Kaas J H
Department of Psychology, Vanderbilt University, Nashville, TN 37240.
J Exp Biol. 1989 Sep;146:165-76. doi: 10.1242/jeb.146.1.165.
Much of the forebrain of many extant species of mammals appears to be sensory-perceptual in nature. Thus, much of the forebrain, especially the dorsal thalamus and neocortex, consists of nuclei and areas that are parts of complex systems that analyze sensory information and allow behavior to be guided by accurate inferences about the external world. Since mammals vary tremendously in brain size, they vary in the amount of tissue devoted to sensory processing. In addition, mammals vary in the sizes and numbers of processing nuclei and areas, and in how neurons and neuron groups (modules) are differentiated within such structures. Sensory-perceptual systems with more, larger and more differentiated parts may allow more stimulus parameters to be considered, experience to play a greater role, and speed calculations through increased parallel processing. The evolution of species differences in brain size, the sizes of individual parts, and internal structure of these parts are potentially understandable within a theoretical framework of gradual modifications of developmental processes. In addition to changes in the generation and specialization of neurons, alterations in the developmental timing that modify internal and external influences on neuron activity patterns seem to have a major role in the construction and maintenance of organization in the nervous system. Because similar selection pressures may arise over and over again and the mechanisms for producing changes may be few, similar changes in the nervous system are likely to occur in independent lines of evolution. It is uncertain how new cortical areas and nuclei evolve. Comparative studies suggest that: (1) all mammals have a few basic sensory areas and nuclei in common, (2) the number of areas and nuclei has increased independently in several lines of mammalian evolution, and (3) new areas have been added to the middle levels of cortical processing sequences. New areas and nuclei may have evolved as a result of sudden duplications and/or by the process of single areas or nuclei gradually differentiating into two or more areas or nuclei. The process of gradual differentiation may have involved the initial step of differentiating functionally distinct classes of cells that are mixed in a representation, followed by the local groupings of such cells into functionally distinct sets, and finally the fusion of cell groups of the same types to form separate representations.
许多现存哺乳动物物种的前脑在本质上似乎是感觉-感知性的。因此，大部分前脑，尤其是背侧丘脑和新皮层，由一些核团和区域组成，这些核团和区域是复杂系统的一部分，这些系统分析感觉信息，并使行为能够基于对外部世界的准确推断来进行引导。由于哺乳动物的脑大小差异极大，它们在用于感觉处理的组织数量上也有所不同。此外，哺乳动物在处理核团和区域的大小及数量上存在差异，在这些结构中神经元和神经元群体（模块）的分化方式也有所不同。具有更多、更大且分化程度更高的部分的感觉-感知系统可能允许考虑更多的刺激参数，使经验发挥更大作用，并通过增加并行处理来加快计算速度。在发育过程逐渐改变的理论框架内，物种在脑大小、各个部分的大小以及这些部分的内部结构方面的差异演变可能是可以理解的。除了神经元的产生和特化变化外，发育时间的改变会影响对神经元活动模式的内部和外部影响，这似乎在神经系统组织的构建和维持中起着重要作用。由于相似的选择压力可能会反复出现，而产生变化的机制可能很少，因此神经系统中相似的变化很可能在独立的进化谱系中发生。新的皮层区域和核团是如何进化的尚不确定。比较研究表明：（1）所有哺乳动物都有一些共同的基本感觉区域和核团，（2）在哺乳动物进化的几个谱系中，区域和核团的数量独立增加，（3）新的区域已添加到皮层处理序列的中间层次。新的区域和核团可能是由于突然复制和/或单个区域或核团逐渐分化为两个或更多区域或核团的过程而进化形成的。逐渐分化的过程可能涉及以下步骤：首先是将混合在一种表征中的功能不同的细胞类别区分开来，然后将这些细胞局部聚集成功能不同的集合，最后将相同类型的细胞群融合形成单独的表征。
