Waveforms and Artifacts

Abstract In the practice of clinical neurophysiology, spontaneously generated or stimulus-induced electrical signals from neural tissue in the central, peripheral, and autonomic systems are recorded and displayed as waveforms through digital equipment. Each recorded waveform represents a change over time in the potential difference between two recording points and reflects the summated static or propagating electrical fields generated by or along nerve or muscle structures. If no change occurs in the potential difference, a flat line with no signal is recorded, even if there is a voltage difference between the points. Changes in potential difference are broadly classified as continuous or intermittent. Continuous waveforms are characterized by a relatively smooth, continuously varying appearance. These are most commonly seen in electroencephalogram (EEG) and related studies. Intermittent waveforms are single, discrete events that occur on a flat background. These are typical of those seen in electromyography, such as the waveform of a fibrillation potential or a motor unit potential. However, there is overlap in these patterns such that an EEG can have discrete, intermittent waveforms superimposed on continuous background waveforms (e.g., the spike discharges in epilepsy). Each type of waveform can be characterized by several different parameters: firing frequency, amplitude, and morphology. Changes in each of these waveform parameters may occur in diseases, and discrete waveforms themselves may be abnormal. Artifacts can alter all of the variables used to describe the continuous and discrete waveforms recorded in clinical neurophysiology. This chapter reviews the basic concepts of waveforms recorded during clinical neurophysiology testing.