The psychophysical research (N=106) on loudness distinction of tonal signals (method 2AFC) has been carried out. Applying the principle of subject`s activity and qualitative analysis to traditional psychophysical research was accomplished in the frameworks of differential psychological approach in psychophysics. The influence of self-regulation (questionnaires: HAKEMP-90, Style of behaviour self-regulation, Self- organization of behavior) on sensory sensitivity index A’ was observed. The relationship between loudness distinction effect and self-regulation processes mediating the sensory task decision were revealed. Applying the theoretical principle of subject`s activity to traditional psychophysical research was accomplished in the frameworks of differential psychology approach in psychophysics (A.N. Gusev). The idea of stimuli variation uncertainty results in appropriate transformation of the functional organ (A.N. Leontiev, A.A. Ukhtomskiy) that is relevant to sensory discrimination task performance.