( redux) Think Different The ways in which brains differ from one another shows up in the way their owners perceive the world By

Perceptual psychology and the brain sciences emphasize the communal- ity in the way that people experience re- ality. Leaving aside cases of brain dam- age or mental disease, we all see the sun rise in the east, enjoy the scent of a rose and experience a jolt of fear when we are woken up in the middle of the night by the sound of breaking glass. This is a re- flection of the great similarities of our brains compared with the brains of our close cousins on the evolutionary tree, the great apes. Laboratory science rein- forces this bias by lumping together the performance of its subjects on any one experiment and reporting only the aver- age and the variation around this mean. This conflation is also true for the telltale hot spots that show up in functional magnetic resonance brain images that Consciousness Redux column, I dis- Ponzo perceives the upper bar as larger we are used to seeing in newspapers, in cussed color blindness—the fact that than the lower one, the magnitude of magazines such as this one, on television about 7 percent of men lack one of the this effect differs substantially across in- and in the movies. genes for the retinal photopigments dividuals. (The size of the illusion is es- Yet as we know from our own life, needed to see hue. But what about differ- tablished by asking how much larger the each one of us has his or her own prefer- ences in the brain proper? Do they influ- lower bar has to be to make it look the ences, likes and dislikes. Some people ence consciousness in measurable ways? same size as the upper one.) Surprising- are acutely sensitive to flashing lights, To answer this question, scientists must ly, these differences are reflected in the some have perfect pitch, some cannot plumb the minds of many individuals surface area of the primary visual cortex see in depth, some can introspect and and relate them to measures of their (V1) at the back of the head. For un- analyze their own failures and triumphs, brains. The widespread availability of known reasons, the area of V1 can differ whereas others—remarkably frequently, fMR I scanners makes such a project fea- by a factor of three among people (un- public figures such as politicians—lack sible today. folded, the size and width of a typical V1 this knack. Take me. I am hopelessly at- Cognitive neuroscientist Geraint compares with that of a credit card). tracted to brilliant colors. As a magpie is Rees, a professor at the Rees and his collaborators discovered drawn to anything glittering, I am Center for Neuroimaging at University that the smaller a person’s V1, the more drawn to school-bus yellow, tangerine College London—undoubtedly the powerfully he or she experiences the il-

orange, burgundy red, rich magenta, world’s leading fMR I center—published lusion. Those individuals with a large ) electric violet, imperial purple and navy a trio of studies that relates differences V1 judged the size of the bars to be more rose (

blue. My love of the garish is reflected in in the way people experience things with similar than those with a smaller V1. IS B my flowery shirts and pants and, I’m differences in gross aspects of their cere- Curiously, the size of the two immedi- R O

sure, in an enhanced cortical representa- bral neocortex, the highly convoluted ately adjacent visual areas did not influ- ); C

tion of these hues. part of the forebrain that crowns the ence the amplitude of the illusion. och K It is obvious that if the apparatus that brains of all mammals. senses the world differs between two in- In one study 30 subjects looked at the Clues from Illusions ( C H KO F

dividuals, then the conscious experience Ponzo [see illustration on opposite page] Bistable illusions are those delightful O of the brains wired to these sensors can- illusion while their brains were scanned. images that can be seen in one of two RIST

not be the same either. In a previous Whereas everybody who looks at the ways. Probably the best known is the C H

16 scientific american mind January/February 2011 Can differences in this higher-order aspect of consciousness ( be tied to differences in brain structures? Yes. )

