It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind (17 page)

The tendency of neurons to inhibit neurons they adjoin is called
lateral inhibition
. The discovery of lateral inhibition proved to be enormously important in neuroscience because it revealed how the interplay of neurons in simple circuits could give rise to adaptive and often surprising results, such as hypersensitivity to edges.

Inspired by the discovery of lateral inhibition, many neuroscientists embarked on the study of neural circuits. They appreciated that neural wiring diagrams, using lateral inhibition and other interactions, could underlie the workings of the brain.

The study of neural circuits has emerged as one of the most exciting frontiers of science. That said, my interest here is less on the details of the circuits than in the larger principle they embody—that the brain follows a neural dialectic, a balance of competition and cooperation from which mental and behavioral phenomena emerge.

Is the balance of neural cooperation and competition peculiar to the visual system of horseshoe crabs? Consider another phenomenon known as
center
-surround inhibition. This phenomenon also reflects the excitatory-inhibitory drama sketched above, but it is also evident in mammals, including people.

If you work in a neurophysiology lab and cast a spot of light on the eye of a cat or other mammal, the neural responses you can record from the animal’s retinal ganglion cells are similar to those picked up from the horseshoe crab’s ganglion cells. Ganglion cells called “on-center” cells are excited by light cast directly
on
them. But they’re also excited by the relative
lack
of light cast
around
them. By contrast, ganglion cells called “off-center” cells are excited by the relative
lack
of light cast directly on them, and also by the presence of light cast in surrounding areas. The underlying mechanism in both cases is lateral inhibition.

Of what functional consequence are these two sorts of cells—these “rooster” cells that wake up to light and are turned on when their neighbors find themselves in the dark (the on-center cells), and their “vampire” cousins that revel in darkness and are turned on when their neighbors find themselves in the spotlight (the off-center cells)?

Having on-center cells and off-center cells promotes edge detection, which is no less important for landlubbers than for crabs and other aqua-dwellers. Edges help you identify objects and locate their boundaries. If you’re unsure of the importance of edge detection for vision, ask any computer scientist who works on robot vision whether he or she thinks edges are important. The roboticist will tell you that without edges, it would be well-nigh impossible to recognize objects. Computer-vision systems designed to identify objects often exploit on-center and off-center units to detect object edges.

Is there evidence that on-center cells and off-center have observable consequences for visual experience, or are the consequences of these cells seen only in neurophysiology labs? It turns out that the effects of on-center cells and off-center cells can be observed in the comfort of your home.

Consider Mach bands, discovered by the Austrian physicist, Ernst Mach, who, among other things, was the first person to measure the speed of sound. Mach bands are illusory, but when you first see them, it’s hard to believe they are (
Figure 6
).
²

The physical display used to show Mach bands has adjacent regions that get progressively darker.
Figure 6
shows a series of such regions. Wherever the regions touch, you may see a bright region on the dark side and a darker region on the brighter side. This may cause you to see depth. When you look at the image in
Figure 6
, you may see ridges at the boundaries.

This illusion is created by the visual system’s reliance on lateral inhibition. On-center cells are bombarded by inputs that cause them to get excited in the light, causing you to see a white band. Off-center cells, meanwhile, are bombarded by inputs that cause them to get excited in the dark, causing you to see a dark band. Neither of these bands is actually present. They are figments
of your imagination, as it were, or, more specifically, of the lateral inhibition in your visual system.

FIGURE 6.
Mach bands.

You can experience a similar visual illusion by looking at an image made up of black tiles with narrow white borders (
Figure 7
).
³
If you look at such an image, you will probably see gray squares at the intersections of the white boundaries. These gray squares are not present in the display. They are created by you, or by neural interactions in your visual system. Cells turned on by the large dark regions send massive excitatory inputs to on-center cells as well as massive inhibitory inputs to off-center cells. Meanwhile, the on-center cells are excited by the bright strips, and the off-center cells are inhibited by those same bright strips. The combined output of the on-center cells and the off-center cells is a weighted sum of the outputs of the two kinds of cells. The perceptual consequence is a shade of gray that, like Mach bands, reflects lateral inhibition.
⁴

Another perceptual consequence of “inner jungleism” is one that may be so startling you may break out in a sweat when you read this section.
I confess I was really quite shaken up when I learned what I’m about to tell you, so be prepared.

FIGURE 7.
The grid illusion.

Presumably, you believe the world has color. Consider the following verse, however, which I composed for this occasion:

Roses are red
,

Violets are blue
,

Grass is green
,

But all through you
.

The world, it turns out, has no color! Nothing inherent in the wavelengths of light produces the colors we see.

You may not believe this. “What’s that you’re saying?” you may exclaim. “Are you color blind?” Fortunately, I do see color. I don’t suffer from color blindness, though as a male I’m statistically more likely than females to have this syndrome. In saying the world is colorless, I mean that color is an emergent feature of brain activity. Only by virtue of the internal pulls and pushes of neural friends and foes do we see the colors we do.

