Mental chronometry is the scientific study of processing speed or reaction time on cognitive tasks to infer the content, duration, and temporal sequencing of mental operations. Reaction time (RT; sometimes referred to as "response time") is measured by the elapsed time between stimulus onset and an individual's response on elementary cognitive tasks (ETCs), which are relatively simple perceptual-motor tasks typically administered in a laboratory setting. Mental chronometry is one of the core methodological paradigms of human experimental, cognitive, and differential psychology, but is also commonly analyzed in psychophysiology, cognitive neuroscience, and behavioral neuroscience to help elucidate the biological mechanisms underlying perception, attention, and decision-making in humans and other species. Mental chronometry uses measurements of elapsed time between sensory stimulus onsets and subsequent behavioral responses to study the time course of information processing in the nervous system. Distributional characteristics of response times such as means and variance are considered useful indices of processing speed and efficiency, indicating how fast an individual can execute task-relevant mental operations. Behavioral responses are typically button presses, but eye movements, vocal responses, and other observable behaviors are often used. Reaction time is thought to be constrained by the speed of signal transmission in white matter as well as the processing efficiency of neocortical gray matter. The use of mental chronometry in psychological research is far ranging, encompassing nomothetic models of information processing in the human auditory and visual systems, as well as differential psychology topics such as the role of individual differences in RT in human cognitive ability, aging, and a variety of clinical and psychiatric outcomes. The experimental approach to mental chronometry includes topics such as the empirical study of vocal and manual latencies, visual and auditory attention, temporal judgment and integration, language and reading, movement time and motor response, perceptual and decision time, memory, and subjective time perception. Conclusions about information processing drawn from RT are often made with consideration of task experimental design, limitations in measurement technology, and mathematical modeling. History and early observations The conception of human reaction to an external stimulus being mediated by a biological interface (such as a nerve) is nearly as old as the philosophical discipline of science itself. Enlightenment thinkers like René Descartes proposed that the reflexive response to pain, for example, is carried by some sort of fiber—what we would recognize as part of the nervous system today—up to the brain, where it is then processed