Assessment of three techniques for measurement of multiunit neural activity in sympathetic nerves

Multiunit sympathetic neural activity is commonly measured by rectifying recorded voltage signals and integrating the resulting record, or estimating peak counts within a rectified record above a certain voltage threshold. A less often employed alternative is to measure the amplitude variance of the signal. These three techniques are examined here, both theoretically and using computer simulation studies, to assess the degree of linear relationship they have to true activity, defined as number of unit (nerve fiber) firings per unit time. The integration and variance techniques are also compared using activity recorded from the rat splanchnic nerve following posterior hypothalamic stimulation. It is concluded that the variance measurement alone bears true linearity for an unrestricted range of activity. Such linearity can only be obtained approximately over restricted activity ranges with the integration technique, although if the integration values are squared, a true linear relationship to activity over almost any range is produced. Peak count estimation fails even to approximate this linearity except over very large ranges and/or when multiple voltage thresholds are used.