The colors we see are due to three teams of neural elements in the retina. Some are activated by the wavelengths of light we call “red” (long wavelengths), some are activated by the wavelengths of light we call “green” (medium
wavelengths), and some are activated by the wavelengths of light we call “blue” (short wavelengths). The interactions among these three elements produce all the colors we see. Especially important for the theme of this book, the interactions reflect cooperation among members of the same retinal team, as well as competition among members of opposing retinal teams.

Understanding color perception in its full form requires an
opponent-process
model of color perception. This model was first proposed by a nineteenth-century physiologist named Ewald Hering and was later refined by two vision scientists, Leo Hurvich and Dorothea Jameson, who spent most of their careers at the University of Pennsylvania.
⁵
The main idea of the opponent-process model is that neurons responding to different dimensions of visual input at levels higher than the retina send excitatory signals to their friends and inhibitory signals to their enemies. The dimensions of the opponent processes turn out to be red-green, blue-yellow, and black-white. These dimensions refer to the avenues of animosity. Red and green are opponents, blue and yellow are opponents, and black and white are opponents.

Having a model like this explains the so-called negative aftereffect of color. Stare at a green field for a while and then look at a white field. The white field will appear to be a shade of red. If you do the exercise the other way around, you will get the opposite result. If you stare at a red field for a while and then look at the same white field, it will appear to be a shade of green.

This is a strange outcome if you think about it. It’s surprising that after seeing one color for a long time, you should see some other color. It’s also surprising that the color you see on a chromatically neutral surface (a white sheet of paper, for example) after prolonged exposure to a chromatic hue such as red or green is some
other
particular color (red following green, or green following red).

What accounts for this? If one color is present for a long time, neural receptors for that color undergo a change due to their extended stimulation. It would be convenient to say that the receptors become fatigued, but that’s a bit of an oversimplification. Among other things, saying that receptors get tuckered out doesn’t explain why you continue to see a color as long as it’s present. You’d expect the color to disappear if the receptors simply became exhausted. Claiming that receptors simply get tired also doesn’t explain why you see a complementary color when the first, long-exposed color is replaced by a chromatically neutral stimulus.

So what actually happens? For present purposes, it suffices to say that there’s a duel between receptors for one color and receptors for the color’s opponent. If the receptors for green get stimulated for a long time, their ability to inhibit their opponents reaches some steady, low level. Then, when the green
receptors get no more visual excitation, they stop inhibiting their red opponents. The perceptual result is perceived redness (or a tint thereof).

A number of other visual phenomena are explained by the opponent-process theory of color vision. I won’t review them here because this book is meant just to give a taste of various mental and behavioral phenomena and the internal cooperation and competition that yield them. Nevertheless, I do want to mention that direct evidence has been obtained for the kinds of color receptors assumed in the opponent-process theory. That evidence has been obtained through physiological recordings similar to the ones taken in the study with which this chapter began—where recordings were taken from visual-processing cells of the horseshoe crab. The color-receptor studies were done in mammalian visual systems.
⁶

Still more support for the understanding of color perception comes from color blindness, a topic I mentioned briefly. In some individuals, receptors for particular wavelengths are missing due to genetic mutations. These mutations happen to be sex-linked. In humans, the inability to see some colors is more common in males than in females. The way color blindness is tested is to determine whether an individual suspected of color blindness misses objects differing from other objects with respect to hue alone. Failing to notice those objects provides the telltale sign that receptors for that hue are missing. Which colors can and can’t be seen by color-blind individuals, plus the negative aftereffects of color they do or don’t experience, fit with the model of color perception sketched above, an opponent-process model that, as its name implies, relies on inner competition (and cooperation, too).
⁷

In the last section, you saw that one form of evidence for opponent processes in color perception is the negative aftereffect of color. Because I adduced this phenomenon as a source of evidence for “inner jungleism,” it would be useful to know that analogous phenomena exist for other aspects of perception. In this connection, consider the so-called negative aftereffect of motion. To a first approximation, this is the motion equivalent of the negative aftereffect of color.

To understand the negative aftereffect of motion, suppose you’re in a tranquil place. Instead of dodging dangerous dogs or evading electrifying eels, you take a break. You sit down beside a peaceful waterfall, watching its languid tendrils descend from the brook above. You sigh. You relax. You enjoy the gentle summer breeze. At long last you’ve found a place where you can relax.

After watching the waterfall for a while, you look past it, glancing non-chalantly at the hill beyond, expecting to take in another calm vista. But the sight you behold baffles you. The slope beyond the stream seems to stream upward! It does so for several seconds. No earthquake has occurred. No gigantic UFO, à la
Close Encounters of the Third Kind
, has floated above, lifting the hillside to take it home for study.
⁸