The orienting response (OR), also called orienting reflex, is an organism's immediate response to a change in its environment, when that change is not sudden enough to elicit the startle reflex. The phenomenon was first described by Russian physiologist Ivan Sechenov in his 1863 book Reflexes of the Brain, and the term ('ориентировочный рефлекс' in Russian) was coined by Ivan Pavlov, who also referred to it as the Shto takoye? (Что такое? or What is it?) reflex. The orienting response is a reaction to novel or significant stimuli. In the 1950s the orienting response was studied systematically by the Russian scientist Evgeny Sokolov, who documented the phenomenon called "habituation", referring to a gradual "familiarity effect" and reduction of the orienting response with repeated stimulus presentations. Researchers have found a number of physiological mechanisms associated with OR, including changes in phasic and tonic skin conductance response (SCR), electroencephalogram (EEG), and heart rate following a novel or significant stimulus. These observations all occur within seconds of stimulus introduction. In particular, EEG studies of OR have corresponded particularly with the P300 wave and P3a component of the OR-related event-related potential (ERP). Neural correlates Current understanding of the localization of OR in the brain is still unclear. In one study using fMRI and SCR, researchers found novel visual stimuli associated with SCR responses typical of an OR also corresponded to activation in the hippocampus, anterior cingulate gyrus, and ventromedial prefrontal cortex. These regions are also believed to be largely responsible for emotion, decision making, and memory. Increases in cerebellar and extrastriate cortex were also recorded, which are significantly implicated in visual perception and processing. Function When an individual encounters a novel environmental stimulus, such as a bright flash of light or a sudden loud noise, they will pay attention to it even before identifying it. This orienting reflex seems to be present early in development, as babies will turn their head toward an environmental change (Nelson Cowan, 1995). From an evolutionary perspective, this mechanism is useful in reacting quickly to events that call for immediate action. Habituation Sokolov's investigation of OR was primarily motivated in understanding habituation. Provided the first introduction of a novel stimulus, defined in Sokolovian terms as any change from the "currently active neuronal model" (what the individual is currently focused on), results in OR. However, with repeated introduction of the same stimulus, the orienting response will decrease in intensity and