by Larry Witham
There is another way that scientists have tried to explain what the brain is doing in visual perception. Rather than focus on modules, this second approach charts two kinds of general functions across the brain: the “What” and “Where” systems of visual perception.20 The “Where” system is the most fundamental facet of the biology of seeing. Although it is color-blind and has less power of detail, it is highly sensitive to small differences in brightness, namely luminescence. The Where system—which combines a number of cellular functions in the brain—picks up contrast, and therefore edges. In short, this system gives the seeing brain the reality of form in the world based on shape, size, depth, and distance. The Where system may be more basic, for it is possessed by many other animals, all of whom must be able to visually make out forms and spaces to survive in the world.
By contrast, the “What” system sets the human primate apart from all other organisms with a sense of sight. The What system is the power of visual precision and recognition. It detects color with great exactness, although it requires large differences in brightness to distinguish one shape from another. Cooperation between the Where and What systems underwrites all the powers and wonders of vision. Behind this simpler description, further complexity lurks, as is to be expected. To do its job, for example, the What system operates with two subsidiary functions. One sees forms by detecting both color and brightness. The other focuses on color alone; it typically apprehends fields of color as they appear on surfaces of objects. Such subdivisions and specializations are evident to science and medicine by the fact that specific parts of visual perception can be absent in humans, usually due to brain damage. Some people have Where powers, but no specific What powers, and the opposite is equally true.21
At the center of everything in the visual system, however, sits the ability to judge luminescence. “I suspect light-dark contours are the most important component of our perception, but they are surely not the only component,” says David Hubel, the modern Nobelist in visual neuroscience.22 Centuries earlier, Alberti had arrived at a similar observation. In On Painting, he says the artist’s mastery of dark and light is most important for an effective work of art. Such contrasts seem to speak ultimately to the needs of the brain, and here is a start at understanding why paintings such as those by Piero can evoke mental interest, even pleasure.
After luminescence is detected, however, something more must happen. The mind must recognize, interpret, and connect those visual experiences with all else that is mental—memory, emotion, insight, and even inspiration. How does that take place? Short of finding a completely anatomical answer, the Platonist philosophy of mind retains a certain modern relevance. The Renaissance Platonist Ficino offers an overview that, while poetic, continues to ring true: “When anyone sees a man with his eyes, he creates an image of the man in his imagination and then ponders for a long time, trying to judge that image. Then he raises the eye of his intellect to look up to the Reason of Man, which is present in the divine light. Then suddenly from the divine light a spark shines forth to his intellect and the true nature of Man is understood.”23
This metaphysical view of the mind is hard to swallow for modern psychology and neuroscience, of course. In social psychology, moreover, tastes in art and culture are said to arise primarily from the outside: from social conditioning, competition, economics, and such mundane and ephemeral realities as fashion. There is little room here to search for an innate—let alone metaphysical or transcendental—apprehension of Beauty as a universal essence. Even more than social psychologists, perhaps, neuroscientists will roll their eyes at the Platonist prose typified by Ficino. Nevertheless, it continues to be part of the lineage of idealist philosophy, which includes modern names such as Immanuel Kant and Ernst Cassirer, and it remains an alternative to a purely naturalistic explanation of the mind.
Even in the sciences, there is disagreement on how far the scientific method, including its ethics of experimenting on humans, can go to crack the mystery of human consciousness and mental life. For some, it will always be a frontier that cannot be crossed for reasons of the sheer interiority of human perception.24 In the nineteenth century, Hermann Helmholtz had cited these limitations: “We cannot at present offer any complete scientific explanation of the mental processes involved, and there is no immediate prospect of our doing so.”25 For a number of reasons, a complete explanation may forever be elusive. In the meantime, one fairly solid finding about mental perception has proved to be remarkably similar to the traditional Platonist assessment, the basic finding that the physical brain seeks essences and constants.26
Essences and constants—these have recently become terms in the toolbox of psychology. And within the study of vision, even neuroscience is sounding like a Platonist doctrine. Where Platonism and neuroscience can agree is that the brain, finally, is sorting through the flux of impressions in search of essences, both for the survival of the organism it serves and in apprehending a pleasure that traditionally has been called beauty. Platonism and neuroscience are both looking inside the “mind” for the seat of our deepest perceptions.27 The stark difference is this, of course: Platonism acknowledges a kind of transcendental intuition in the brain, while philosophical materialists and neuroscience itself reject such a “Ghost in the Machine,” as one twentieth-century philosopher cleverly put the case.28
Ghost or no ghost, for both neuroscience and Platonism the experience of beauty is said to arise from the recognition of what is essential and constant. This goes further than Helmholtz’s tentative theory that comfort to the eye produces the pleasure of beauty. Adding to that, neuroscience now suggests that the greatest works of art may be those that are able to capture the greatest number of the essential qualities related to the needs of the brain.29 Operating very much on its own, the brain seizes upon these qualities and they produce an automatic sense of satisfaction. Perhaps this is why, historically, beauty has been tied to a contemplative mode, not to merely satiating an appetite such as food, sleep, or sex. The brain’s automatic recognition of visual constants—and thus features it deems pleasant—may explain why philosophers have called the experience of judging beauty “disinterested,” done for its own sake and not mixed with other motives.30
A corollary to the brain’s easy recognition of constancy is its encounter with anomalies in the world—that is, visual events that run counter to what the mind expects: the mind is taken aback by things that defy normal expectations, or that defy what seems to be natural. An optical illusion, for example, or a “counterintuitive” event, puts the mind on alert. This is a well-recognized form of mental stimulation, and the resulting alertness has been linked to both the nature of aesthetic experience, in which shapes defy normal expectation, and the nature of religious experience, in which events that go against the grain of nature seem supernatural.31
Platonism and neuroscience find consonance in one other important area, it seems. Although the basic structure of the brain has not changed since the emergence of modern humans, the neural networks in each individual brain can change by experience. The brain, it turns out, is both constant and “plastic.” This is one way to reconcile the fact that, while humans perceive beauty in some common ways, each culture or period in history may see, or define, specific kinds of beauty based on local experience. In his philosophy of judging beauty, Immanuel Kant grappled with this very conundrum: how individual judgment must be free, and yet humans tend to arrive at a similar conclusion about what truly is beautiful regardless of specific cultural norms.
