When the process of retrieval for concepts fails expressively, we may find that we canÅt think of a thing to say--the, "My mind had gone blank," feeling.
Even normal individuals experience a total blank under heightened stress.
Sometimes when giving a lecture, for example, I lose my train of thought and experience a mental void for what seems like hours. In this lapse I must struggle to draw up a concept to discuss.
This experience, however, even, when pathologically severe, is NOT called agnosia. Remember, Agnosia is a receptive problem.
When it's expressive it is called the, "I can't think of a thing to say!" feeling.
These conceptual thought processes alone (when they work) are skills enough to make humans masters of their world.
But humans take it one step further. They substitute in their minds fragments of sounds or sights for many if not most of these concepts. We call this the symbolic process.