Central pathway of photic reflex myoclonus.

Direct, new evidence for the cortical origin of photic reflex myoclonus in a patient with "posterior cortical atrophy" is provided. Photic stimulation elicited myoclonic jerks in the right upper limb muscles. An H2(15)O-PET activation study with photic stimulation showed increased regional cerebral blood flow not only in both striate cortices but in the left premotor and primary motor areas as well. Transcranial magnetic stimulation over the area of the left occipital cortex elicited motor evoked potentials in the right upper limb muscles. It is concluded that in the central pathway of photic reflex myoclonus the contralateral occipital cortex is activated first, then the impulses propagate intrahemispherically to the primary motor cortex, to elicit myoclonic jerks.