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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.

The use of somatosensory evoked potentials (SEPs) for determining spinal cord integrity in a clinical setting is well known. The poor signal-to-noise (SNR) ratio of such measurements has led to ensemble averaging being employed to extract the signal from the background noise. Over the thoracic region, the poor SNR is largely the result of interference generated by the cardiac musculature. Therefore, any reduction in the level of this cardiac interference will greatly improve the performance of any SEP monitoring system. Three methods were investigated as techniques to reduce the cardiac interference in SEP measurements. These were clipping, gating and adaptive noise cancellation (ANC). It was found that, although clipping and gating performed as well as, if not better than, ANC, these techniques were both very sensitive to the setting of a threshold level. The linear ANC scheme employed circumvented the problem associated with a threshold level, and with an order of 50, SNR improvement figures of approximately sixfold were achievable over the entire thoracic region.

In some clinical centres, somatosensory evoked potentials are used for the assessment of neurological function during surgical procedures on the spine. As these potentials are heavily contaminated in background noise, ensemble averaging coupled with invasive instrumentation is routinely used to enhance the signal. However, this procedure is very time consuming, often taking several minutes. In this paper, an adaptive matched filter has been used to dramatically reduce this measurement time to around 20 seconds, even when employing non-invasive surface electrodes. This filter has been implemented in real-time by using a TMS320C25 digital signal processor and results are presented for signal acquisition both at the L1-T12 and T5-T6 spinal levels. In addition, the adaptive nature of this filter allows the tracking of slowly changing parameters within the evoked potential with time.