Mind & Brain / Memory, Emotions, & Decisions

Here's a fascinating question that can never be asked scientifically: What is the nature of consciousness? No experiment can even show that consciousness exists (see my June 2006 column to be reminded why). Fortunately, there are ways to get closer and closer to this impossible topic. For instance, it is possible to ask what meaning is, even if we cannot ask about the experience of meaning.

V. S. Ramachandran, a neuroscientist at the University of California at San Diego and the Salk Institute, has come up with a research program to approach the question of meaning with remarkable concreteness. Like many of the best scientists, Rama (as he is known to his colleagues) is engaged in an extension of his childhood curiosities. When he was 11, he wondered about the digestive system of the Venus flytrap, the carnivorous plant. Are the digestive enzymes in its leaves triggered by proteins, by sugars, or both? Would saccharin fool the traps the way it fools our taste buds? Later Rama graduated to studying vision and published his first paper in the journal Nature when he was 20. He is best known for the work that overlaps with my own interests: using mirrors as a low-tech form of virtual reality to treat phantom-limb pain and stroke paralysis. His research has also sparked a fruitful ongoing dialogue between the two of us about language and meaning.

In my May 2006 column, I described how parts of the brain's cerebral cortex are specialized for particular sensory systems, such as vision, and how there are also overlapping regions between these parts, known as cross-modal areas. Rama has been interested in how the cross-modal areas of the brain may give rise to a core element of language and meaning: the metaphor.

His canonical example is encapsulated in an experiment known as bouba/kiki. Rama presents test subjects with two words, which are pronounceable but meaningless in most languages: bouba and kiki. Then he shows the subjects two images, one a spiky, hystricine shape and the other a rounded cloud form. Match the words and the images! Of course the spiky shape goes with kiki and the cloud matches bouba. This correlation is cross-cultural and appears to be a general truth for all of humankind.

Can you tell which shape is bouba and which is kiki?

The bouba/kiki experiment isolates one form of linguistic abstraction. "Boubaness" or "kikiness" arises between two stimuli that are otherwise utterly dissimilar: an image formed on the retina versus a sound activated in the cochlea of the ear. Such abstractions seem to be linked to the mental phenomenon of metaphor. For instance, Rama finds that patients who have lesions in a cross-modal brain region called the inferior parietal lobule have difficulty both with the bouba/kiki task and with interpreting proverbs or stories that have nonliteral meanings.

Rama's experiments suggest that some metaphors can be understood as mild forms of synesthesia. In its more severe forms, synesthesia is an intriguing neurological anomaly in which a person's sensory systems are crossed; a color might be perceived as a sound. (It could be argued that Shakespeare also linked synesthesia, in a broader form, to meaning: Bottom, in A Midsummer Night's Dream, describes his dream as being indescribable because "The eye of man hath not heard, the ear of man hath not seen . . . what my dream was.")

What is the connection between the images and the sounds in Rama's experiment? Well, from a mathematical point of view, kiki and the spiky shape both have "sharp" components that are not so pronounced in bouba; similar sharp components are present in the tongue and hand motions needed to make the kiki sound or draw the kiki picture.

Rama suggests that cross-modal abstraction—the ability to make consistent connections across senses—may have initially evolved in lower primates as a better way to grasp branches. Here's how it could have happened: The cross-modal area of the brain might have evolved to link an oblique image hitting the retina (caused by viewing a tilted branch) with an "oblique" sequence of muscle twitches (leading the animal to grab the branch at an angle). The remapping ability then became co-opted for other kinds of abstraction that humans excel in, like the bouba/kiki metaphor. This is a common phenomenon in evolution: A preexisting structure, slightly modified, takes on analogous, yet dissimilar functions.