Cardiac psychophysiology and autonomic space in humans: empirical perspectives and conceptual implications.

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. We here empirically instantiate a quantitative bivariate model for the chronotropic control of the heart in humans. 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.