For centuries, the classical and Platonist traditions have acknowledged that people make claims in regard to seeing absolute and relative beauty.32 In a Platonist psychology, too, the mind is both constant and plastic. The mind recognizes the flux of reality. Plato called this the assertion of “opinion” on the way to true “understanding,” which exists in unchanging and transcendent Ideas. This kind of mind—situated between the m
aterial and transcendent worlds—is the very premise of religion. In artistic practice, moreover, the quest of a painter such as Piero della Francesca is no different. The maker of art stands between some unchanging ideal—either transcendent or a product of imagination—and the world of material transience.
On the foundation of neuroscience, the field of neuroaesthetics is attempting to explain the satisfaction—namely, the experience of beauty—that is felt through various kinds of artworks.33 So far, few art historians have pushed this approach as far as it might go. One of them has been Michael Baxandall, a British historian of Renaissance art. In the positivist tradition of Heinrich Wölfflin and Gombrich, Baxandall proposed the notion of the “period eye,” which is a biologically based visual preference of people at particular times and in particular cultures (based on the presumption that their brains have evolved slightly to established tastes for what they experience as satisfying art).34 Nevertheless, even amenable art historians such as Baxandall have gone down this path of neuroaesthetics carefully, even reluctantly. Of the biological brain’s perception of art, Baxandall says:
The process is indescribably complex and still obscure in its physiological detail… . Higher levels of the attentive visual process introduce different kinds of problems, particularly when the attention is to complex paintings, and for various reasons I do not feel the cognitive sciences invoked here [in his overview essay] offer art criticism as much broadening suggestion for dealing with those higher levels: for that we must go elsewhere.35
Baxandall is saying that social, cultural, and educational factors are the “higher levels” that finally may determine why, for example, the Italian Renaissance celebrated its type of art while art patrons in China or Africa had different tastes. With such major caveats in mind, the field of neuroaesthetics nevertheless presents a range of theories based on experiments with the visual system. How, then, have these theories interpreted some popular types of paintings? Second, what might neuroaesthetics say about the works of Piero della Francesca (whose works, meanwhile, have not been subjected to a full neuroaesthetic analysis)? Some of the published neuroaesthetic conclusions about well-known kinds of art include:
Paintings done in the Renaissance tradition of perspective and naturalism offer a geometric reality that satisfies the visual system’s quest for clarity, essences, and constancy.36
Realistic paintings that offer a degree of ambiguity also satisfy the brain’s ability to fill in constants where a familiar setting is otherwise not too precise. One example is a Vermeer domestic scene that does not reveal all the details behind the human circumstances. In such cases, the mind can insert into the visual scene an archetype from memory (or, from a metaphysical standpoint, what Plato might call a universal Idea).37
Many kinds of paintings play off the eye’s focal and peripheral vision and thus send a kind of unique stimulation to the brain. These optical effects would include, for example, the way peripheral vision picks up a shadow around the Mona Lisa’s mouth that makes it seem like she is smiling—when she is not exactly doing that. The peripheral vision of viewers is also active in filling in colored areas in Impressionist-style paintings that otherwise lack precise shapes and edges; this visual filling-in gives even sketchy Impressionism a solidity and reality that satisfies the mind.38
Abstract art has been useful in studying neuro-responses because it activates the most basic feature-detector neurons in the visual system, those that detect strong edges, directions of lines, and contrasts in luminosity, including color contrast. In studies of blood flow in the brain, abstract art stimulates certain visual-detector cell areas without fail, whereas art that is more representational—adding a complex story to be interpreted by the viewer—will disperse the blood flow to other higher areas of mental perceptions as well.39
Many kinds of modern paintings stimulate the mind by confounding its What and Where systems. For example, Impressionism and Post-Impressionism—from a Monet to a Seurat—create their famous flickering sense of movement and light because the What system finds recognizable objects, but the Where system cannot always find precise edges. Similarly, when confronted with Cubist paintings that suggest a fragmented reality with many viewpoints, the What and Where systems are stimulated by the discordant perceptions. The brain attempts to arrive at an identifiable essence: a violin or portrait hidden in the Cubist painting, for example. The brain must compose the essence out of multiple viewpoints and memory, much as the brain over time creates knowledge and memory out of constant fragmentary views of well-known things in the world.40
Paintings with a wide range of luminosity strike visual perceptions as particularly rich because they emulate all the subtleties of seeing in the real world. The range of dark and light used by Leonardo da Vinci and Rembrandt, for example, exploits the satisfaction that the brain normally experiences from familiar luminosity contrasts found every day in nature.41 The same is obviously true the world over today; but historically, such three-dimensional modeling with luminosity is a unique innovation of European art (only later adopted, for example, by Japanese painters, or painters in India, who added this range of modeled luminescence to their flat and high-contrast prints, miniatures, and screen paintings). Hence, the old European masters continue to be admired today for their subtlety of using a scale of values of luminescence to create a sense of space (rivaled only by photography, centuries later).
