schizophrenia

     Many of the symptoms of schizophrenia (passivity experiences, such as delusions of control) seem to reflect a confusion between changes in sensation caused by actions of the self and changes with external causes. Examples include, hearing my thoughts spoken aloud, experiencing emotions that are not mine, believing that my actions are caused by alien forces. In all these cases the patient is attributing effects generated by the self to external forces.

     Perhaps we should experience surprise, not at the strange ideas of the patients, but at the ease with which the rest of us make the distinction between self and other generated sensation. When an image moves across our retina, how do we know whether this is the world moving across our eyes, or our eyes moving across the world? Helmholtz suggested that an active movement, moving the eyes across the world, will be preceded by a motor command to move the eyes, which does not occur when the world moves in front of our eyes. Information about this motor command, subsequently termed corollary discharge or reafference copy, can be used to  predict the sensory consequences of the eye movement. The predicted and observed outcomes of the movement can be compared. If they match, then the changes were self generated.

     Given this account, it seems plausible that symptoms, such as delusions of control, might occur because something goes wrong with this comparator mechanism. Over last few decades this idea has been confirmed experimentally and become much more sophisticated.

     The first increase in sophistication came from the development of engineering approaches to the study of motor control. In order to overcome the inherent delays in feedback signals associated with movement, the system needs to generate a forward model. The forward model predicts the consequences of implementing motor commands in terms of both kinematics (where and when our hand will arrive) and in terms of sensations (what it will feel like). If we can predict where our hand is going to be after the motor command has been implemented, then we can check whether we have chosen the correct command before we actually make the movement. If we can predict the sensations we are going to feel, then they are unimportant and can be attenuated. Experiments suggest that patients with delusions of control have problems with both the kinematic and sensory aspects of the forward model, but the precise nature of these problems remain to be specified. Recently it has been shown that the problem lies in the precision of the prediction of the sensory consequences of action: the less the precision the greater the severity of delusions of control. As a result of this lack of precision, patients rely more strongly on external cues for agency, leading to better than normal adaptation to external feedback.

     One problem with the proposal that patients with delusions of control fail to compute a forward model is that such a failure would be associated with major problems in motor control. This is clearly not evident in the every-day behaviour of patients with delusions of control. Furthermore, there is evidence that the automatic predictions that occur well below awareness are intact in these patients. The problems arise at a higher cognitive level, in particular in relation to agency: the conscious experience of motor control. In other words, the problem relates, not to the control of actions, but to the patients’ subjective experience of actions.

     Before we can understand the symptoms associated with schizophrenia we need a much better understanding of the nature and mechanisms of conscious experience.


Frith C. Explaining delusions of control: The comparator model 20 years on. Conscious Cogn. 2011

Frith, C.D. & Friston, K.J. False perceptions & false beliefs: Understanding schizophrenia. Pontifical Academy of Sciences, 2013

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