation of a context-dependent representation of ongoing acoustic scenes.
生物体检测和区分感觉输入的能力取决于近期的刺激历史。例如，在掩蔽刺激之后，短暂纯音的感知检测阈值可提高多达50分贝。先前的研究表明，这种前向掩蔽并非外周神经适应的直接结果；中枢通路显然以一种进一步减弱输入对短探测信号反应的方式修改表征。在这里，我们表明这种转换的大部分在中脑下丘（IC）水平就已完成。在清醒狨猴IC的中央核中记录了单神经元细胞外反应。针对各种掩蔽音 - 探测音延迟，在一系列掩蔽音声压级（SPL）和频率范围内，确定了在最佳频率呈现的20毫秒探测纯音的阈值。数据最显著的方面是，尽管在相同声压级范围内对掩蔽音的兴奋性反应常常饱和或呈非单调变化，但随着掩蔽音SPL的增加，前向掩蔽的效力增强。这导致在高掩蔽音水平下的探测阈值几乎总是高于在听神经中观察到的阈值。探测阈值的变化通常不是由对掩蔽音的持续兴奋性反应引起的；相反，我们提出一种锁定声音抵消的宽动态范围抑制机制来解释数据的几个关键方面。这些发现进一步阐明了皮层下听觉处理在生成依赖于上下文的正在进行的声学场景表征中的作用。
