Contemporary findings reveal that autonomic control of dually innervated visceral organs does not lie along a single continuum extending from parasympathetic to sympathetic dominance. Rather, a bivariate autonomic space bounded by sympathetic and parasympathetic axes is the minimal representation necessary to capture the modes of autonomic control. A quantitative bivariate model for the chronotropic control of the heart in humans is empirically instantiated. This model provides a more comprehensive characterization of psychophysiological response than simple measures of end-organ state and permits a differentiation of behavioral states and processes that would otherwise remain obscure. The model also illuminates and subsumes general principles such as the law of initial values and reveals a fundamental physiological rationale for the selection of heart period over heart rate as a metric for cardiac chronotropy. The present article also considers strategies for psychophysiological investigations within the autonomic space model, the limitations of these methods, and analytical tools for assessing their validity.(PsycINFO Database Record (c) 2016 APA, all rights reserved)