NATURE|VOL 434|17 MARCH 2005|www.nature.com/nature 301Although we rarely confuse a painting forthe scene it presents, we are often taken inby the vividness of the lighting and the three-dimensional (3D) layout it captures. This isnot surprising for a photorealistic painting,but even very abstract paintings can convey astriking sense of space and light, despiteremarkable deviations from realism.The rules of physics that apply in a realscene are optional in a painting; they can beobeyed or ignored at the discretion of theartist to further the painting’s intendedeffect. Some deviations, such as Picasso’sskewed faces or the wildly coloured shad-ows in the works of Matisse and otherImpressionists of the Fauvist school, aremeant to be noticed as part of the style andmessage of the painting. There is, however,an ‘alternative physics’ operating in manypaintings that few of us ever notice butwhich is just as improbable. These trans-gressions of standard physics — impossibleshadows,colours,reflections or contours —often pass unnoticed by the viewer and donot interfere with the viewer’s understand-ing of the scene. This is what makes themdiscoveries of neuroscience. Because we donot notice them, they reveal that our visualbrain uses a simpler, reduced physics tounderstand the world.Artists use this alter-native physics because these particulardeviations from true physics do not matterto the viewer: the artist can take shortcuts,presenting cues more economically, andarranging surfaces and lights to suit themessage of the piece rather than therequirements of the physical world.In discovering these shortcuts artists actas research neuroscientists, and there is agreat deal to be learned from tracking downtheir discoveries. The goal is not to expose the‘slip-ups’ of the masters, entertaining as thatmight be, but to understand the human brain.Art in this sense is a type of found science —science we can do simply by looking.To c ount as a ‘discovery’ in this art-basedneuroscience, deviations from standardphysics must be mostly invisible to thehuman eye in casual viewing. A paintingthat, despite physical impossibilities in thedepiction, gives an unhindered sense of thespace and objects within it, says somethingabout our brain. For example, a shadow thatlooks like a convincing shadow, eventhough its shape does not match the objectthat cast it, suggests the physics of light andshadow used by our visual brain is simplerthan true physics1,2.This simplified internal physics employedby our visual brain is used not just to appreci-ate paintings, but to enable our rapid andefficient perception of the real world. Realshadows are subject to an extensive set ofconstraints, but few of these seem to bechecked by our vision; that is why an artistcan use an unrealistic representation withsuch great impact. It is important to notethat the simplified rules of physics thatinterest us (and the artists’ shortcuts thatexploit them), are not based on the ever-changing conventions of artistic represen-tation, as they hold for monkeys andinfants3,4,both quite immune to the con-ventions of art. These simplified rules aregrounded instead in the physiology of thevisual brain.Darkness alone requiredCast shadows have appeared on and off inWestern art from the early classical Greek5and Roman paintings and mosaics to thebeginning of the modern era6.In contrast,with the exception of a single drawing, castshadows did not appear in Eastern art untilmodern times7.Artists take many libertieswhen depicting shadows, using the wrongcolour or shape, without disturbing theapparent light, space or form of the depictedscene. These physical impossibilities that slipby unnoticed (Fig. 1) are important forunderstanding vision. They reveal that thevisual brain recognizes shadows using only asmall subset of the criteria that constrain realThe artist asneuroscientistArtistic licence taps into thesimplified physics used byour brain to recognizeeveryday scenes, saysPatrick Cavanagh.Figure 1 By 1467, artists such as Fra Carnevalehad mastered consistent perspective but notconsistent lighting. The people in theforeground cast deep shadows but those on theplaza above and to the left do not. The alcove onthe right is brightly lit but the only opening inits left wall is a small door. The shadows on theright wall of the alcove rise mysteriouslyupwards. These severe inconsistencies are notevident or jarring to the human viewer. (Detailof The Birth of the Virgin by Fra Carnevale.)ROGERS AND GWYNNE ANDREWS FUNDS,1935 (35.121) PHOTOGRAPH  1983 THE METROPOLITAN MUSEUM OF ARTscientists on artCavanagh 4/3/05 3:11 pm Page 301Nature PublishingGroup© 2005shadows. Unsurprisingly, one criterion usedunfailingly by artists is that the shadowsmust be darker than their immediate sur-roundings. This finding has been confirmedby perceptual experiments1that examinethe recovery of shapes defined by shadows.Such experiments have also shown, as haveartists many times over, that few if anyother deviations from realism affect therecovery of shape from shadows. Exceptionsto this broad tolerance can be found inpaintings where shadows fail to be convinc-ing. Specifically, shadows should not appearto have volume or substance of their own(Fig. 2), a criterion that has yet to be exam-ined scientifically.Scientific studies of the perception ofshadows,and shape from shadows1have sup-ported other discoveries made by painters. Inthe two-tone images of Fig. 3, the shadowsbelow the nose,eyebrows and chin define thedepth of the face. When the shadows violatethe rules required by the visual system, theface is no longer seen as such a strong 3Dstructure. These studies show, for example,that the shadows must be darker, but do nothave to be of physically possible colours.Studies of lighting direction supportpainters’ intuition that inconsistent direc-tion of lighting is not readily noticed. Thecubes in Fig. 4 are all lit from one direction,with one exception.Subjects take a long timeto pick out the oddly lit cube8.302 NATURE|VOL 434|17 MARCH 2005|www.nature.com/natureFigure 3 The dark red areas of the two-toneimage of a man’s face on the left include bothregions of dark shadow and dark pigment(eyebrows, hair, moustache). These areas, inappropriate lighting, should be darker than thesurrounding green areas. (If this is not the case,move to a location with fluorescent or naturallighting.) In the version on the right, the samered areas are now brighter