Middlebrooks J C, Zook J M
J Neurosci. 1983 Jan;3(1):203-24. doi: 10.1523/JNEUROSCI.03-01-00203.1983.
The area of the cat's primary auditory cortex (AI) within which high frequency sounds are represented can be subdivided using functional criteria. Within each subdivision, or "binaural interaction band," all recorded neurons display similar responses to binaural stimulation. The current study distinguishes the thalamic sources of input to these subdivisions of AI and characterizes the topography within the thalamic projection to each class of bands. The borders of binaural bands in AI were mapped using microelectrode recording with diotic tonal stimulation, then injections of one to three retrograde tracers were introduced into identified bands. Within the ventral division (V) of the medial geniculate body (the major thalamic source of input to AI), the neuronal populations that projected to different classes of binaural bands were strictly segregated from each other. This segregation of class-specific thalamic sources constitutes a laminar organization within an axis of V that is orthogonal to the previously described tonotopic organization. Excitatory/excitatory (EE) binaural neurons in AI were found to be segregated from excitatory/inhibitory (EI) neurons in alternating "bands." We consistently identified: (1) a ventral pair of rostrocaudally continuous EI and EE bands, (2) a middle area within which the pattern of binaural subdivisions was more variable and within which bands often were discontinuous rostrocaudally, and (3) a dorsal zone (DZ) within which the responses of neurons differed in binaural properties and in frequency specificity from the response patterns that were characteristic of neurons elsewhere in AI. Each EI band apparently derived input that converged from three thickened laminae of cells in V that were oriented approximately horizontally. The most ventral of these laminae encompassed the ovoidal part of V (Vo), suggesting that EI bands are the only recipients in AI of a projection from Vo. All of the EE bands and DZ derived their input from a single continuous structure which included the dorsal two-thirds of the rostral pole of V and a horizontal lamina interposed between the two dorsalmost EI-projecting laminae. Restricted portions of the complex EE-projecting structure in V projected preferentially to particular EE subdivisions of AI. The V-to-AI thalamocortical topography exhibited a high degree of convergence and divergence within the projections to each cortical binaural band and within the projections to each class of bands. These observations indicate that the high frequency representation in AI and its principal thalamic source of input, the ventral division of the medial geniculate body, may be thought of as assemblies of spatially discrete, functionally distinguishable subunits. The significance of this intrinsic organization is discussed in regard to the requirements for analysis of sound stimuli.
猫的初级听觉皮层（AI）中代表高频声音的区域可根据功能标准进行细分。在每个细分区域或“双耳相互作用带”内，所有记录的神经元对双耳刺激都表现出相似的反应。本研究区分了AI这些细分区域的丘脑输入源，并描绘了丘脑向每类频段投射的拓扑结构。使用微电极记录和双耳纯音刺激绘制了AI中双耳带的边界，然后将一到三种逆行示踪剂注入已确定的频段。在内侧膝状体的腹侧部（V）（AI的主要丘脑输入源）内，投射到不同类双耳带的神经元群体彼此严格分离。这种类别特异性丘脑源的分离在V轴内构成了一种层状组织，该轴与先前描述的音频拓扑组织正交。在AI中，兴奋性/兴奋性（EE）双耳神经元与兴奋性/抑制性（EI）神经元在交替的“带”中分离。我们一致发现：（1）一对腹侧的前后连续的EI和EE带，（2）一个中间区域，其中双耳细分模式更具变异性，并且带在前后方向上通常不连续，以及（3）一个背侧区（DZ），其中神经元的反应在双耳特性和频率特异性方面与AI其他部位神经元的特征反应模式不同。每个EI带显然接收了从V中三个大致水平排列的增厚细胞层汇聚而来的输入。这些层中最腹侧的一层包括V的卵形部分（Vo），这表明EI带是AI中来自Vo投射的唯一接收者。所有的EE带和DZ都从一个单一的连续结构接收输入，该结构包括V的嘴侧极的背侧三分之二以及介于两个最背侧的EI投射层之间的一个水平层。V中复杂的EE投射结构的受限部分优先投射到AI的特定EE细分区域。从V到AI的丘脑皮质拓扑结构在向每个皮质双耳带的投射以及向每类频段的投射中表现出高度的汇聚和发散。这些观察结果表明，AI中的高频表征及其主要丘脑输入源，即内侧膝状体的腹侧部，可被视为空间离散、功能可区分的亚单位的集合。本文讨论了这种内在组织对于声音刺激分析要求的意义。
