The effectiveness of low-frequency stimulation for mapping cortical function

Objective: To establish the efficacy and safety of low-frequency electrical stimulation for cortical brain mapping. Methods: Cortical function was mapped using electrical stimulation in epilepsy patients with chronically implanted intracranial subdural electrodes. Contacts overlying motor, sensory, visual, and language cortex were stimulated at frequencies of 5, 10, and 50 Hz, using current levels ranging from 1 to 17.5 mA for 3-5 s. The current intensity and incidence at which functional alterations and afterdischarges (ADs) occurred were recorded. The modified McNemar test for nonindependent measures was used to analyze the data. Results: 122 electrode contact pairs were electrically stimulated at least two different frequencies in 14 patients. Functional alterations were obtained at all stimulation frequencies (5, 10, and 50 Hz) at generally similar rates. The likelihood of producing an AD correlated with stimulation frequency, and lower-frequency stimulation was less likely to provoke an AD. Higher current intensity was required to induce both functional responses and ADs at low-frequency stimulation than high-frequency stimulation. While overall rates of producing functional changes were similar, differences in functional response with regard to frequency were noted at individual cortical sites. Conclusion: 5- and 10-Hz stimulation are as effective for mapping cortical function as 50-Hz stimulation and produce fewer ADs. We recommend that mapping of cortical function be started with 5-Hz-frequency stimulation. Higher frequencies should be used in suspect cortex if no symptoms or signs are produced with 5-Hz stimulation.