NYU Vision Scientist Denis Pelli Says Close's Paintings Refute Long-Standing Assumptions About How Shape Is Perceived
Pelli's Findings Are Published in the August 6th Issue of Science
The work of painter Chuck Close is well known. For 25 years, he has created gigantic "block portraits." Up close, his paintings appear to be orderly arrays of painted squares. But as the viewer backs away from the canvas, a human face emerges. The common explanation for this visual effect is that the edges of the individual blocks blur together as the viewer backs away.
But an article by NYU vision scientist Denis Pelli in the August 6th issue of Science challenges this explanation. According to Pelli, the transition from grid to face occurs at relatively short distances from the canvas (typically less than six meters). At these distances, the individual blocks are still in sharp focus for a person with normal vision. Something else is driving the effect.
According to Pelli, the dramatic effect of viewing distance on the appearance of Close's paintings shows that perception of shape depends on visual size. This finding, which Pelli credits to Close, refutes the age-old assumption that shape is seen the same way at all sizes.
Pelli's data were collected at the Chuck Close Retrospective, when it was at the Museum of Modern Art in New York City in the spring of 1998. (The retrospective is now at the Hayward Gallery in London, July 22 to September 19.) Pelli's main experiment used just a tape measure.
He notes that "It is commonly assumed that scientific research today can only be done in laboratories with complex equipment. But the essence of science is careful observation of the world around us. And in this sense, the best science and the best art share a common methodology."
Each of 5 observers was asked to do a "nose test" on 33 Close paintings to measure the transition from grid to face. While looking at a given painting, the observer moved forward and backward to find the point where the nose emerges from the canvas. These "critical distances" were recorded for each observer. Pelli also measured the size of the blocks making up the painting, and calculated the angle subtended at the observer's eye at the critical distance. To his surprise, this critical angle was always about 0.3 degree, for all the paintings, despite their great variety in face size, block size, and number of blocks per face.
There are two existing theories--proposed by Harmon and Julesz in an influential 1973 Science article--for why the face is obscured by the blocks when seen from near, and revealed when seen from far. Pelli's results refute both theories.
The first theory proposes that a certain number of blocks per face are required to adequately represent the information that our visual system uses to see the face. According to this "critical band" theory, coarser blocks interfere by introducing visual "noise" into the "critical band." This theory predicts that faces with fewer than, say, 10 blocks across the face (from cheek to cheek), should never be recognizable as faces, and that faces with more blocks will always be recognizable. But, in fact, Close's portraits have variable numbers of blocks across the face, and every one is recognizable at some distances and not at others. The second theory, as mentioned above, assumes that backing away reveals the face because our eye optically blurs the blocks together. But this would predict a much smaller critical angle, about 0.1 deg, rather than the 0.3 deg Pelli found.
Instead, Pelli's results suggest that when the marks exceed the critical 0.3 degree size they are perceived as discrete objects, preventing the observer from seeing the face.
In the Science article, Pelli argues strenuously that Close deserves credit for these findings, "One might suppose that he was a naïve artist, obsessed by grids, who innocently produced the coarsely gridded paintings that we use here to reveal the size dependence of shape perception. In fact, Close has devoted his career to studying just that . . . He was more thorough than his scientific colleagues; the size of the marks in his block portraits increased by 15 percent per year from 1973 (0.4 cm) to 1997 (9 cm). He made sure that exhibitions of his work would convey the idea, canceling a retrospective that could not provide long viewing distances. So credit Chuck Close with discovering this size-dependent breakdown of our ability to extract shape from shading, well within the bounds of our visual field and acuity."
Pelli also noted the distinction between the work of Close and that of other artists. He said, "There are well known antecedents to block painting: Lichtenstein's cartoons, Seurat's pointillism and the ancient mosaics at Delos and Pompeii. However, none used coarse grids to render three-dimensional shapes, so there is no duality. The mosaics and the pointillist grids are too fine to readily disintegrate into flat marks, and Lichtenstein's benday screens are uniform, so they're always flat, at all distances. The only precendent for the duality of Close's recent works may be the long-lost skiagraphia of Apollodorus."
This research was supported by the National Eye Institute.
Denis G. Pelli is an NYU professor of psychology and neural science. He received his Ph.D. in 1981 from Cambridge. He is the co-designer of an eye chart -- the Pelli-Robson contrast sensitivity chart -- that measures the faintest letter we can see, complementing the traditional eye chart measurement of the smallest letter we can see, to provide a more complete assessment of visual function.
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