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In psychophysical experiments subjects always produce responses that are not nearly as constant as the stimuli presented in laboratory conditions. In this study the fluctuation of responses is described as originating in the nervous system. This description requires a definition of three quantities: a measure for the total neural activity elicited by the stimulus, the fluctuation of this activity about the mean, and the criterion value set by the subject. The method is used in the case of electrical and tactile pulses applied to the skin. This experiment is extended to the case where the test pulse is masked by another simultaneous pulse at an adjacent location. Also, in many learning experiments the response does not improve from trial to trial but it is the mean of fluctuating responses, which shows a learning phenomenon. Here again a theoretical description requires the definition of three quantities: a measure for the state of the nervous system telling its aptitude to show the required response, the fluctuation of this aptitude about the mean, and the criterion value set by the experimental environment. These theoretical considerations are applied to the learning of a compensatory tracking task with two different displays: a visual display and a tactile display.