There is No Such Thing As a “just Noticeable” Difference

The Just Noticeable Difference, or JND, is defined as the smallest perceptible difference between two stimuli. Despite SDT clearly eliminating this notion of a JND, the vast majority of people still maintain this intuitive belief about discrimination (see below). We investigated whether this belief is substantiated by evaluating whether the JND construct holds in 20 different datasets from a variety of species (including humans, mice, seals, bats, and butterflies) tested on a variety of dimensions on different senses (including visual contrast, auditory velocity, rubber smell, and salt taste). We fit data from each domain with two models, a No JND model and a Weibull function, and compared fits using BIC. We found that the No JND Model provided a better fit to the data in nearly every case. Given that the data are uniformly consistent with No JND, we turned to the question of why so many still believe in the JND. We hypothesized that the belief that some differences are too small to perceive is an intuitive belief about perception. To evaluate this, we surveyed naïve undergraduate subjects to assess their intuitive beliefs about perceptual limits. The survey evaluated peoples’ beliefs about perception generally as well as about specific domains such as visual number, visual color, and felt weight. Overwhelmingly, regardless of domain, people endorsed the idea of a JND. This suggests that one reason for the prevalence and longevity of the JND in psychological theorizing may be because it aligns with our intuitive beliefs about the limits of our perceptual abilities.