This research is concerned with the principles controlling the evocation of simple responses and is guided by a form of decision theory in which variability of the decision criterion plays a dynamic role. The criterion is assumed to be influenced by variables generally regarded as controlling motivation, attention, adaptation, and set. Response probability and latency are determined by the level and variability of the criterion and by associative and sensory strength. Experimental methods include eyelid conditioning and simple, disjunctive and choice reaction time. CS intensity and RT signal intensity are major experimental variables. Other variables include UCS intensity, inter-stimulus interval, stimulus similarity, and instructions controlling speed of response. One major goal is to determine by scaling procedures the functions of time describing the processing of sensory and associative information.