The mirror focus and secondary epileptogenesis

Abstract

Morrell coined the term secondary epileptogenesis and subsequently referred to it as the mirror focus. He demonstrated that following the creation of an epileptic primary focus, an independent epileptic focus can develop in the homotopic area of the contralateral hemisphere—that is, that region to which the primary focus sends a direct synaptic projection. In its early stages of development, this secondary epileptic zone is dependent on the integrity of the corpus callosum and subcortical connections. When in the mirror focus (“matures”), abolition of the primary focus, or section of the corpus callosum and/or interruption of subcortical connections, does not abolish the secondary epileptic zone. Thus, for the development of the mirror focus both callosal direct and subcortical polysynaptic connections are necessary; interruption of either during the formation of the mirror focus prevents its development. The secondary epileptic or mirror focus is a “pure culture” of epileptic neurons, uncontaminated by the agent used for creating the primary lesion or by the kindling electrode introduced into the primary site. In addition to serving as a model of epileptogenesis, the development of a secondary mirror focus has been used to investigate underlying mechanisms of information transfer and storage. Morrell devoted much of his research to the study of the synthesis of macromolecules (RNA and DNA) and the lasting changes in synaptic organization that occur during the development of secondary epileptic foci.