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

Now I’d like you to try another experiment. Go back to Figure 2.4, but this time, instead of rotating the page, hold it upright and tilt your body and head to the right, so your right ear almost touches your right shoulder and your head is parallel to the ground. What happens? The ambiguity disappears. The top row always looks like bumps and the bottom row as cavities. This is because the top row is now light on the top with reference to your head and retina, even though it’s still light on the right in reference to the world. Another way of saying this is that the overhead lighting assumption is head centered, not world centered or body-axis centered. It’s as if your brain assumes that the sun is stuck to the top of your head and remains stuck to it when you tilt your head 90 degrees! Why such a silly assumption? Because statistically speaking, your head is upright most of the time. Your ape ancestors rarely walked around looking at the world with their heads tilted. Your visual system therefore takes a shortcut; it makes the simplifying assumption that the sun is stuck to your head. The goal of vision is not to get things perfectly right all the time, but to do get it right often enough and quickly enough to survive as long as possible to leave behind as many babies as you can. As far as evolution is concerned, that’s all that matters. Of course, this shortcut makes you vulnerable to certain incorrect judgments, as when you tilt your head, but this happens so rarely in real life that your brain can get away with being lazy like this. The explanation of this visual illusion illustrates how you can begin with a relatively simple set of displays, ask questions of the kind that your grandmother might ask, and gain real insights, in a matter of minutes, into how we perceive the world.

Illusions are an example of the black-box approach to the brain. The metaphor of the black box comes to us from engineering. An engineering student might be given a sealed box with electrical terminals and lightbulbs studding the surface. Running electricity through certain terminals causes certain bulbs to light up, but not in a straightforward or one-to-one relationship. The assignment is for the student to try different combinations of electrical inputs, noting which lightbulbs are activated in each case, and from this trial-and-error process deduce the wiring diagram of the circuit inside the box without opening it.

In perceptual psychology we are often faced with the same basic problem. To narrow down the range of hypotheses about how the brain processes certain kinds of visual information, we simply try varying the sensory inputs and noting what people see or believe they see. Such experiments enable us discover the laws of visual function, in much the same way Gregor Mendel was able to discover the laws heredity by cross-breeding plants with various traits, even though he had no way to know anything about the molecular and genetic mechanisms that made them true. In the case of vision, I think the best example is one we’ve already considered, in which Thomas Young predicted the existence of three kinds of color receptors in the eye based on playing around with colored lights.

When studying perception and discovering the underlying laws, sooner or later one wants to know how these laws actually arise from the activity of neurons. The only way to find out is by opening the black box—that is, by directly experimenting on the brain. Traditionally there are three ways to approach this: neurology (studying patients with brain lesions), neurophysiology (monitoring the activity of neural circuits or even of single cells), and brain imaging. Specialists in each of these areas are mutually contemptuous and have tended to see their own methodology as the most important window on brain functioning, but in recent decades there has been a growing realization that a combined attack on the problem is needed. Even philosophers have now joined the fray. Some of them, like Pat Churchland and Daniel Dennett, have a broad vision, which can be a valuable antidote to the narrow cul-de-sacs of specialization that the majority of neuroscientists find themselves trapped in.

IN PRIMATES, INCLUDING humans, a large chunk of the brain—comprising the occipital lobes and parts of the temporal and parietal lobes—is devoted to vision. Each of the thirty or so visual areas within this chunk contains either a complete or partial map of the visual world. Anyone who thinks vision is simple should look at one of David Van Essen’s anatomical diagrams depicting the structure of the visual pathways in monkeys (Figure 2.6), bearing in mind that they are likely to be even more complex in humans.