The effect of hypnotic anaesthesia on cortical responses

The phenomenon of hypnotic anaesthesia has been well established for over a century, since Esdaile (1846) published his original account of 73 surgical operations carried out painlessly under hypnosis, and the technique has established a modest, but definite, place in contemporary medicine and dentistry (Mason, 1960, 1964). In view of this, it is surprising how little is known of the physiology of the hypnotic state. Studies of the E.E.G. during hypnosis, reviewed by Ellingson (1956) and by Barber (1961), suggest that there is no specific change in the background activity, the record showing a normal waking pattern unless the subject actually appeared to go to sleep or was put to sleep by suggestion, when a typical sleep record supervened. There have been several investigations of the physiological responses to painful stimuli under hypnosis, recording such measures as changes in the pulse rate and respiration, vasomotor responses, or facial flinching. The literature has been reviewed by Gorton (1949) and Barber (1961), but this work and some other papers (Das, 1958; Black and Wigan, 1961; Black, Edholm, Fox, and Kidd, 1960; Barber and Hahn, 1962) have added little to our understanding of the mode of action of hypnotic suggestion in producing a loss of sensibility, although they substantiate the existence of differences in the objective responses to 'painful' stimulation in the hypnotic state. Dawson (1958b) suggested that the loss of sensation under hypnosis might be associated with blocking or gross attenuation of the afferent sensory volley before it reached the cortex. There are centrifugal fibres going from the cortex to the synapses in the sensory pathways and it is known that a conditioning stimulus to the cortex can greatly reduce the size of the post-synaptic response to a test stimulus, given peripherally. Dawson himself, for instance (1958a), recording the post-synaptic response to a stimulus to the forepaw of the rat, found that it could be reduced to 50% of its original size by a

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