Sometimes, very large parts of the person may not be able to be articulated. But sometimes it is very specific. Patients become silent. I think this is quite likely to represent an aspect of the person's being where caretakers have not been able to hold the emotional experience, so that the subsequent articulation and expression of it hasn't developed. I've come to believe that it is sometimes useful to actively work with creative media.
Words are easier, to be sure.
Also, I think this connects with some of the work of neurophysiologists and traumatologists. Thought isn't really involved at that level. This apparently can be measured through scanning technology. It is hard to share experience at this level, particularly with words. But it shows up quite clearly in behavior.