Barbour Dennis L, Wang Xiaoqin
Laboratory of Auditory Neurophysiology, Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
J Neurophysiol. 2002 Nov;88(5):2684-99. doi: 10.1152/jn.00253.2002.
Natural sounds often contain energy over a broad spectral range and consequently overlap in frequency when they occur simultaneously; however, such sounds under normal circumstances can be distinguished perceptually (e.g., the cocktail party effect). Sound components arising from different sources have distinct (i.e., incoherent) modulations, and incoherence appears to be one important cue used by the auditory system to segregate sounds into separately perceived acoustic objects. Here we show that, in the primary auditory cortex of awake marmoset monkeys, many neurons responsive to amplitude- or frequency-modulated tones at a particular carrier frequency [the characteristic frequency (CF)] also demonstrate sensitivity to the relative modulation phase between two otherwise identically modulated tones: one at CF and one at a different carrier frequency. Changes in relative modulation phase reflect alterations in temporal coherence between the two tones, and the most common neuronal response was found to be a maximum of suppression for the coherent condition. Coherence sensitivity was generally found in a narrow frequency range in the inhibitory portions of the frequency response areas (FRA), indicating that only some off-CF neuronal inputs into these cortical neurons interact with on-CF inputs on the same time scales. Over the population of neurons studied, carrier frequencies showing coherence sensitivity were found to coincide with the carrier frequencies of inhibition, implying that inhibitory inputs create the effect. The lack of strong coherence-induced facilitation also supports this interpretation. Coherence sensitivity was found to be greatest for modulation frequencies of 16-128 Hz, which is higher than the phase-locking capability of most cortical neurons, implying that subcortical neurons could play a role in the phenomenon. Collectively, these results reveal that auditory cortical neurons receive some off-CF inputs temporally matched and some temporally unmatched to the on-CF input(s) and respond in a fashion that could be utilized by the auditory system to segregate natural sounds containing similar spectral components (such as vocalizations from multiple conspecifics) based on stimulus coherence.
自然声音通常在很宽的频谱范围内包含能量，因此当它们同时出现时会在频率上重叠；然而，在正常情况下，这样的声音在感知上是可以区分的（例如鸡尾酒会效应）。来自不同声源的声音成分具有不同的（即非相干的）调制，并且非相干性似乎是听觉系统用于将声音分离为可单独感知的声学对象的一个重要线索。在这里我们表明，在清醒的狨猴初级听觉皮层中，许多对特定载波频率[特征频率（CF）]的调幅或调频音调有反应的神经元，也表现出对两个其他方面相同调制的音调之间的相对调制相位的敏感性：一个在CF，另一个在不同的载波频率。相对调制相位的变化反映了两个音调之间时间相干性的改变，并且发现最常见的神经元反应是在相干条件下抑制达到最大值。相干敏感性通常在频率响应区域（FRA）的抑制部分的一个窄频率范围内被发现，这表明只有一些偏离CF的神经元输入到这些皮层神经元中，在相同的时间尺度上与CF上的输入相互作用。在所研究的神经元群体中，显示相干敏感性的载波频率与抑制的载波频率一致，这意味着抑制性输入产生了这种效应。缺乏强烈的相干诱导的易化也支持这种解释。发现相干敏感性在16 - 128 Hz的调制频率下最大，这高于大多数皮层神经元的锁相能力，这意味着皮层下神经元可能在这种现象中起作用。总的来说，这些结果表明，听觉皮层神经元接收一些在时间上与CF上的输入匹配和一些不匹配的偏离CF的输入，并以一种听觉系统可以利用的方式做出反应，以基于刺激相干性分离包含相似频谱成分的自然声音（例如来自多个同种个体的发声）。
