Shipp S
Wellcome Department of Cognitive Neurology, University College London, Gower Street, London WC1E 6BT, UK.
Philos Trans R Soc Lond B Biol Sci. 2003 Oct 29;358(1438):1605-24. doi: 10.1098/rstb.2002.1213.
The pulvinar is an 'associative' thalamic nucleus, meaning that most of its input and output relationships are formed with the cerebral cortex. The function of this circuitry is little understood and its anatomy, though much investigated, is notably recondite. This is because pulvinar connection patterns disrespect the architectural subunits (anterior, medial, lateral and inferior pulvinar nuclei) that have been the traditional reference system. This article presents a simplified, global model of the organization of cortico-pulvinar connections so as to pursue their structure-function relationships. Connections between the cortex and pulvinar are topographically organized, and as a result the pulvinar contains a 'map' of the cortical sheet. However, the topography is very blurred. Hence the pulvinar connection zones of nearby cortical areas overlap, allowing indirect transcortical communication via the pulvinar. A general observation is that indirect cortico-pulvino-cortical circuits tend to mimic direct cortico-cortical pathways: this is termed 'the replication principle'. It is equally apt for certain pairs (or groups) of nearby cortical areas that happen not to connect with each other. The 'replication' of this non-connection is achieved by discontinuities and dislocations of the cortical topography within the pulvinar, such that the associated pair of connection zones do not overlap. Certain of these deformations can be used to divide the global cortical topography into specific sub-domains, which form the natural units of a connectional subdivision of the pulvinar. A substantial part of the pulvinar also expresses visual topography, reflecting visual maps in occipital cortex. There are just two well-ordered visual maps in the pulvinar, that both receive projections from area V1, and several other occipital areas; the resulting duplication of cortical topography means that each visual map also acts as a separate connection domain. In summary, the model identifies four topographically ordered connection domains, and reconciles the coexistence of visual and cortical maps in two of them. The replication principle operates at and below the level of domain structure. It is argued that cortico-pulvinar circuitry replicates the pattern of cortical circuitry but not its function, playing a more regulatory role instead. Thalamic neurons differ from cortical neurons in their inherent rhythmicity, and the pattern of cortico-thalamic connections must govern the formation of specific resonant circuits. The broad implication is that the pulvinar acts to coordinate cortical information processing by facilitating and sustaining the formation of synchronized trans-areal assemblies; a more pointed suggestion is that, owing to the considerable blurring of cortical topography in the pulvinar, rival cortical assemblies may be in competition to recruit thalamic elements in order to outlast each other in activity.
丘脑枕是一个“联合性”丘脑核团,这意味着它的大部分输入和输出关系是与大脑皮层形成的。这种神经回路的功能鲜为人知,其解剖结构虽经过大量研究,但仍极为深奥。这是因为丘脑枕的连接模式并不遵循一直以来作为传统参照系统的结构亚单位(丘脑枕前核、内侧核、外侧核和下核)。本文提出了一个简化的、整体的皮质 - 丘脑枕连接组织模型,以探究它们的结构 - 功能关系。皮层与丘脑枕之间的连接是按拓扑结构组织的,因此丘脑枕包含了一张皮质层的“地图”。然而,这种拓扑结构非常模糊。因此,相邻皮质区域的丘脑枕连接区相互重叠,从而允许通过丘脑枕进行间接的跨皮质交流。一个普遍的观察结果是,间接的皮质 - 丘脑枕 - 皮质回路往往模仿直接的皮质 - 皮质通路:这被称为“复制原则”。对于某些碰巧不相互连接的相邻皮质区域对(或组)来说同样适用。这种不连接的“复制”是通过丘脑枕内皮质拓扑结构的间断和错位来实现的,这样相关的一对连接区就不会重叠。这些变形中的某些可用于将整体皮质拓扑结构划分为特定的子区域,这些子区域构成了丘脑枕连接细分的自然单位。丘脑枕的很大一部分也表现出视觉拓扑结构,反映了枕叶皮质中的视觉图谱。丘脑枕中只有两个排列有序的视觉图谱,它们都接收来自V1区以及其他几个枕叶区域的投射;由此产生的皮质拓扑结构的重复意味着每个视觉图谱也充当一个单独的连接域。总之,该模型识别出四个按拓扑结构排序的连接域,并协调了其中两个域中视觉图谱和皮质图谱的共存。复制原则在域结构层面及以下起作用。有人认为皮质 - 丘脑枕神经回路复制了皮质神经回路的模式,但不是其功能,而是起到了更具调节性的作用。丘脑神经元在其固有节律性方面与皮质神经元不同,皮质 - 丘脑连接模式必定控制着特定共振回路的形成。一个更广泛的含义是,丘脑枕通过促进和维持同步跨区域集合的形成来协调皮质信息处理;一个更明确的观点是,由于丘脑枕中皮质拓扑结构的显著模糊,相互竞争的皮质集合可能会竞相招募丘脑元素,以便在活动中比对方持续更长时间。