Two important parameters for Symbols are Retention and Retrieval.
When we hear words in running discourse, despite the need for the brain to search it's store of retained words (around 100,000) for each match, recognition seems instantaneous.
There is furious activity, however, in the brain as the processes of retrieval occurs with lightning speed. Listening is a very active and fatiguing activity. It requires as much attention and care, if not more, than weight lifting.
It is interesting to note, that neurons require more protein than muscle fibers to function effectively.
Like any process, the symbolic processes of retrieval can be disrupted. When this happens, we may be unable to substitute symbols for our thoughts, or derive meaning from symbols we receive.
This is a condition called aphasia.