This is the smallest change in stimuli that can be perceived. As stated above, the JND dS is proportional to the initial stimuli intensity S. Mathematically, it can be described as d S = K ⋅ S {\displaystyle dS=K\cdot S} where S {\displaystyle S} is the reference stimulus and K {\displaystyle K} is a constant.[5] It may be written as Ψ = k logS, with Ψ being the sensation, k {\displaystyle k} being a constant, and S {\displaystyle S} being the physical intensity of the stimulus.