evolved into paradigms that are still in use today. These domains are broadly described as sensory factors, response characteristics, preparation, choice, and conscious accompaniments. Sensory factors Early researchers noted that varying the sensory qualities of the stimulus affected response times, wherein increasing the perceptual salience of stimuli tends to decrease reaction times. This variation can be brought about by a number of manipulations, several of which are discussed below. In general, the variation in reaction times produced by manipulating sensory factors is likely more a result of differences in peripheral mechanisms than of central processes. Strength of stimulus One of the earliest attempts to mathematically model the effects of the sensory qualities of stimuli on reaction time duration came from the observation that increasing the intensity of a stimulus tended to produce shorter response times. For example, Henri Piéron (1920) proposed formulae to model this relationship of the general form: , where represents stimulus intensity, represents a reducible time value, represents an irreducible time value, and represents a variable exponent that differs across senses and conditions. This formulation reflects the observation that reaction time will decrease as stimulus intensity increases down to the constant , which represents a theoretical lower limit below which human physiology cannot meaningfully operate. The effects of stimulus intensity on reducing RTs was found to be relative rather than absolute in the early 1930s. One of the first observations of this phenomenon comes from the research of Carl Hovland, who demonstrated with a series of candles placed at different focal distances that the effects of stimulus intensity on RT depended on previous level of adaptation. In addition to stimulus intensity, varying stimulus strength (that is, "amount" of stimulus available to the sensory apparatus per unit time) can also be achieved by increasing both the area and duration of the presented stimulus in an RT task. This effect was documented in early research for response times to sense of taste by varying the area over taste buds for detection of a taste stimulus, and for the size of visual stimuli as amount of area in the visual field. Similarly, increasing the duration of a stimulus available in a reaction time task was found to produce slightly faster reaction times to visual and auditory stimuli, though these effects tend to be small and are largely consequent of the sensitivity to sensory receptors. Sensory modality The sensory modality over