Thought blocking is a neuropsychological symptom expressing a sudden and involuntary silence within a speech, and eventually an abrupt switch to another topic.[1] Persons undergoing thought blocking may utter incomprehensible speech; they may also repeat words involuntarily or make up new words.[citation needed] The main causes of thought blocking are schizophrenia, anxiety disorders, petit mal seizures, post-traumatic stress disorder, bradyphrenia, aphasia, dementia and delirium.[2]