A custom designed system to measure corticospinal tract jitter

Typical latency of an individual limb muscle response to magnetic or electric stimulation of the human cortex is in the range of 10-50 ms. For the latency variability, i.e., jitter studies, a resolution of at least 20 micros is needed. Commercially available EMG equipment needs custom-designed upgrading to allow for such studies. Two solutions were designed: (i) a hardware unit allowing an adjustable delay of data acquisition after the delivered stimuli; and (ii) diverting of the amplified biological signal and the EMG equipment trigger to an external computer equipped with an analogue-to-digital conversion (ADC) module. Custom-designed software made fast ADC possible during the whole period of data acquisition. Both concepts were applied to a Vickers Medical Mystro electromyograph, and have been successfully used in the Ljubljana (Slovenia) Institute of Clinical Neurophysiology for the last 2 years.