Sensori-motor reaction time is the interval that elapses between the stimulation of a sense organ and a motor response. The physiological process involved consists of (a) an afferent factor,—the stimulation of a sensory terminal, and transmission of an impulse along sensory nerve fibres to the brain; (b) a psychical factor, involving an act of sensory perception and the voluntary production of a motor impulse; (c) an efferent factor,—the transmission of an impulse along motor nerve fibres, and consequent contraction of muscle.