Necker cube, or the “old woman, young tied to differences in brain structures? girl illusion.” These two interpretations Yes, as a just published third study by flip back and forth. The time it takes for Rees and his colleagues concludes. the percept to flip differs consistently Thirty-two healthy volunteers car- across individuals. One person might ried out a difficult visual task in the scan- see the figure alternate every five sec- ner. They had to judge which one of a onds; another sees it flip every 10. number of faint patches was a tad more Rees and his group used a dynamic salient than the other ones; this judgment version of such bistable illusions: a cloud was purposefully made demanding. Fol- of moving dots perceived as a cylinder lowing each trial, subjects had to choose rotating either to the left or to the right. a number between one and six, indicat- Here the scientists correlated the width ing the confidence they had in their own The Ponzo illusion provides a visual cue of of the cortical sheet—the thickness of its judgment. A six indicated that they were converging railroad ties, so the upper blue bar gray matter—with how long each stable very confident of their judgment, where- is perceived to be farther away—and thus percept lasts before it switches. Scan- as a one implied a guess. That is, they much larger—than the lower blue bar. Yet they are exactly the same dimensions. The ning the brains of 52 subjects—in a field were asked to introspect: Are you sure smaller the size of your primary visual cortex, dominated by studies that come to grand you just saw the bright patch here? Psy- the more dramatic the illusion. conclusions by querying a handful of chologists know this as meta-cognition: brains—they found only a single region, thinking about thinking. creased. Patients with lesions in these re- the left and right superior parietal lobe Not surprisingly, subjects differed gions typically lose the ability to intro- (SPL), in which the thickness of the gray greatly in the accuracy of their judg- spect. And this part of the neocortex has matter (and its density) significantly and ments (independent of the level of their expanded more than any other region in consistently correlated negatively with performance). Think of the game show primates. Again, the neuronal mecha- the perceptual duration. In other words, Who Wants to Be a Millionaire, where nisms underlying this correlation remain the thicker the SPL cortex, the faster two contestants have to judge whether they unknown for now. interpretations switch back and forth. It want to use a lifeline before they know Rees’s studies establish that differenc- is known from other imaging and clini- the answer, depending on their confi- es in the morphology, or shape, of our cal studies that the SPL in the back of the dence. Some people are astute, using the brains are mirrored in differences in the brain controls selective visual attention, lifelines wisely; other people fritter them way we consciously experience and ap- but how the thickness and density of SPL away. The cognitive scientists extracted prehend the world, including our own gray matter should be important is any- a measure of variability of introspection brains and bodies. In this way, neurosci- body’s guess. and discovered that this measure corre- ence maps the physical structure of the Ask people what they believe to be lated with variability in gray matter vol- material brain onto the inner geometry of the defining feature of consciousness, ume in the right anterior prefrontal cor- phenomenal and ineffable experience. M and most will point to self-awareness. tex. The more neurons you have in this To be capable of being aware of your region in the front of the brain, the bet- CHRISTOF KOCH is Lois and Victor Troendle hopes, to worry about your spouse’s ill- ter your introspection. Not that your Professor of Cognitive and Behavioral Biology ness, to wonder why you feel despondent performance goes up, but the insight you at the California Institute of Technology. He or why he provoked you is taken to be have into your performance—whether serves on Scientific American Mind’s board the pinnacle of sentience. Self-awareness you thought you did well or not—in- of advisers. is, by and large, absent in nonprimates. Although my dog—as with many and, (Further Reading) perhaps, all animals—experiences the ◆◆Human Parietal Cortex Structure Predicts Individual Differences in Perceptual Rivalry. sights, sounds and, in particular, the Ryota Kanai, Bahador Bahrami and Geraint Rees in Current Biology, Vol. 20, No. 18, smells of life, she doesn’t worry why her pages 1626–1630; August 19, 2010. tail isn’t wagging as it used to or wheth- ◆◆Relating Introspective Accuracy to Individual Differences in Brain Structure. Stephen er tomorrow’s food will appear. Fleming, Rimona Weil, Zoltan Nagy, Raymond Dolan and Geraint Rees in Science, Vol. 329, pages 1541–1543; September 17, 2010. So can differences in this elusive ◆◆The Surface Area of Human V1 Predicts the Subjective Experience of Object Size.

scientific american mind american scientific higher-order aspect of consciousness be Samuel Schwarzkopf, Chen Song and Geraint Rees in Nature Neuroscience (in press).

www.ScientificAmerican.com/Mind scientific american mind 17