Lack of correlation between neuronal hyperexcitability and electrocorticographic responsiveness in epileptogenic human neocortex

Object. The purpose of this study was to determine whether intrinsic neuron al properties and synaptic responses differed between interictally active a nd inactive tissue removed in neocortical resections from patients undergoi ng surgical treatment for epilepsy. Methods. Whole-cell patch recordings were performed in layer 2 or 3 and lay er 5 pyramidal cells in neocortical slices obtained from tissue surgically removed from patients for the treatment of medically intractable seizures. Synaptic responses to stimulation at the layer 6-white matter border were u sed to classify cells as nonbursting if they responded with only a single a ction potential for all above-threshold stimuli (80%). These responses were usually followed by biphasic inhibitory postsynaptic potentials (IPSPs). C ells were classified as bursting if they fired at least three action potent ials in response to synaptic stimulation (20%). These cells typically showe d no IPSPs and responded in either an all-or-nothing or graded fashion. App roximately twice as many cells at layer 2 or 3 (29%) than cells at layer 5 (14%) fired synaptic bursts. Synaptic bursting was not associated with an a lteration in a cell's response properties to gamma-aminobutyric acid. It wa s notable that, in tissue samples determined by electrocorticography (ECoG) to be either interictally active or not active, the proportion of cells th at burst was exactly the same in both groups (24%). We found no cells with intrinsic burst firing. Conclusions. We conclude that synaptic bursting was characteristic of a sma ll proportion of cells from epileptic tissue; however, this did not correla te with interictal spikes on ECoG.