究竟什么导致他们被绑架，至今仍不清楚。
研究人员至今仍不清楚大脑的极限、做梦的原因以及梦的含义。
Researchers are still not sure what our brains are up to, why we dream, or what those dreams even mean.
尽管进行了大量的临床和实验研究，但其确切机理至今仍不清楚。
Despite a number of clinical and experimental studies, the pathogenesis of NPE is not entirely understood yet.
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