temporal lobes and gray matter are very important brain areas.

The brain areas are critical in the sense of....

这二个句子很像是讲解定语从句的构成的。以上二个句子合并为一个句子，用关系代词替换the brain areas, 则得到：

Temporal lobes and gray matter are very important brain areas,  which are critical in the sense of....

这种讲解定语从句构成的句子，通常用定冠词the。

如果不是讲解定语从句的构成，则the brain areas应该用and they are..., 而不是重复名词。英语最忌讳的就是无谓的重复。