Davis Simon W, Dennis Nancy A, Buchler Norbou G, White Leonard E, Madden David J, Cabeza Roberto
Center for Cognitive Neuroscience, Duke University, PO Box 90999, LSRC Bldg., Room B254, Durham, NC 27708, USA.
Neuroimage. 2009 Jun;46(2):530-41. doi: 10.1016/j.neuroimage.2009.01.068.
Aging is associated with significant white matter deterioration and this deterioration is assumed to be at least partly a consequence of myelin degeneration. The present study investigated specific predictions of the myelodegeneration hypothesis using diffusion tensor tractography. This technique has several advantages over other methods of assessing white matter architecture, including the possibility of isolating individual white matter tracts and measuring effects along the whole extent of each tract. The study yielded three main findings. First, age-related white matter deficits increased gradually from posterior to anterior segments within specific fiber tracts traversing frontal and parietal, but not temporal cortex. This pattern inverts the sequence of myelination during childhood and early development observed in previous studies and lends support to a "last-in-first-out" theory of the white matter health across the lifespan. Second, both the effects of aging on white matter and their impact on cognitive performance were stronger for radial diffusivity (RD) than for axial diffusivity (AD). Given that RD has previously been shown to be more sensitive to myelin integrity than AD, this second finding is also consistent with the myelodegeneration hypothesis. Finally, the effects of aging on select white matter tracts were associated with age difference in specific cognitive functions. Specifically, FA in anterior tracts was shown to be primarily associated with executive tasks and FA in posterior tracts mainly associated with visual memory tasks. Furthermore, these correlations were mirrored in RD, but not AD, suggesting that RD is more sensitive to age-related changes in cognition. Taken together, the results help to clarify how age-related white matter decline impairs cognitive performance.
衰老与显著的白质退化相关，并且这种退化被认为至少部分是髓鞘变性的结果。本研究使用扩散张量纤维束成像技术对髓鞘变性假说的具体预测进行了研究。与其他评估白质结构的方法相比，该技术具有多个优点，包括能够分离单个白质纤维束并测量沿每个纤维束全长的影响。该研究得出了三个主要发现。第一，在穿过额叶和顶叶而非颞叶皮质的特定纤维束中，与年龄相关的白质缺陷从后段到前段逐渐增加。这种模式与先前研究中观察到的儿童期和早期发育过程中的髓鞘形成顺序相反，并支持了关于白质健康在整个生命周期中的“后进先出”理论。第二，衰老对白质的影响及其对认知表现的影响，径向扩散率（RD）比轴向扩散率（AD）更强。鉴于先前已表明RD比AD对髓鞘完整性更敏感，这第二个发现也与髓鞘变性假说一致。最后，衰老对选定白质纤维束的影响与特定认知功能的年龄差异相关。具体而言，前纤维束中的分数各向异性（FA）主要与执行任务相关，而后纤维束中的FA主要与视觉记忆任务相关。此外，这些相关性在RD中有所体现，但在AD中没有，这表明RD对与年龄相关的认知变化更敏感。综上所述，这些结果有助于阐明与年龄相关的白质衰退如何损害认知表现。
