Components of heart rate variability have attracted considerable attention in psychology and medicine and have become important dependent measures in psychophysiology and behavioral medicine. Quantification and interpretation of heart rate variability, however, remain complex issues and are fraught with pitfalls. The present report (a) examines the physiological origins and mechanisms of heart rate variability, (b) considers quantitative approaches to measurement, and (c) highlights important caveats in the interpretation of heart rate variability. Summary guidelines for research in this area are outlined, and suggestions and prospects for future developments are considered.

Evaluative processes refer to the operations by which organisms discriminate threatening from nurturant environments. Low activation of positive and negative evaluative processes by a stimulus reflects neutrality, whereas high activation of such processes reflects maximal conflict. Attitudes, an important class of manifestations of evaluative processes, have traditionally been conceptualized as falling along a bipolar dimension, and the positive and negative evaluative processes underlying attitudes have been conceptualized as being reciprocally activated, making the bipolar rating scale the measure of choice. Research is reviewed suggesting that this bipolar dimension is insufficient to portray comprehensively positive and negative evaluative processes and that the question is not whether such processes are reciprocally activated but under what conditions they are reciprocally, nonreciprocally, or independently activated.

All organisms must be capable of differentiating hostile from hospitable stimuli to survive. Typically, this evaluative discrimination is conceptualized as being bipolar (hostile-hospitable). This conceptualization is certainly evident in the area of attitudes, where the ubiquitous bipolar attitude measure, by gauging the net affective predisposition toward a stimulus, treats positive and negative evaluative processes as equivalent, reciprocally activated, and interchangeable. Contrary to conceptualizations of this evaluative process as bipolar, recent evidence suggests that distinguishable motivational systems underlie assessments of the positive and negative significance of a stimulus. Thus, a stimulus may vary in terms of the strength of positive evaluative activation and the strength of negative evaluative activation it evokes. Low activation of positive and negative evaluative processes by a stimulus reflects attitude neutrality or indifference, whereas high activation of positive and negative evaluative processes reflects attitude ambivalence. As such, attitudes can be represented more completely within a bivariate space than along a bipolar continuum. Evidence is reviewed showing that the positive and negative evaluative processes underlying many attitudes are distinguishable (stochastically and functionally independent), are characterized by distinct activation functions (positivity offset and negativity bias principles), are related differentially to attitude ambivalence (corollary of ambivalence asymmetries), have distinguishable antecedents (heteroscedacity principle), and tend to gravitate from a bivariate toward a bipolar structure when the underlying beliefs are the target of deliberation or a guide for behavior (principle of motivational certainty). The implications for society phenomena such as political elections and democratic structures are discussed.

The affect system has been shaped by the hammer and chisel of adaptation and natural selection such that form follows function. The characteristics of the system thus differ across the nervous system as a function of the unique constraints existent at each level. For instance, although physical limitations constrain behavioral expressions and incline behavioral predispositions toward a bipolar (good-bad, approach-withdraw) organization, these limiting conditions lose their power at the level of underlying mechanisms. According to the authors' model of evaluative space (J.

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