Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago 7800003, Chile.
Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago 7800003, Chile; Facultad de Medicina, Universidad Finis Terrae, Santiago 7501015, Chile.
Curr Biol. 2023 Jan 9;33(1):1-14.e4. doi: 10.1016/j.cub.2022.10.070. Epub 2022 Nov 28.
Re-entrant connections are inherent to nervous system organization; however, a comprehensive understanding of their operation is still lacking. In birds, topographically organized re-entrant signals, carried by axons from the nucleus-isthmi-parvocellularis (Ipc), are distinctly recorded as bursting discharges across the optic tectum (TeO). Here, we used up to 48 microelectrodes regularly spaced on the superficial tectal layers of anesthetized pigeons to characterize the spatial-temporal pattern of this axonal re-entrant activity in response to different visual stimulation. We found that a brief luminous spot triggered repetitive waves of bursting discharges that, appearing from initial sources, propagated horizontally to areas representing up to 28° of visual space, widely exceeding the area activated by the retinal fibers. In response to visual motion, successive burst waves started along and around the stimulated tectal path, tracking the stimulus in discontinuous steps. When two stimuli were presented, the burst-wave sources alternated between the activated tectal loci, as if only one source could be active at any given time. Because these re-entrant signals boost the retinal input to higher visual areas, their peculiar dynamics mimic a blinking "spotlight," similar to the internal searching mechanism classically used to explain spatial attention. Tectal re-entry from Ipc is thus highly structured and intrinsically discontinuous, and higher tectofugal areas, which lack retinotopic organization, will thus receive incoming visual activity in a sequential and piecemeal fashion. We anticipate that analogous re-entrant patterns, perhaps hidden in less bi-dimensionally organized topographies, may organize the flow of neural activity in other parts of the brain as well.
折返连接是神经系统组织的固有特征；然而，人们对其运作机制仍缺乏全面的了解。在鸟类中，由来自 nucleus-isthmi-parvocellularis（Ipc）的轴突携带的拓扑组织折返信号，作为 across the optic tectum（TeO）的爆发放电而被清晰地记录下来。在这里，我们使用多达 48 个微电极，这些微电极均匀分布在麻醉鸽子的浅层脑皮层上，以描述这种轴突折返活动的时空模式，以响应不同的视觉刺激。我们发现，一个短暂的光亮斑点引发了重复的爆发放电波，这些波从初始源开始传播，水平传播到代表多达 28°的视觉空间的区域，远远超过了由视网膜纤维激活的区域。对视觉运动的反应，连续的爆发波沿着和围绕刺激的脑皮层路径开始，以不连续的步骤跟踪刺激。当呈现两个刺激时，爆发波的源在激活的脑皮层区域之间交替，好像在任何给定时间只有一个源可以活跃。由于这些折返信号增强了视网膜输入到更高的视觉区域的信号，它们的特殊动力学类似于经典的内部搜索机制，用于解释空间注意力。因此，来自 Ipc 的脑皮层折返是高度结构化和内在不连续的，而缺乏视网膜组织的更高脑皮层区域将以连续和零碎的方式接收传入的视觉活动。我们预计，类似的折返模式，也许隐藏在组织程度较低的拓扑结构中，也可能会组织大脑其他部分的神经活动的流动。
