An in vivo technique for 3-dimensional field mapping in the rodent spinal cord

The purpose of this investigation was to develop a reliable method for in vivo mapping of small exogenously applied electrical fields in the mammalian spinal cord. Experiments were performed in vitro to validate the technique in addition to recordings made in vivo on normal and spinal cord injured rats. Sine waves of 1.5-50 microA and 0.5-50 Hz were used for stimulation. Field gradients were measured with a lock-in differential amplifier. Glass recording micro-electrodes were used for in vivo experiments. The accuracy of the technique was found to be highly dependent on the waveform of the current and impedance matching between the recording electrodes. Recordings made in vivo demonstrated that measured fields were linearly related to the intensity of stimulation. Field gradients were not influenced by stimulation frequencies of 5-50 Hz. Estimates were made of the influence of the surgical exposure on the measured gradients. Our results demonstrate that small electrical fields can be accurately mapped in vivo and interpolated throughout a broad range of intensities and frequencies.