At a certain abstract level, there is no difference between acoustic and spatial forms, colors, postures, gestures. We sense this when we compare sign systems, for example we call a color, a sound, and a form ‘sharp’. When this phenomenon occurs spontaneously, it is called synesthesia; the simultaneous experience of an emotion in e.g., color and smell, or sound and form. The fact that this is not arbitrary is demonstrated by the Bouba-Kiki effect. Wolfgang Köhler, a German Gestalt psychologist who experimented with perception and movement, first observed the Bouba-Kiki effect in 1929. It has since been replicated all over the world. The effect consists of the fact that when people have to place the words Bouba and Kiki next to two forms (a convex, round form and a sharp, pointed form), 95 to 98% label the convex, round form as Bouba, and the sharp form as Kiki (Köhler, 1947; Ramachandran, 2001). This suggests that the human brain is capable of extracting abstract properties of both shape and sound.

Synesthesia occurs in the SCS on the dimensional level. Parameters from one sensory domain transgress via the dimensions to the parameters of a different sensory domain. In the SCS synesthetic links were forged using scientific research that demonstrated connections between certain sensory domains. For example, researchers from different domains independently have demonstrated that light colors such as pastels are experienced as being lighter in weight than dark colors. (Meerwein 2007; Alexander and Shansky 1976; Heller 1989). Knowledge of synesthetics is important for designers who by its application are given bigger control over the power and impact of an anticipated emotional reaction.