The affect system has parallel and integrative processing components: Formfollows function

The affect system has been shaped by the hammer and chisel of adaptation an d natural selection such that form follows function. The characteristics of the system thus differ across the nervous system as a function of the uniq ue constraints existent at each level For instance, although physical limit ations constrain behavioral expressions and incline behavioral predispositi ons toward a bipolar (good-bad, approach-withdraw) organization, these limi ting conditions lose their power at the level of underlying mechanisms. Acc ording to the authors' model of evaluative space (J. T. Cacioppo & G. G. Be rntson, 1994; J. T. Cacioppo, W. L. Gardner, & G. G. Berntson, 1997), the c ommon metric governing approach-withdrawal is generally a single dimension at response stages that itself is the consequence of multiple operations, s uch as the activation function for positivity (appetition) and the activati on function for negativity (aversion), at earlier affective processing stag es.