• COGNITION
• NEURAL NETWORKS
• VISION

Usually, the eye and the brain can easily differentiate between a flat, two-dimensional picture and a typical three-dimensional sculpture in the round. Yet the emergence of an unusual form of Renaissance relief sculpture suggests that this observation merits further consideration. In Florence around 1415, that most accomplished of sculptors Donato dei Bardi, commonly known as Donatello, devised a variety of very low sculptural relief known by the Italian term rilievo schiacciato—‘flattened out’ or ‘squashed’ relief.1 An extreme form of bas-relief, this form of sculpture comprises carving with a depth of just a few millimetres and has been described as ‘a kind of engraving on the stone’ in which the marble surface of the sculpture is chiselled ‘to create indentations and protrusions that will catch the light…creating highlights and shadows. Rilievo schiacciato is rather like drawing with light and shadow, and this new relief style was developed during the same period that cast shadows were being rediscovered by the painters of Florence.’2

Although bas-relief is a very old form of image-making dating back at least to Assyro-Babylonian times,1 carving the nearest figures in higher relief compared with the background carving in the extremely shallow rilievo schiacciato form was not only a new and highly challenging creative endeavour but presents a challenge too for the visual brain, which must ‘process’ both two and three dimensions simultaneously to perceive a single sculptural entity. Thus, on the one hand, the brain needs to engage with the perception of a conventional, almost two-dimensional engraving in the background, which may or may not include three-dimensional features such as linear perspective that had just been described by Donatello’s friend Brunelleschi (figure 1).3 On the other hand, at the same time, the brain needs to process the perception of real depth which …