Interactive reportExplaining the symptoms of schizophrenia: Abnormalities in the awareness of action
Section snippets
Contrasting delusions of control and the anarchic hand sign
Rather than attempting to elucidate a biological basis for schizophrenia, our aim in this essay is to try and explain one symptom or class of symptoms. We shall concentrate on delusions of control. This is an example of a passivity experience in which a patient feels that his own actions are being created, not by himself, but by some outside force. The actions in question can be very trivial, such as picking up a cup or combing one's hair. Other examples of passivity include thoughts or
Explaining symptoms in cognitive, physiological and experiential terms
In order to understand symptoms like delusions of control we require an explanation at least three levels. First, at the cognitive level, we must understand how the symptom arises in terms of a model of motor control that can be applied to normal and abnormal cases and which makes a distinction between those aspects of motor control which reach awareness and those which do not. Second, at the physiological level we need to consider how the cognitive components of the model relate to underlying
Control of action and awareness of action
The motor system can be considered a control system in which the input is the motor command that produces a movement and the output is the sensory consequence of that movement (see Fig. 1). In order to produce a goal-directed movement the system must be able to estimate its current state (e.g. the current position of a limb) and must also represent its goal (e.g. the desired position of the limb). On the basis of these two representations the system can compute a sequence of motor commands that
The cognitive basis of the anarchic hand sign
In a patient with the anarchic hand sign the movements of the hand are no longer determined by the goals of the patient, but solely by the current context. The appearance of a door knob in the patient's line of vision is sufficient to elicit a reaching and grasping movement. Such movements are normally inhibited because they are incompatible with current goals. These inhibitory signals are no longer transmitted to the cortical regions controlling the movement of the hand because of damage to
The cognitive basis of delusions of control
In the patient with delusions of control something is wrong with that part of the motor system concerned with the generation of a forward model and the representation of the predicted state of the system. Since the rest of the system remains intact, the patient can represent the desired state, can calculate and carry out the motor commands required to achieve the desired state, and can check that the desired state has been reached (see Fig. 2).
However, in the absence of an awareness of the
Behavioural evidence
The anarchic hand sign is directly observable. The behaviour of the anarchic hand can be studied, as can the strategies adopted by the patient to cope with this aberrant behaviour. This is not the case with delusions of control. There is nothing obviously abnormal about the patient's actions. It is only from the patient's report of his anomalous experiences that we know that there is something wrong with the motor system. A major advantage of specifying the disorder in cognitive terms is that
The physiological under pinnings of motor control
At present we have only a very crude idea of how the motor control system illustrated in Fig. 1 can be mapped onto the central nervous system. Most of this knowledge comes from the study of neurological patients with circumscribed lesions. From such studies we know that prefrontal cortex has a major role in the formulation of goals and plans, see e.g. 20, 30, and that medial premotor cortex (supplementary motor cortex, SMA) has a role in developing appropriate sequences of motor commands and
Physiological abnormalities associated with delusions of control
If patients with delusions of control fail to predict the sensory consequences of their actions, then, at the physiological level, we would expect to see over-activity in regions concerned with analysis of the relevant sensations. Spence et al. [31]scanned patients with delusions of control while they performed a simple motor task in which they were required to move a joy stick in one of four directions, chosen at random, in time with a pacing tone. In comparison to a control task involving
Understanding delusions of control
At the cognitive level we suggest that delusions of control arise because of a failure to form a representation of the predicted consequences of an action. In addition to an abnormal experience of the control of movements this leads to measurable behavioural consequences in tasks involving mental practice, error correction and memory for actions.
At the physiological level we suggest that delusions of control arise because of a disconnection between frontal brain regions where actions are
Acknowledgements
This work was supported by the Wellcome Trust. S.-J. Blakemore is supported by a Wellcome Trust four-year PhD programme in neuroscience at University College London.
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