Influence of recording instrumentation on the stimulus artifact tail in the surface acquisition of somatosensory evoked potentials.

Surface recorded somatosensory evoked potentials (SEPs) are neural signals elicited by an external stimulus. In the case of electrically induced SEPs, the artifact generated by the stimulation process can severely distort the signal. The artifact is characterized by a large impulse followed by a slowly decaying tail. In some cases, the artifact tail often lasts well into the initiation of the SEP making the determination of absolute latency very difficult. While the literature often states that the recording instrumentation plays a part in the generation of this artifact tail, no firm evidence has ever been presented. In this work, comparisons are made between three instrumentation systems (BJT, JFET and CMOS) with differing input impedances in an attempt to quantify the effects on the artifact tail. The conclusions from this investigation show that there is no significant interaction between the input impedance of the recording instrumentation and the duration of the artifact tail. Each amplifier type produced results with no significant statistical differences. It was also found that while stimulation amplitude has a weak effect on the artifact tail, the greatest contribution to variation has an inter-subject origin. Consequently, it is concluded that the time constant of the artifact tail must originate from other sources that are subject dependent.