Researchers have established that when people are mentally,bio-chemical changes occur in the brain

题目

Researchers have established that when people are mentally,bio-chemical changes occur in the brain
请问这个句子中brain后的that属于句子的那个成分,还有其后的指代词it 指代的是什么,这个句子该怎么分析成分,谢谢!
Researchers have established that when people are mentally engaged，bio-chemical changes occur in the brain that allow it to act more effeetively in cognitive（认知的）areas such as attention and memory. 不好意思刚才句子不完整，请大家帮我分析一下，谢谢！

答案

研究者们已经确定：当人们精神上(此处少了东西,应该是个形容词),头脑中就会发生生化变化.
Researchers have established that （引导宾语从句,后边全部是宾语从句部分）when people are mentally（是不是漏了什么词,这半句是宾语从句里的状语成分）,bio-chemical changes occur in the brain （宾语从句的主干部分）
另外：没有见到您说的it啊