What do brain damaged patients, adults saying tongue twisters and all children have in common? They're more likely to make speech errors than an adult who is speaking under normal circumstances.
A common type of speech error in children occurs when one sound persists and interferes with the rest of their phrase. For example, if a child wanted to say the "silly dog," it might come out the "silly sog."
Young speakers often get stuck on a single sound when what they have just said intrudes on what they're currently saying.
Adult speakers, on the other hand, make far fewer errors and the ones they do make tend to be anticipatory; for these speakers, what they are about to say intrudes on what they're currently saying.
When adults make this kind of speech error, they would say "dilly dog" instead of "silly dog."
Suppress What We've Said
The difference between these two mistakes may seem unimportant to a lay person, but to a cognitive psychologist it is crucial for understanding how language development occurs.
In order for us to construct meaningful language, we must learn how to suppress things we've already said. In addition, we must learn how to activate in memory what we are currently trying to say and what we are about to say.
Children haven't learned how to suppress in memory what they've just said or how to activate in memory what they're currently saying. A brain-damaged patient may make similar errors if the part of his or her brain responsible for this kind of memory suppression and activation has been harmed.
And an adult repeating a tongue twister might make this kind of error because the brain is processing so much unusual information that it temporarily loses its ability to perform these activating and suppressing activities.