Detection of abnormal cerebral excitability by coincident stimulation and recording

Objective: A method for mapping brain excitability and detecting abnormalities, by concurrently stimulating and recording ‘focal’ compound responses through one microelectrode, was evaluated in three rat epilepsy models in comparison with distal stimulation of perforant path afferents. Methods: A fixed trajectory from neocortex to dentate gyrus was mapped under halothane anesthesia. Several weeks earlier, tetanus toxin or vehicle was microinjected into the dentate polymorphic layer, or else rats were genetically epilepsy-prone (GEPR-9) or epilepsy-resistant (GERR-0). Other (unmapped) rats received acute penicillin microinjections within the dentate granular layer. Results: Focal responses, although widespread, proved largest in the dentate (>±0.5 mV). Tetanus toxin diminished focal responses near the microinjection site versus vehicle-microinjected (66%) or contralateral controls (55%), but enhanced them elsewhere in the dentate. It enhanced distal responses at all hippocampal locations. Focal but not distal responses were higher in GEPR-9 than in GERR-0 rats at widespread forebrain locations (mean 233%). Penicillin facilitated both focal and distal dentate responses, but the focal facilitation peaked sooner (about 75 versus 180 min). Conclusions: Focal responses better uncover pervasive or discrete excitability differences. Significance: Focal mapping may aid in diagnostic imaging and intraoperative targeting, offering high resolution, rapid performance, low stimulus currents and minimal invasion.