Cerebral evoked potentials in patients with dissociated sensory loss.

After stimulation of a peripheral nerve in man a small evoked response can be recorded from a surface electrode placed over the contralateral postcentral gyrus, as was first shown by Dawson (1947a). In normal healthy subjects the response is only a few microvolts in size, and its detection is uncertain unless use is made of a summation technique of the kind which Dawson himself subsequently introduced (Dawson, 1954). Recorded in this way, however , a clear response can be obtained from any normal healthy subject. It has a latency, constant to within a few milliseconds, characteristic for the site of stimulation employed. Furthermore, while quite marked individual variations may occur in the amplitude and wave-form recorded from different subjects, the early components of the response appear to change minimally in the same subject from time to time, provided that the conditions of the experiment are not materially altered. Not many investigations of the effect of neuro-logical disease on this cortical response have so far been published. Dawson (1947b) showed that in a case of myoclonic epilepsy the amplitude of the potential was five to 10 times larger than normal but that the latency and duration of the response was the same as that in healthy subjects. Alajouanine, Scherrer, Barbizet, Calvet, and Verley (1958) were able to record the responses in 26 out of a series of 50 patients suffering from disturbances of somatic sensation due to a large variety of causes. These included 10 cases with cortical lesions (two with a thalamic syndrome), five cases with lesions of brain-stem or cord, three with root lesions, one case of congenital insensitivity to pain, and seven cases of hysterical hemianaesthesia. The limitations of their recording technique did not allow them to study the latency, wave-form, or distribution of the response and they were concerned only with amplitude changes. They reported no cases in which the potential was larger than normal and found it to be of normal amplitude in the patients with hysterical anaesthesia or congenital indifference to pain. In 'Member of the external staff of the Medical Research Council "In receipt of a grant from the Tnstitute of Neurology the other cases it was apparently unaffected if the sensory impairment was mild, but was either absent or appeared only with stronger stimulation than normal if the sensory loss was severe; this was so whether the lesion was central or peripheral. They …