About 15–20 per cent. of cases of head injury of moderate or severe degree are followed by persisting intellectual impairment, causing loss of efficiency often increased by psychoneurotic symptoms arising from such incapacity. Deterioration of powers of sustained attention, of recent memory and of emotional control are characteristic of post-traumatic states after severe head injury, but ignorance of the patient's previous capacity and personality makes it difficult to assess psychological and physical factors—to distinguish between the patient who is ineffective because he is neurotic and the one who is neurotic because he is ineffective—as a result of cerebral damage. Therefore any reliable index of damage to the “matrix of the mind” will be helpful in prognosis and in deciding disposal.