“Light creates space”—this is how the art theorist and perceptual psychologist Rudolf Arnheim boiled down the essential meaning of depicting light in paintings (1). Space, however, comes along with the possibility to disambiguate the shape of objects, so light also assists perception of three-dimensional structures. This disambiguation is not very effective as long as the location of the light source is unknown or unreliably assessed (2). There are only rare cases where we can directly observe the source of light in paintings, e.g. explicitly showing the sun as often done in Van Gogh’s Wheat Field series of oil paintings (see Fig. S2A), or by showing a human-made light source such as candles in the famous Georges de La Tour paintings (see Fig. …show more content…
2). We found that the preferential location of the light source was synchronized with the beginning of the Early Renaissance era, starting from approximately at 1420 and on through the Cinquecento until the end of the 19th century. Our data shows that from the Renaissance on, Western art history had “its bias” to the left regarding the light source. Paintings for which we found clear initial, but also singular, laterality effects were created by: Simone Martini (1284–1344), Giotto (1266/1267–1337), and Duccio di Buoninsegna (1255/1260–1318/1319). This is very compatible with the notion of art historians that lighting and shadowing effects were identified as a basic and innovative topic of Western art history with the rise of Early Renaissance painters such as Masaccio (1401–1428, Andrea Mantegna (1431–1506) and Andrea del Verrocchio (1435–1488). Important techniques to realize lighting and shadowing effects were developed during this period, most importantly a technique from the later 15th century where strong contrasts between light and dark are used, known as chiaroscuro (from Italian chiaro, “light,” and scuro, “dark”). Critically, some preliminary works also dealing with clear lighting and shadowing effects could have been covered in the statistical analysis by the mere fact that before 1400 a comparatively smaller number of paintings were (and are) available (see Fig. 2A). Still, based on participants’ confidence (Fig. 2BC), we can clearly state that overall there was not a clear and reliable way of positioning the light source before the Early