threshold

n. The lowest level of a sensory stimulus or a change in the intensity of a stimulus which a subject can reliably report. Originally psychophysicists believed there was some absolute level which corresponded with human consciousness which defined this limit, but close study revealed variability in subject responses, which were partially random, partially dependent on the instructions given to the subject, and partially dependent on previous observation of stimuli. Subsequent study revealed that there is a gradual onset of stimulus detection which can be described using signal detection methods which give probabilities of response to a stimulus at different intensities or levels of the stimulus. Also called a limen.