Given this range of findings—and there are more—what can neuroaesthetics say about the enduring attraction to the works of Piero della Francesca? Connoisseurs such as Roberto Longhi have described Piero’s unique visual effect as “color-form,” Bernard Berenson spoke of his monumental stillness as the “ineloquent in art,” and Roger Fry pointed to the marvelous “plasticity” of his paintings. Now that neuroaesthetics is providing an additional vocabulary in art appreciation, we can follow that line of speculation to Piero and ask: How might his art conspire with the strengths of the visual system to evoke mental satisfaction?
Like a good many painters, Piero starts by simply hitting upon rudimentary features in biology that make visual art compelling. His perspective provides a clarity that satisfies the mind, and, by the care in which he organizes his objects, Piero creates a unique illusion of space, certainly quite novel in the Renaissance, but still engaging for the human eye today. Closely related to this, he portrays reality with a naturalism that very nearly becomes pure geometric shape. These set up a wide variety of edges—vertical, horizontal, curved, and diagonal—that in turn set off feature-detection cells up the line toward the visual cortex and beyond.
Piero’s realism activates the mind’s power of depth perception, as has been analyzed in perhaps the only study that attempts to probe the effect of a Piero painting on the brain. This is Baxandall’s analysis of Piero’s Resurrection. While it is not based on a laboratory experiment (in which, for example, subjects are wired to brain scanners as they look at a Piero painting), Baxandall brings to bear much that is known about how depth perception works. It is new, but also old, for Baxandall covers tricks well known by some Renaissance painters and by methods that Hermann von Helmholtz, a century earlier, had summarized in his lectures on physiological optics and painting. In this updated survey, Baxandall credits the Resurrection with providing ten cues to the brain—from foreshortening to shadows and object sizes—to make it think it is seeing authentic depth. One new feature identified in research on visual psychology is called the “texture gradient,” which Baxandall also adds; it is the way a surface’s visual texture changes as it recedes into the distance, which a skilled painter may replicate.42
This sort of analysis may also suggest the effectiveness of Piero’s geometric use of perspective, his famous “power of lines.” However, it has today been argued that modern viewers don’t need mathematical perspective; they just need enough in the way of visual cues—a few suggestive lines, for example, or some visual tricks on
the Baxandall list—for their brain, conditioned for generations by photography, to decide it is seeing the full-blown illusion of depth. If Piero were painting today, in other words, he could put aside his Euclid, compass, and ruler and apply his brush more casually. According to experimental psychology, the modernly conditioned brain will fill in the details in even an ambiguous painting.
This is the theory of “mental sets,” advocated by Ernst Gombrich, who said that the “laborious constructions [of Piero] ceased to be necessary for the suggestion of space and solidity when the public was prepared to ‘take them as read’ [i.e., take them on faith].” Piero’s hyper-perspective, with its overbearing power of lines, might even be repellent, Gombrich suggested: “Once the requisite mental set was established among beholders, the careful observation of all [perspectivist painter] clues was not only redundant but something of a hindrance.”43 Nevertheless, while such laborious geometric constructions may no longer be needed to suggest space, especially in modern or Impressionist painting, the value of Piero’s clarity seems undiminished. Our over-acquaintance with perspective has come primarily from still photography, with its images of railroad tracks meeting on the distant horizon. But it remains clear that Piero achieved his unique effects by way of a difficult manual craft.
Although very few of Piero’s works use a strong chiaroscuro, which provides the highest contrasts of luminescence, Piero strikes a consistent balance of dark and light. The effect is one of full sunlight, generally. This often is accented by a cool range of colors, all of which produces a pale harmony that, for reasons beyond the detection of science, is acknowledged as pleasurable. Perhaps deep in the human brain, the full light of day is greatly valued (for effective living and survival), and on that basis the paintings of Piero have been spoken of as idealized portrayals. They turn organic forms into geometric or ideal Platonic shapes. They offer utter clarity across a visual field.