Researchershaveestablishedthatwhenpeoplearementally,bio-chemicalchangesoccurinthebrain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!

　　Researchershaveestablishedthatwhenpeoplearementally,bio-chemicalchangesoccurinthebrain

　　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!

　　Researchershaveestablishedthatwhenpeoplearementallyengaged，bio-chemicalchangesoccurinthebrainthatallowittoactmoreeffeetivelyincognitive（认知的）areassuchasattentionandmemory.不好意思刚才句子不完整，请大家帮我分析一下，谢谢！