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Although we rarely confuse a paintingfor the scene it presents, we are often

taken in by the vividness of the lightingand the three-dimensional layout it captures.This is not surprising for a photorealisticpainting, but even very abstract paintingscan convey a striking sense of space andlight, despite remarkable deviations fromrealism. The rules of physics that apply in areal scene are optional in a painting; theycan be obeyed or ignored at the discretionof the artist to further the painting’sintended effect. Some deviations such asPicasso’s skewed faces or the wildlycoloured shadows in the works of Matisseand other Impressionists of the Fauvistschool are meant to be noticed as part ofthe style and message of the painting.There is, however, an ‘alternative physics’operating in many paintings that few of usever notice but which is just as improbable.These transgressions of standard physics— impossible shadows, colours, reflectionsor contours — often pass unnoticed by theviewer and do not interfere with the viewer’sunderstanding of the scene. This is whatmakes them discoveries of neuroscience.Because we do not notice them, they revealthat our visual brain uses a simpler,reduced physics to understand the world.Artists use this alternative physics becausethese particular deviations from truephysics do not matter to the viewer: theartist can take shortcuts, presenting cuesmore economically, and arranging sur-faces and lights to suit the message of thepiece rather than the requirements of thephysical world.

In discovering these shortcuts artists actas research neuroscientists, and there is agreat deal to be learned from tracking down

their 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 count 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 physicsemployed by our visual brain is not used

just to appreciate paintings, but to enableour rapid and efficient perception of thereal world. Real shadows are subject to anextensive set of constraints, but few of theseseem to be checked by our vision; that is whyan artist can use an unrealistic representa-tion with such great impact. It is importantto note that the simplified rules of physicsthat interest us (and the artists’ shortcutsthat exploit them), are not based on theever-changing conventions of artistic repre-sentation, as they hold for monkeys andinfants3,4, both quite immune to the conven-tions 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 Greek5

and 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 recognises shadows using only a

The 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.)

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small subset of the criteria that constrain realshadows. Unsurprisingly, one criterion usedunfailingly by artists is that the shadows mustbe darker than their immediate surround-ings. This finding has been confirmed byperceptual experiments1 that examine therecovery of shapes defined by shadows. Suchexperiments have also shown, as have artistsmany times over, that few if any other devia-tions from realism affect the recovery ofshape from shadows. Exceptions to this broad

tolerance can be found in paintings whereshadows fail to be convincing. Specifically,shadows should not appear to have volumeor substance of their own (Fig. 2), a criterionthat has yet to be examined scientifically.

Scientific studies of the perception ofshadows, and shape from shadows1 have 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 violate

the 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.

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Figure 3 The dark red aimage of a man’s face on the left include both

reas of the two-tone

regions of dark shadow and dark pigment (eyebrows, hair, mustaappropriate lighting, should be darker than the

che). These areas, in

surrounding 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 than the greensurround. Shadows have to be darker to supportthe recovery of object shape from shadow cuesso the face on the right is much less 3D. Butnotice that the shadows, as long as they aredarker, do not have to be the right colour1 (the ambient red light seen in the shadows should

Figure 2 Signorelli takes great liberty withshadows, but goes too far here in making theguard’s shadow cross over the satyr’s shadow asif it were paint. Although shadows can lie in thewrong direction and have the wrong shape, theycannot look opaque and still appear as shadows.(Detail from The Assumption of the Virgin withSaints Michael and Benedict by Luca Signorelli.)

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Surrogate boundariesMuch of our earliest recorded art takes theform of line drawings and, remarkably, theelements of this type of representation haveremained unchanged. But given that linesdo not divide objects from their back-grounds in the real world9, why do linedrawings work?

The effectiveness of line drawings is notsimply attributable to learned convention,passed on through culture. Infants10, stone-age tribesmen11 and even monkeys12 arecapable of interpreting line drawings as wedo. So what do lines represent to the brain?Artists do not just trace the brightness dis-continuities in an image. Conventional linedrawings do not include the outlines of castshadows or pigment contours; rather, theytrace out the contours that characterizeshape (Fig. 5). Artists have discoveredwhich key contours must be perceived bythe visual brain for the viewer to identify theessential structure of an object. By studyingthe nature of lines used in line drawings, sci-entists may eventually gain access to thisnatural knowledge base.

Seeing through paintIt is not easy to draw or paint a material thatis barely visible and through which back-ground patterns are only slightly altered.Artists do this by making a reasonable ver-sion of the background surface appearthrough the transparent surface. This super-position involves crossing the contours of

the transparent object with the contours inthe background. For example, in Fig. 6 thefront rim of the glass crosses the backwaterline, and in Fig. 7 the hem of the sheercotton garment crosses the outline of thelegs that are visible both above and belowthe hem. Experiments by F. Metelli haveshown how these crossings or ‘X-junctions’are critical cues for the successful depictionof transparency13. When the X-junctions are

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misaligned, the impression of transparencyis lost (Fig. 8).

