Ramachandran R, Davis K A, May B J
Department of Biomedical Engineering and Center for Hearing and Balance, Johns Hopkins University, Baltimore, MD 21205, USA.
J Assoc Res Otolaryngol. 2000 Sep;1(2):144-60. doi: 10.1007/s101620010029.
Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound-pressure levels (SPLs) are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow excitatory area at best frequency (BF: a unit's most sensitive frequency) and surrounding inhibition at higher and lower frequencies. As a result of this receptive field organization, type I units exhibit strong excitatory responses to BF tones but respond only weakly to broadband noise (BBN). These response characteristics predict that type I units are well suited to encode narrowband signals in the presence of background noise. To test this hypothesis, the dynamic range properties of ICC unit types were measured under quiet conditions and in multiple levels of continuous noise. As observed in previous studies of the auditory nerve and cochlear nucleus, type I units showed upward threshold shifts and discharge rate compression in background noise that partially degraded the dynamic range properties of neural representations at high noise levels. Although the other two unit types in the ICC showed similar trends in threshold shift and noise compression, their ability to encode auditory signals was compromised more severely in increasing noise levels. When binaural masking effects were simulated, only type I units showed an enhanced representation of spatially separated signals and maskers that was consistent with human perceptual performance in independent psychoacoustic observations. These results support the interpretation that type I units play an important role in the auditory processing of narrowband signals in background noise and suggest a physiological basis for spatial factors that govern signal detection under free-field listening conditions.
给去大脑猫的下丘中央核（ICC）中的神经元对侧耳呈现不同频率和声压级（SPL）的纯音时，会表现出三种主要反应模式。I型神经元的频率反应图由其最佳频率（BF：神经元最敏感的频率）处狭窄的兴奋性区域以及更高和更低频率处的周围抑制唯一确定。由于这种感受野组织，I型神经元对BF纯音表现出强烈的兴奋性反应，但对宽带噪声（BBN）的反应较弱。这些反应特性表明，I型神经元非常适合在存在背景噪声的情况下编码窄带信号。为了验证这一假设，在安静条件下和多个连续噪声水平下测量了ICC神经元类型的动态范围特性。正如之前在听神经和耳蜗核研究中所观察到的，I型神经元在背景噪声中表现出阈值上移和放电率压缩，这在高噪声水平下部分降低了神经表征的动态范围特性。尽管ICC中的其他两种神经元类型在阈值移动和噪声压缩方面表现出类似趋势，但在噪声水平增加时，它们编码听觉信号的能力受到的损害更为严重。当模拟双耳掩蔽效应时，只有I型神经元表现出与独立心理声学观察中人类感知性能一致的空间分离信号和掩蔽声的增强表征。这些结果支持了I型神经元在背景噪声中窄带信号的听觉处理中起重要作用的解释，并为自由场聆听条件下控制信号检测的空间因素提供了生理基础。
