Perceptual psychology and the brain sciences emphasize the communality in the way that people experience reality. Leaving aside cases of brain damage 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 reflection of the great similarities of our brains compared with the brains of our close cousins on the evolutionary tree, the great apes. Laboratory science reinforces this bias by lumping together the performance of its subjects on any one experiment and reporting only the average 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 we are used to seeing in newspapers, in magazines such as this one, on television and in the movies.
Yet as we know from our own life, each one of us has his or her own preferences, likes and dislikes. Some people are acutely sensitive to flashing lights, some have perfect pitch, some cannot see in depth, some can introspect and analyze their own failures and triumphs, whereas others—remarkably frequently, public figures such as politicians—lack this knack. Take me. I am hopelessly attracted to brilliant colors. As a magpie is drawn to anything glittering, I am drawn to school-bus yellow, tangerine orange, burgundy red, rich magenta, electric violet, imperial purple and navy blue. My love of the garish is reflected in my flowery shirts and pants and, I’m sure, in an enhanced cortical representation of these hues.
It is obvious that if the apparatus that senses the world differs between two individuals, then the conscious experience of the brains wired to these sensors cannot be the same either. In a previous Consciousness Redux column, I discussed color blindness—the fact that about 7 percent of men lack one of the genes for the retinal photopigments needed to see hue. But what about differences in the brain proper? Do they influence consciousness in measurable ways? To answer this question, scientists must plumb the minds of many individuals and relate them to measures of their brains. The widespread availability of fMRI scanners makes such a project feasible today.
Cognitive neuroscientist Geraint Rees, a professor at the Wellcome Trust Center for Neuroimaging at University College London—undoubtedly the world’s leading fMRI center—published a trio of studies that relates differences in the way people experience things with differences in gross aspects of their cerebral neocortex, the highly convoluted part of the forebrain that crowns the brains of all mammals.
In one study 30 subjects looked at the Ponzo illusion while their brains were scanned. Whereas everybody who looks at the Ponzo perceives the upper bar as larger than the lower one, the magnitude of this effect differs substantially across individuals. (The size of the illusion is established by asking how much larger the lower bar has to be to make it look the same size as the upper one.) Surprisingly, these differences are reflected in the surface area of the primary visual cortex (V1) at the back of the head. For unknown reasons, the area of V1 can differ by a factor of three among people (unfolded, the size and width of a typical V1 compares with that of a credit card). Rees and his collaborators discovered that the smaller a person’s V1, the more powerfully he or she experiences the illusion. Those individuals with a large V1 judged the size of the bars to be more similar than those with a smaller V1. Curiously, the size of the two immediately adjacent visual areas did not influence the amplitude of the illusion.
Clues from Illusions
Bistable illusions are those delightful images that can be seen in one of two ways. Probably the best known is the Necker cube, or the “old woman, young girl illusion.” These two interpretations flip back and forth. The time it takes for the percept to flip differs consistently across individuals. One person might see the figure alternate every five seconds; another sees it flip every 10.