Although the X-junctions must be pre-sent to successfully convey transparency,other properties of the transparent materialare not critical. For example, in paintings ofwater and glass, gross deviations from theoptics of refraction (Fig. 6) are rarely noticedby the viewer, indicating again that the visualbrain only computes a small set of the possible

Figure 4 An array of cubes all lit from one direction except one. Subjects take an average of eightseconds to find the odd item (bottom right here) and make many errors (30%), suggesting thatinconsistent lighting is not readily noticed. (From ref. 8.)

Figure 5 Lines are used to convey the outer contours of the horses in a very similar way in these two drawings, one from 15,000 BCE (left: Chinese Horse,paleolithic cave paining at Lascaux) and the other from 1300 CE (right: Jen Jen-fa, detail from The Lean Horse and the Fat Horse, Peking Museum).

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physical properties of a transparent materialin assessing whether or not a surface istransparent.

Filling in the gapsMany paintings only hint at the elements ina scene and depend on the viewer’s memo-ries to construct meaningful images fromthe fragments. Impressionism and Cubismin particular rely on this memory basedreconstruction to complete scenes frompartial representations. These paintingsdemonstrate the minimal skeletons of visualforms that are capable of evoking remem-bered images (Fig. 9).

No need for 3DFlat paintings are so commonplace that weseldom ask why flat representations work sowell. If we really experienced the world as3D, an image seen in a flat picture woulddistort jarringly when we moved in front ofit. But it does not as long as it is flat. A foldedpicture, in contrast, distorts as we movearound it (Fig. 10). Our ability to interpretrepresentations that are less than 3D indi-cates that we do not experience the visualworld as truly 3D (refs 14–16), and hasallowed flat pictures (and movies) to domi-nate our visual environment as an econom-ical and convenient substitute for 3Drepresentations. This tolerance of flat repre-sentations is found in all cultures10, infants3,and in other species4 so it cannot result fromlearning a convention of representation.Imagine how different our culture would beif we could not make sense of flat representa-tions. Visual art would all be 3D: there would

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Figure 6 No optical distortion of the lemon in thewater is shown here and yet the glass and thewater appear convincingly transparent.(Implement Blue, M. Preston, 1927; oil on canvason paperboard, 42.5 x 43 cm; gift of the artist 1960collection; Art Gallery of New South Wales.)

Figure 7 (right) Egyptian artists were the first todepict transparency. They needed to show theelegance of the fine transparent cottons worn bythe wealthy (Pharaoh Sethi I on the right). Thetransparency of the cotton tunic is capturedthrough overlapping contours and contrasts.

Figure 8 (left) When a transparent surfacecovers a contour in the object behind it, thecontour of the transparent surface and theunderlying contour cross to form an X-junction. Two of these X-junctions are seen inthe top panel. As Metelli showed13, if thecontours are displaced to eliminate the X-junction, as on the bottom panel, the samepatches of light and dark look more opaquethan transparent. T

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be no paintings or movies. Our pocketswould be bulging with little statuettes ofloved ones rather than photographs.

Less is moreImpressionists used minimal detail in theirpaintings, yet their pieces evoke a strongsense of place and mood. A photograph ofan equivalent scene might be unexceptional,but the inaccurate splashes of colour andhints of contour are often moving despite,or perhaps because of, discrepancies from arealistic portrayal (Fig. 11).

Why is this style so effective? Recentneuroscience studies of the connectionbetween vision and the centres of emotionsuggest a possible reason. Brain imaging17

of subjects presented with faces expressingfear show that the amygdala (a centre ofemotion) responds strongly to a blurry ver-sion of the faces. In contrast, areas respon-sible for conscious face recognitionrespond weakly to blurry faces and best tofaces presented in sharp detail (Fig. 12).Impressionist works may connect moredirectly to emotional centres than to con-scious image-recognition areas because theunrealistic patchwork of brush strokes and

mottled colouring distract consciousvision (Fig. 11).

Depicting reflectionMirrors have been depicted in art sinceGreek and Roman times but, inevitably,artists commit fascinating errors when rep-resenting what is reflected by the mirror18.Having encountered reflections in mirrorsthroughout our lives, we might assume we

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understand how they work — what objectsshould be visible in the mirror given ourposition, the angle of the mirror and thelocation of other objects around us. Realmirrors never test this knowledge becausethey are always correct. But painters do testour knowledge of mirrors and reflection,and reveal that basically we have none. In apainting, almost any reflection will do, withonly a few limits. Artists can depict people

Figure 9 Two dancers are made up of some of the isolated swatches of colour. The arrangements aresufficiently similar to familiar human shapes to trigger the integration of the marks as legs, arms,heads and bodies of single figures. Before the development of brain imaging, similarly disconnectedimages23 were used by neurologists to identify brain injury to the parietal lobe. (The Yellow Dancersby Gino Severini, circa 1911–1912; oil on canvas, 45.7 x 61 x 2.3 cm.)

Figure 10 When a flat picture is viewed from different angles, the 3D scene can still be perceived without jarring distortions. In contrast, when a foldedpicture of a face is tilted, striking changes of expression are seen. (From ref. 16.)

