The Meaning of Motion Lines? A Review of Theoretical and Empirical Research on Static Depiction of Motion

Static depiction of motion, particularly lines trailing behind a mover, has long been of interest in the psychology literature. Empirical research has demonstrated that these "motion lines" benefited motion comprehension in static images by disambiguating the direction of movement. Yet, there is no consensus on how those lines derive their meaning. In this article, we review three accounts suggesting different interpretations of what motion lines represent. While a perceptual account considers motion lines originating from motion streaks in the primary visual cortex, metaphorical and lexical accounts propose them as graphical conventions that should be learned-either through resemblance to sensory experiences (e.g., natural path marks) or directly being mapped to a conceptual category of paths. To contrast these three accounts, we integrate empirical research on motion lines and their understanding. Overall, developmental, proficiency, and cross-cultural variances indicate that the understanding of motion lines is neither innate nor universal, thus providing less support for lines having a purely perceptual origin. Rather, we argue the empirical findings suggest that motion lines are not iconic depictions of visual percepts but are graphical conventions indexing conceptual path information, which need to be learned and encoded in a visual lexicon.