I know the title seems insane, and this is most likely impossible, but is there a mental illness that almost stops a person from thinking? By thinking I mean the "talking in your head" thing. I'm not looking for a mental illness that makes them unconscious, but rather a mental illness that keeps them conscious and almost stops them from thinking. I have been saying this a few times, but I feel like it's false, since thinking is a person can't stop thinking if they are alive and conscious(right?). A more realistic question is: "is there a mental illness that causes one's thinking abilities to severely decline?"
Thinking is mainly supported by the frontal lobes (although the ARAS – the Ascending Reticular Activating System – is essential for its arousal and other cortical areas are essential for its sensory inputs and other associative functions, such as language, spatial orientation, and emotion, which are closely associated with thinking). So, anything that affects the frontal lobes can affect the thinking.
But frontal lobes never stops function if one is still conscious – neurons in the frontal lobes will keep firing as long as there’s a stimulation from the ARAS. So, to literally stop thinking (or all neurons in the frontal lobes stop firing) in still-conscious people is impossible.
However, frontal lobes can be interfered to the degree that the thinking is significantly impaired or retarded and the person seems to stop thinking. This can happen in a varieties of conditions, such as:
Psychological: severe emotional overload (e.g., by a sudden loss of something so precious, an unexpected devastating event, or an overwhelming rage), schizophrenia in some stages (e.g. catatonic stage), severe depression
Pharmacological: overdosage of antipsychotic, sedative, or other psychoactive drugs
- all metalbolic derangements (hypoglycemia, hyponatremia, hypercalcemia, etc.)
- some medical conditions (hypothyroidism, hypocorticolism, hypothermia, etc.)
- frontal lobes disorders: frontal lobe tumor, infarction, hemorrhage, encephalits, dementing diseases
- other causes: post- cerebral contusion, post-ictal stage
At some stages of the above conditions, either before the patients lapse into coma or after they emerge from coma, the patients will appear dazed, muted, unable to think, and unable to respond. That is, apparently, they seem to stop thinking in these conditions.
Thought blocking is a common phenomenon in schizophrenia and other psychotic disorders. The patient experience a short stopping of all thoughts, they report that all thoughts disappear for a short while (seconds, rarely more than 20-30 seconds or so in my experience). This has been described in the psychopathological literature at least since the publication of Fish's Clinical Psychopathology in 1967, but presumably a lot earlier.
I'm not aware of other psychiatric or otherwise pathological conditions in which patients remain conscious and report the loss of thoughts. There are some conditions, as listed by user287279 above, and while it's plausible that there might be a loss of verbal thinking in severe cases of some of them, I have never heard patients report this, neither during the condition nor after recovery.
Consciousness is mostly affective. Affective consciousness and emotional response (albeit changed) is preserved in dementia. Cognitive tasks progressively become nearly impossible. Thoughts, if they arise, become unpronounceable or impossible to communicate. In other psychiatric disorders that disrupt thinking - such as major psychoses - some kind of thinking is preserved, even grandiose or very persecutory delusions have some internal logic and mostly can be communicated.