日本理化研究所的一项最新研究显示，大脑保持可塑性的时间比原先估计的要长。这一成果将修改科学界对大脑临界期的定义，而且为研究婴幼儿早教提供了重要线索。
20世纪60年代，科学家们提出临界期的概念, 这一概念的内容是，哺乳动物的新生儿在出生后一段短暂的时间内，其未成熟的大脑能够根据生活环境发生结构性改变。
当时的实验证据是，包括人类在内的哺乳动物新生儿，如果一只眼睛被遮挡一段时间，那么即使遮挡物被移开，新生儿大脑皮质视觉区的神经细胞也不会对这只眼睛传回的视觉信号发生反应。也就是说，这只健康的眼睛弱视了。
日本科学家此次使用强大的双光子激发钙成像技术，对大脑皮质视觉区的两种神经元——抑制性神经元和兴奋性神经元分开来进行观察。结果显示，即使临界期结束，实验鼠的大脑仍然具有可塑性，而不是完全消失了。
科学家选择出生后第50天的实验鼠进行实验。这些实验鼠已经过了临界期。研究人员把3只实验鼠的右眼遮挡起来饲养，另外3只实验鼠正常饲养，7天后比较大脑皮质视觉区的抑制性神经元和兴奋性神经元对实验鼠双眼的反应情况。另外，他们选择了出生后第27天的实验鼠进行对比实验，同样是把3只实验鼠的右眼遮挡起来饲养，另外3只实验鼠正常饲养，两天后比较大脑皮质视觉区的抑制性神经元和兴奋性神经元对双眼反应情况。出生后第27天的实验鼠，大脑临界期仍未结束。
对比结果显示，在临界期内，不论抑制性神经元还是兴奋性神经元都具有可塑性。过了临界期，只有抑制性神经元保持着可塑性。
公报说，本项研究发现，即便大脑过了临界期，其视觉区中依然有部分神经细胞群保持可塑性，而大脑的其他领域或许也存在同样的现象。这一成果不仅将修改临界期概念，而且为研究婴幼儿早教、成年人教育提供了重要线索。
日本科学家的论文发表在最新一期美国《神经科学杂志》上。（生物谷Bioon.com）
生物谷推荐原始出处：
The Journal of Neuroscience, January 27, 2010, 30(4):1551-1559; doi:10.1523/JNEUROSCI.5025-09.2010
Difference in Binocularity and Ocular Dominance Plasticity between GABAergic and Excitatory Cortical Neurons
Katsuro Kameyama,1,2 * Kazuhiro Sohya,1 * Teppei Ebina,1,3 Atsuo Fukuda,4 Yuchio Yanagawa,5,6 and Tadaharu Tsumoto1
1Brain Science Institute, RIKEN, Wako 351-0198 Japan, 2Tottori University Graduate School of Medical Sciences, Yonago 683-8503 Japan, 3Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei 184-8588 Japan, 4Department of Physiology, Hamamatsu University School of Medicine, Hamamatsu 431-3192 Japan, 5Gunma University Graduate School of Medicine, Maebashi 371-8511 Japan, and 6Japan Science and Technology Agency, Core Research for Evolutional Science and Technology, Tokyo 102-0075 Japan
Neuronal circuits in the cerebral cortex consist mainly of glutamatergic/excitatory and GABAergic/inhibitory neurons. In the visual cortex, the binocular responsiveness of neurons is modified by monocular visual deprivation during the critical period of postnatal development. Although GABAergic neurons are considered to play a key role in the expression of the critical period, it is not known whether their binocular responsiveness and ocular dominance plasticity are different from those of excitatory neurons. Recently, the end of the critical period was found to be not strict so that cortical neurons in the adult still have some ocular dominance plasticity. It is not known, however, which type of neurons or both maintain such plasticity in adulthood. To address these issues, we applied in vivo two-photon functional Ca2+ imaging to transgenic mice whose GABAergic neurons express a yellow fluorescent protein called Venus. We found that GABAergic neurons are more binocular than excitatory neurons in the normal visual cortex, and both types of neurons show the same degree of modifiability to monocular visual deprivation during the critical period, but the modifiability of GABAergic neurons is stronger than that of excitatory neurons after the end of the critical period.
