The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

If you look in standard physiology and psychology textbooks, you will learn that a sketch is effective because cells in your primary visual cortex, where the earliest stage of visual processing occurs, only care about lines. These cells respond to the boundaries and edges of things but are insensitive to the feature-poor fill regions of an image. This fact about the circuitry of the primary visual area is true, but does it explain why a mere outline sketch can convey an extra vivid impression of what’s being depicted? Surely not. It only predicts that an outline sketch should be adequate, that it should be as effective as a halftone (the reproduction of a black-and-white photo). It doesn’t tell you why it’s more effective.

A sketch can be more effective because there is an attentional bottleneck in your brain. You can pay attention to only one aspect of an image or one entity at a time (although what we mean by “aspect” or “entity” is far from clear). Even though your brain has 100 billion nerve cells, only a small subset of them can be active at any given instant. In the dynamics of perception, one stable percept (perceived image) automatically excludes others. Overlapping patterns of neural activity and the neural networks in your brain constantly compete for limited attentional resources. Thus when you look at a full-color picture, your attention is distracted by the clutter of texture and other details in the image. But a sketch of the same object allows you to allocate all your attentional resources to the outline, where the action is.

FIGURE 8.1 Comparison between (a) Nadia’s drawing of a horse, (b) da Vinci’s drawing, and (c) the drawing of a normal eight-year-old.

Conversely, if an artist wants to evoke the rasa of color by introducing peak shifts and ultranormal stimuli in color space, then she would be better off playing down the outlines. She might deemphasize boundaries, deliberately smudging the outlines or leaving them out entirely. This reduces the competitive bid from outlines on your attentional resources, freeing up your brain to focus on color space. As mentioned in Chapter 7, that is what Van Gogh and Monet do. It’s called impressionism.

Great artists intuitively tap into the law of isolation, but evidence for it also comes from neurology—cases in which many areas in the brain are dysfunctional—and the “isolation” of a single brain module allows the brain to gain effortless access to its limited attentional resources, without the patient even trying.

One striking example comes from an unexpected source: autistic children. Compare the three illustrations of horses in Figure 8.1. The one on the right (Figure 8.1c) is by a normal eight-year-old child. Pardon me for saying so, but it’s quite hideous—completely lifeless, like a cardboard cutout. The one on the left (Figure 8.1a), amazingly, is by a seven-year-old mentally retarded autistic child named Nadia. Nadia can’t converse with people and can barely tie a shoelace, yet her drawing brilliantly conveys the rasa of a horse; the beast seems to almost leap out of the canvas. Finally, in the middle (Figure 8.1b) is a horse drawn by Leonardo da Vinci. When giving lectures, I often conduct informal polls by asking the audience to rank-order the three horses by how well they are drawn without telling them in advance who drew them. Surprisingly, more people prefer Nadia’s horse to da Vinci’s. Here again we have a paradox. How is it possible that a retarded autistic child who can barely talk can draw better than one of the greatest geniuses of the Renaissance?

The answer comes from the law of isolation as well as the brain’s modular organization. (Modularity is a fancy term for the notion that different brain structures are specialized for different functions.) Nadia’s social awkwardness, emotional immaturity, language deficits, and retardation all stem from the fact that many areas in her brain are damaged and function abnormally. But maybe—as I suggested in my book Phantoms in the Brain—there is a spared island of cortical tissue in her right parietal lobe, a region known to be involved in many spatial skills, including our sense of artistic proportion. If the right parietal lobe is damaged by a stroke or tumor, a patient often loses the ability to draw even a simple sketch. The pictures they manage to draw are usually detailed but lack fluidity of line and vividness. Conversely, I have noticed that when a patient’s left parietal lobe is damaged, his drawings sometimes actually improve. He starts leaving out irrelevant details. You might wonder if the right parietal lobe is the brain’s rasa module for artistic expression.