Origins of baseline variance and the Law of Initial Values.

The Law of Initial Value (LIV) asserts that the magnitude of a phasic psychophysiological response is dependent on the initial baseline level. Although results in accord with the LIV are often observed, exceptions are frequent, especially for between-subjects analyses. A general assumption in studies of the LIV is that a given baseline difference is equivalent regardless of its functional origin. The present study examined the relationships between basal heart period variance, arising from alternate sources, and the magnitude of the chronotropic response to a speech stressor. Results reveal that baseline differences due to orthostatic manipulations, which are known to be largely of autonomic origin, yielded larger LIV effects than did individual differences in basal heart period, which include a significant nonautonomic component. The minimal LIV-like effects of baseline differences associated with non-autonomic factors, relative to variations in autonomic control, may contribute to the inconsistent appearance of between-subjects LIV effects.