The Spanish Journal of Psychology has an interesting English language article [pdf] on the neuropsychology of private thoughts – still one of the most mysterious and poorly understood aspects of our mental life.
Neuropsychology is especially good at looking at how differences in brain function relate to objectively observable behaviour. Private thoughts are quite hard to study in this way, because they are essentially subjective.
Sometimes, of course, we make our private thoughts ‘public’ by talking to ourselves, and, it seems, this is something we learn to do during childhood.
Infants seem unable to ‘think to themselves’ and instead ‘talk to themselves‘ when solving problems, usually vocalising the most tricky or novel aspects of the situation. As we grow, we develop the ability to internalise this speech, and can eventually have a purely internal monologue.
Understanding inner speech is also important because it becomes distorted in psychotic disorders such as schizophrenia.
People with psychosis can experience effects like ‘thought insertion’, where they experience external thoughts being inserted into their stream of consciousness, or ‘thought withdrawal’, where thoughts seem to be removed from the mind.
This suggests that there must be something that the brain uses to identify thoughts as self-generated, and that this perhaps breaks down in psychosis, so we can have the uncanny experience of having thoughts that don’t seem to be our own.
Why we would need this is an interesting question, as surely all thoughts would be our own.
However, the Spanish Journal of Psychology article notes that inner speech often activates areas of the brain also used for ‘outloud speech’, suggesting that it may be a sort of internal action.
Being able to distinguish bodily movements caused by something external (someone moving your arm) and movements caused by our own will is very important, and, perhaps, this is the sort of mechanism that becomes disturbed for what were originally movements, but have become internalised as we ‘think to ourselves’.