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looking at their reflections and reveal boththe person and the reflection, often whenthis is geometrically impossible (Fig. 13).

Recent vision research also demonstratesthat people have little or no awareness ofwhere reflections ought to be19, or even ofwhat they should look like20. Subjects in oneexperiment had to indicate on a drawingwhere, when walking into a room, theywould first see the reflection of a particularobject in a mirror. Even physics studentswere unable to do this with any accuracy.Experiments also show, as painters haveknown for centuries, that the pattern ofreflection on a surface doesn’t have to matchthe actual scene around it for it to appear as areflection21 (Fig. 14). The pattern only needsto match the average properties of naturalscenes20 and curve in concert with theimplied curvature of the shiny surface22.

The neuroscience of artPaintings and drawings are a 40,000-yearrecord of experiments in visual neuro-science, exploring how depth and structurecan best be conveyed in an artificial medium.Artists are driven by a desire for impact andeconomy: thousands of years of trial anderror have revealed effective techniques thatbend the laws of physics without penalty. We

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Figure 12 Vuilleumier et al.17 found that the blurry, fearful face on the right activated the amygdala more than the sharply detailed or unfiltered versions.This suggests that low spatial frequencies (gross detail) provide the amygdala, an important emotional centre of the brain, with coarse, but rapid, fear-related information. The face-recognition areas of ventral visual cortex showed less activation in response to the blurry versions than to the sharp orunfiltered images. Slower conscious analysis may therefore rely on the high spatial frequencies for face identification. Earlier studies24 showed that theamygdala can respond to the fearful images even in a brief, masked presentation that subjects do not report seeing. The right amygdala has even beenshown to respond to emotional facial expressions in a patient with no primary visual cortex and no conscious visual experience21.

Figure 11 The blurry, global shapes and colours may convey emotional content directly to emotionalcentres of the brain while the irrelevant fine detail typical of Impressionist pieces distracts consciousperception. (Oil on canvas (55.1 x 65.9 cm), Potter Palmer Collection, The Art Institute of Chicago.)

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can look at their work to find a naive physicsthat uncovers deep and ancient insights intothe workings of our brain. Discrepanciesbetween the real world and the worlddepicted by artists reveal as much about thebrain within us as the artist reveals about theworld around us. ■

Patrick Cavanagh is in the Vision SciencesLaboratory, Department of Psychology, HarvardUniversity, 33 Kirkland Street, Cambridge,Massachusetts 02138, USA.1. Cavanagh, P. Leclerc, Y. G. J. Exp. Psychol. Hum. Percept.

Perform. 15, 3–27 (1989).

2. Casati, R. The Shadow Club (Little Brown, New York, 2004).

3. Yonas, A., Cleaves, W. T. & Pettersen, L. Science 200, 77–79 (1978).

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5. Bruno, V. J. Form and Color in Greek Painting (W. W. Norton &

Company Inc., New York, 1977).

6. Hills, P. The Light of Early Italian Painting (Yale Univ. Press,

New Haven 1987).

7. Gombrich, E. H. Shadows (Natl Gallery Pub., London, 1995).

8. Ostrovsky, Y., Sinha, P. & Cavanagh, P. Perception (in the press).

9. Kennedy, J. M. & Silver, J. Perception 3, 313–322 (1974).

10.Yonas, A. & Arterberry, M. E. Perception 23, 1427–1435 (1994).

11.Kennedy, J. M. & Ross, A. S. Perception 4, 391–406 (1975).

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1–11 (1990).

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Hecht, H. Schwartz, R. & Atherton, M.) 239–300 (MIT,

Cambridge, Massachusetts 2003).

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Intl Symp. 3D Data Processing, Visualization and Transmission

35–41 (2004).

17.Vuilleumier, P., Armony, J. L., Driver, J. & Dolan, R. J. Nature

Neurosci. 6, 624–631 (2003).

18.Miller, J. On Reflection (Yale Univ. Press, New Haven, 1998).

19.Croucher, C. J., Bertamini, M. & Hecht, H. J. Exp. Psychol. Hum.

Percept. Perform. 28, 546–562 (2002).

20.Fleming, R. W., Dror, R. O. & Adelson, E. H. J. Vis. 3, 347–68

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Neurosci. 8, 24–25 (2005).

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Proc. 1st Symp. on applied perception in graphics and

visualization. 115–118 (2004).

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Brain Cog. 6, 1–14 (1987).

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Figure 14 These green billiard balls are all shown to reflect the scene that surrounds the ball on the left20. Ino longer correspond to the new surrounds but subjects perceive the balls to be as glossy as the one on the

Figure 13 Try to determine where the woman is looking. Could she be looking at her own reflection or is that physically impossible?

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Acknowledgments: I thank M. Bernson, E. Besancon, M. Carrio,

A. Dietrich, H. Farid, L. Gay, A. Kiely, A. Lonyai, C. Pemberton, D.

Wang, S. Hamlin and R. M. Shapley for their contributions to this work.

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n the central and right examples, the reflections left. (From ref. 20.)