Neurons in human epileptic cortex: Correlation between unit and EEG activity

A total of 90 neurons were recorded extracellularly from 17 awake patients undergoing craniotomy for excision of epileptogenic cortex. Relationships between single-unit activity and gross epileptiform spikes recorded locally by the microelectrode or from the immediate overlying cortical surface by electrocorticography (ECoG) were examined. Similar relationships were also sought between interictal bursts from nearby cells when action potentials from several neurons had been recorded simultaneously by the tungsten electrodes. Although 40 single units fired action potentials in some relation to ECoG spikes, the relationships were variable between units and, often, for the same unit. For many units, action potentials were more consistently related to one phase of the local field potential recorded through tungsten microelectrodes than to the ECoG recorded from the overlying cortical surface. Synchronous firing between single units recorded simultaneously by the same microelectrode was rarely seen except at the onset of an ictal event. In addition, a high degree of synchrony between unit firing and local ECoG spikes was recorded in a few patients, but these patients had frequent focal spontaneous seizures. The data imply that in human epilepsy, unlike some animal models of the disorder, relationships between surface epileptiform events and single-unit burst firing are not easily found in interictal recordings. The data also suggest that synchrony between unit and surface events requires a high degree of synchrony among neurons within the epileptogenic focus.