that 不可能引导非限制性定语从句，那是 which 的活儿。

本句，that 定语从句的先行词是paths 。因为插入了 and even entirely new brain cells, 前后加了两个逗号。去掉后，逗号自然就没了。

But brain researchers have discovered that when we consciously develop new habits, we create parallel synaptic paths that can jump our trains of thought onto new, innovative tracks.