384 CHAPTER 11 A Novel Application of a Brain-Machine prediction exceeded the noise floor by approximately two standard deviations to the stimulus threshold, and the peak EMG prediction to the spillover point or 200 µs, whichever was smaller. Exceptions to these rules occurred when, during a closed-loop experi- ment, muscle fatigue significantly affected task performance. To compensate for the decreasing stimulation-evoked muscle force, we often increased the stimulation frequency to 30 Hz and, if needed, increased the currents by 25% or 50%. 11.3.8 Real-Time Control Online sorting was performed to discriminate action potential waveforms based on the waveform clustering in principal component feature space. We made some effort to identify waveforms that appeared to result from single neurons, based on standard criteria: well-defined waveform clusters and a minimum inter-spike interval greater than 1.5 ms. However, the over- lapping activity of well-modulated multiple units recorded from a single