Neural Recordings

A 16-channel Eckhorn microdrive (Thomas Recording, Giessen, Germany) was used to make up to 14 simultaneous microelectrode penetrations into M1 during daily recording sessions (average number of electrodes used per session: 9, range: 3–14). Electrodes were platinum insulated with quartz glass and had a shaft diameter of 80 μm and impedance of 1–2 MΩ (Thomas Recording). Cells were identified as corticospinal if they responded at a constant latency to stimulation through the chronically implanted PT electrodes (up to 400 μA, 0.2-ms pulse, 1 Hz) and if the evoked spikes could be collided by orthodromic spikes occurring shortly before the stimulus. Single-unit activity (band pass, 300 Hz to 10 kHz, sampled at 25 kHz) was recorded while the animal performed the task, together with lever displacement, force, and EMG activity (band pass, 30 Hz to 2 kHz, sampled at 5 kHz). Off-line, action potential waveforms were discriminated to generate the occurrence times of single spikes with custom-written cluster-cutting software (Baker et al. 1998; Dyball and Bhumbra 2003). Only single units with a consistent spike waveform and no interspike intervals < 1 ms were used in subsequent analysis.

The hand representation of M1 was identified by multiple-pulse stimulation (13 biphasic stimuli, 0.2 ms per phase, 300 Hz train frequency, 1 Hz repetition rate) through the recording electrodes and visual observation of muscle twitches at low (<20 μA) current intensities.