THE DYNAMICS OF CEREBRAL-DOMINANCE DURING UNILATERAL LIMBIC SEIZURES

An apparent discrepancy exists between the remarkable language compete nce of the isolated right cerebral hemisphere in some split-brain pati ents and the lack of language competence in some aphasic patients in w hom only the left cerebral hemisphere has been damaged. This has led t o a revival of the idea that the dominant cerebral hemisphere inhibits the potential functioning of its partner. We tested this model of int erhemispheric inhibition in four patients with unilateral limbic seizu res (three left-sided, one right-sided). While seizure activity was mo nitored with bilaterally implanted stereotactic EEG depth electrodes o r bilateral foramen ovale EEG electrodes, the patients were tested in lateralized hemisphere-specific tachistoscopic-recognition experiments . In two patients performance was correlated with the electrical patte rn during prolonged unilateral subclinical limbic status epilepticus, and in two patients performance and intracranial electrical activity w ere compared before, immediately after and during the recovery phase o f left limbic complex partial seizures with postictal aphasia. Three m ain findings were obtained (i) Focal unilateral limbic seizure activit y in the depth interferes with cognitive functions, even when unnotice d clinically or undetectable on the surface EEG, provided that epilept iform EEG activity lasts for several seconds. (ii) This interference i s specific to the cognitive functions associated with the 'dominant' p rocessing of the discharging hemisphere. (iii) Seizure-induced impairm ent of the 'dominant' functions of the discharging hemisphere is assoc iated with improvement of performance of the same function by the othe r hemisphere. Although each patient's clinical and experimental situat ion differed, the converging results support the hypothesis of hemisph ere interaction based upon functional inhibition for verbal processing in patients with a left hemispheric focus. The reverse situation, nam ely facial processing in a single case with a right hemisphere focus, remains inconclusive.