Schizophrenia affects about 1% of people all over the world. The first symptoms typically appear in a person's mid-twenties, and many people never fully recover. Worse still, we know almost nothing about schizophrenia's causes.
On the one hand, there is strong evidence that schizophrenia has a biological basis, because it runs in families, which indicates a genetic component. There are also subtle abnormalities in brain structure. Treatment with drugs, particularly those that target the neurotransmitter dopamine, can reduce the symptoms, but the mechanism of this effect is unknown and unfortunate side effects can and do occur.
On the other hand, the characteristic symptoms of schizophrenia are firmly in the domain of the mind. Patients report hallucinations (false perceptions) and delusions (false beliefs). A patient may hear his own thoughts spoken aloud or hear voices discussing him. A patient may believe that alien forces are controlling his or her actions or inserting thoughts into his or her mind. The challenge for the neuropsychologist is to show how a disorder in the brain can lead to these bizarre experiences. My own starting point for understanding schizophrenia is the observation that, in some cases, the "voices" that patients hear are clearly their own. This observation puts the problem in a slightly different light: the question is not why patients hear voices, but why they mistake their own voice for that of someone else. This question applies to other symptoms as well. For example, patients with delusions of control report that their movements are alien; they feel as if they were being made by someone else. This is not as startling as it may at first appear. After all, every action we perform causes changes in our sensations. When we speak, we hear the sound of our own voice. When we move our arm, there are changes in kinesthetic and tactile sensations. But there is nothing in the nature of these sensations that distinguishes them from signals caused by external events - the sound of someone else's voice, someone else lifting our arm.
Normally, we have no trouble recognizing whether sensations are caused by our own actions, because those that are can be predicted. Based on the motor commands that we send to our muscles, we can predict precisely the sensations that the action will cause. This is why we can't tickle ourselves. If we stroke our left palm with our right hand the sensation is minimal. But the feeling is very intense if another person or a robot applies the stimulation.
The same phenomenon can be observed in the brain using functional magnetic resonance imaging (FMRI). Stroking the palm causes activity in the part of the brain - the parietal lobe - that processes tactile stimulation. But there is far less brain activity when the person being scanned strokes her own palm. This mechanism, called corollary discharge or reafference, reduces our experience of the sensations that we cause by our own actions. This enables us to identify the more important changes in sensation that are caused by external events and to distinguish them from sensations caused by our own actions.
If this mechanism didn't work properly, we might falsely attribute sensations caused by our own actions to something happening in the outside world over which we have no control. This hypothesis can be tested experimentally. In terms of experience, patients who report that their movements feel alien should be more aware of the sensations caused by their own actions. They should, for example, be able to tickle themselves.
In fact, we have found precisely that: patients with delusions of control report that the sensations that occur when they stroke their own palm are just as intense as the sensations caused when someone else strokes their palm. In terms of physiology, these patients show abnormally high activity in the parietal lobe when making movements that feel alien. Their physiological responses to self-generated sensations are not being reduced. Similar observations have been made for the auditory system. Activity in the part of the brain concerned with sound - the temporal lobe - is reduced when we hear the sound of our own voice while we are speaking. Patients who experience auditory hallucinations do not show this reduction in activity to the sound of their own voice. As yet, we know very little about the mechanisms that allow us to predict the sensations caused by our own actions. The part of the brain that generates actions - the frontal cortex - must be sending signals to regions of the brain where sensations are processed, but we know very little about the nature of these signals or the precise pathways involved.
But we can measure the strength of the connections between brain regions, and the strength of these connections seems to be reduced in patients with schizophrenia. So perhaps the key to understanding the symptoms of schizophrenia at a physiological level will come from studying connections between different brain regions.
There is still much work to be done, both to comprehend schizophrenia and develop more effective treatments. But at least we now have a promising framework for understanding how bizarre experiences in the mind can be linked to abnormal processes in the brain.