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

We began with a simple experiment. We showed Mirabelle a white number 5 on a black computer screen. As expected, she saw it in color—in her case, bright red. We had her fix her gaze on a small white dot in the middle of the screen. (This is called a fixation spot; it keeps the eyes from wandering). We then gradually moved the number farther and farther away from the central spot to see if this did anything to the color that was evoked. Mirabelle pointed out that the red color became progressively less vivid as the number was moved away, eventually becoming a pale desaturated pink. This in itself may not seem very surprising; a number seen off-axis prompts a weaker color. But it was nonetheless telling us something important. Even seen off to the side the number itself was still perfectly identifiable, yet the color was much weaker. In one stroke this result showed that synesthesia can’t be just a childhood memory or a metaphorical association.¹ If the number were merely evoking the memory or the idea of a color, why should it matter where it was placed in the visual field, so long as it is still clearly recognizable?

We then used a second, more direct test called popout, which psychologists employ to determine whether an effect is truly perceptual (or only conceptual). If you look at Figure 3.1 you will see a set of tilted lines scattered amid a forest of vertical lines. The tilted lines stick out like a sore thumb—they “pop out.” Indeed, you can not only pick them out of the crowd almost instantly but can also group them mentally to form a separate plane or cluster. If you do this, you can easily see that the cluster of tilted lines forms the global shape of an X. Similarly in Figure 3.2, red dots scattered among green dots (pictured here as black dots among gray dots) pop out vividly and form the global shape of a triangle.

In contrast, look at Figure 3.3. You see a set of Ts scattered amid the Ls, but unlike the tilted lines and colored dots of the previous two figures, the Ts don’t give you the same vivid, automatic “here I am!” popout effect, in spite of the fact that Ls and Ts are as different from each other as vertical and tilted lines. You also cannot group the Ts nearly as easily, and must instead engage in an item-by-item inspection. We may conclude from this that only certain “primitive,” or elementary, perceptual features such as color and line orientation can provide a basis for grouping and popout. More complex perceptual tokens such as graphemes (letters and numbers) cannot do so, however different they might be from each other.

FIGURE 3.1 Tilted lines embedded in a matrix of vertical lines can be readily detected, grouped, and segregated from the straight lines by your visual system. This type of segregation can occur only with features extracted early in visual processing. (Recall from Chapter 2 that three-dimensional shape from shading can also lead to grouping.)

To take an extreme example, if I showed you a sheet of paper with the word love typed all over it and a few hates scattered about, you could not find the hates very easily. You would have to search for them in a more or less serial fashion. And even as you found them, one by one, they still wouldn’t segregate from the background the way the tilted lines or colors do. Again, this is because linguistic concepts like love and hate cannot serve as a basis for grouping, however dissimilar they might be conceptually.

FIGURE 3.2 Dots of similar colors or shading can also be grouped effortlessly. Color is a feature detected early in visual processing.