Figure 6. Early and late antidromic responses to spinal cord stimulation.

A, a cortical response comprising two components was seen following spinal cord stimulation at 40 μA (average of 120 sweeps). B, both responses followed each of a train of stimuli delivered at 250 Hz (average of 50 sweeps). C, both responses collided completely with a PT stimulus at 2.2 ms, but only the late one collided at 2.4 ms (40 sweeps per interstimulus interval). For clarity, the response to PT stimulation alone, including a large antidromic volley, has been subtracted from each trace. The waveform that can be seen preceding spinal stimulation reflects the incomplete cancellation of large PT stimulation artefacts. Although this residual is slightly different in each trace, the actual PT response (before subtraction) was consistent across all conditions. D, the long latency component was facilitated by a PT stimulus preceding spinal stimulation by 15 ms, whilst the short latency component was unaffected. E, modulation of the early and late responses by a conditioning PT stimulus at different interstimulus intervals (40 sweeps per interstimulus interval).