Rees and his group used a dynamic version of such bistable illusions: a cloud of moving dots perceived as a cylinder rotating either to the left or to the right. Here the scientists correlated the width of the cortical sheet—the thickness of its gray matter—with how long each stable percept lasts before it switches. Scanning the brains of 52 subjects—in a field dominated by studies that come to grand conclusions by querying a handful of brains—they found only a single region, the left and right superior parietal lobe (SPL), in which the thickness of the gray matter (and its density) significantly and consistently correlated negatively with the perceptual duration. In other words, the thicker the SPL cortex, the faster two interpretations switch back and forth. It is known from other imaging and clinical studies that the SPL in the back of the brain controls selective visual attention, but how the thickness and density of SPL gray matter should be important is anybody’s guess.
Ask people what they believe to be the defining feature of consciousness, and most will point to self-awareness. To be capable of being aware of your hopes, to worry about your spouse’s illness, to wonder why you feel despondent or why he provoked you is taken to be the pinnacle of sentience. Self-awareness is, by and large, absent in nonprimates. Although my dog—as with many and, perhaps, all animals—experiences the sights, sounds and, in particular, the smells of life, she doesn’t worry why her tail isn’t wagging as it used to or whether tomorrow’s food will appear.
So can differences in this elusive higher-order aspect of consciousness be tied to differences in brain structures? Yes, as a just published third study by Rees and his colleagues concludes.
Thirty-two healthy volunteers carried out a difficult visual task in the scanner. They had to judge which one of a number of faint patches was a tad more salient than the other ones; this judgment was purposefully made demanding. Following each trial, subjects had to choose a number between one and six, indicating the confidence they had in their own judgment. A six indicated that they were very confident of their judgment, whereas a one implied a guess. That is, they were asked to introspect: Are you sure you just saw the bright patch here? Psychologists know this as meta-cognition: thinking about thinking.
Not surprisingly, subjects differed greatly in the accuracy of their judgments (independent of the level of their performance). Think of the game show Who Wants to Be a Millionaire, where contestants have to judge whether they want to use a lifeline before they know the answer, depending on their confidence. Some people are astute, using the lifelines wisely; other people fritter them away. The cognitive scientists extracted a measure of variability of introspection and discovered that this measure correlated with variability in gray matter volume in the right anterior prefrontal cortex. The more neurons you have in this region in the front of the brain, the better your introspection. Not that your performance goes up, but the insight you have into your performance—whether you thought you did well or not—increased. Patients with lesions in these regions typically lose the ability to introspect. And this part of the neocortex has expanded more than any other region in primates. Again, the neuronal mechanisms underlying this correlation remain unknown for now.
Rees’s studies establish that differences in the morphology, or shape, of our brains are mirrored in differences in the way we consciously experience and apprehend the world, including our own brains and bodies. In this way, neuroscience maps the physical structure of the material brain onto the inner geometry of phenomenal and ineffable experience.