The cerebral bases of the bouba-kiki effect.

The crossmodal correspondence between some speech sounds and some geometrical shapes, known as the bouba-kiki (BK) effect, constitutes a remarkable exception to the general arbitrariness of the links between word meaning and word sounds. We have analyzed the association of shapes and sounds in order to determine whether it occurs at a perceptual or at a decisional level, and whether it takes place in sensory cortices or in supramodal regions. First, using an Implicit Association Test (IAT), we have shown that the BK effect may occur without participants making any explicit decision relative to sound-shape associations. Second, looking for the brain correlates of implicit BK matching, we have found that intermodal matching influences activations in both auditory and visual sensory cortices. Moreover, we found stronger prefrontal activation to mismatching than to matching stimuli, presumably reflecting a modulation of executive processes by crossmodal correspondence. Thus, through its roots in the physiology of object categorization and crossmodal matching, the BK effect provides a unique insight into some non-linguistic components of word formation.