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The mechanism of mirror movements in two patients was investigated; one with congenital mirror movement, the other with schizencephaly. Transcranial magnetic stimulation on one side elicited motor evoked potentials (MEPs) in their thenar muscles on both sides with almost the same latencies, minimal thresholds, and cortical topographies. During voluntary contraction of the thenar muscle on one side, contralateral transcranial magnetic stimulation induced a silent period not only on the voluntary contraction side but on the mirror movement side and of the same duration. By contrast, ipsilateral transcranial magnetic stimulation elicited MEPs without silent periods in both muscles. With intended unilateral finger movements, an H,'50-PET activation study showed that the regional cerebral blood flow increased predominantly in the contralateral sensorimotor cortex, as seen in normal subjects, although mirror movements occurred.It is considered that the ipsilateral motor cortex plays a major part in the generation of mirror movements, which may be induced through the ipsilateral uncrossed corticospinal tract. (JNeurolNeurosurg Psychiatry 1997;62:629-632)

We evaluated motor cortical excitability of the unaffected hemisphere in three patients with intractable epilepsy who underwent hemispherectomy, using transcranial magnetic stimulation (TMS) and PET. TMS of the unaffected hemisphere evoked motor responses not only in the contralateral muscles but also in the ipsilateral ones in all the patients. A PET study in one patient showed activation of the unaffected motor cortex by movement of either arm. All of these responses were enhanced after the hemispherectomy, probably due to motor cortical disinhibition by transection of the corpus callosum. The PET study also showed postoperative activation of the premotor area of the unaffected hemisphere. These phenomena indicate posthemispherectomy neuroplastic reorganization leading to preservation of the motor function after the operation.

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