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.

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.

Rama also wonders about other kinds of metaphors, ones that don't obviously fall into the bouba/kiki category. In his current favorite example, Shakespeare has Romeo declare Juliet to be "the sun." There is no obvious bouba/kiki-like dynamic that would link a young, female, doomed romantic lead with a bright orb in the sky, yet the metaphor is immediately sensible to anyone who hears it.

A few months ago, when we ran into each other at a conference where we were both speaking, I made a simple suggestion to Rama about how to extend the bouba/kiki idea to Juliet and the sun. Suppose you had a vocabulary of only 100 words. (This experience will be familiar if you've ever traveled in a region where you don't speak the language.) In that case, you'd have to use your small vocabulary creatively to get by. Now extend that condition to an extreme. Suppose you had a vocabulary of only four nouns: kiki, bouba, Juliet, and sun. When the choices are reduced, the importance of what might otherwise seem like trivial synesthetic or other elements of commonality is amplified.

Juliet is not spiky, so bouba or the sun, both being rounded, fit better than kiki. (If Juliet were given to angry outbursts of spiky noises, then kiki would be more of a contender, but that's not our girl in this case.) There are a variety of other minor overlaps that make Juliet more sunlike than bouba-ish.

If a tiny vocabulary has to be stretched to cover a lot of territory, then any difference at all between the qualities of words is practically a world of difference. The brain is so desirous of associations that it will then amplify any tiny potential linkage in order to get a usable one. (There's infinitely more to the metaphor as it appears in the play, of course. Juliet sets like the sun, but when she dies, she doesn't come back like it. We can all only wish that she really were more like the sun in this way. Or maybe the archetype of Juliet always returns, like the sun—a good metaphor breeds itself into a growing community of interacting ideas.)

I am not a Shakespeare scholar by any means, but it seems to me that the Bard didn't generally aim for the outer reaches of his vocabulary when he set down a metaphor. His vocabulary was large—almost 30,000 words—but he usually chose small, common words, although in shocking juxtapositions, to describe Juliet, Hamlet, or anything else important.

Likewise, much of the most expressive slang comes from people with poor or inapplicable formal education making creative use of the words they know. This is true of pidgin languages, street slangs, and so on. The most evocative words are often the most common ones that are used in the widest variety of ways. Yiddish: Nu? Spanish: Pues. . . .

One reason the metaphor of the sun fascinates me is that it bears on a conflict that has been at the heart of information science since its inception: Can meaning be described compactly and precisely, or is it something that can emerge only approximately from statistical associations between large numbers of components? Mathematical expressions are compact and precise, and most early computer scientists assumed that at least part of language ought to display those qualities too. In my August 2006 column, I described how statistical approaches to tasks like automatic language translation seem to be working better than compact, precise ones. I also argued against the probability of an initial small, well-defined vocabulary in the evolution of language and in favor of an emergent vocabulary that never became precisely defined. There is, however, at least one more possibility I didn't describe in the August column: Vocabulary could be emergent, but there could be an outside factor that initially makes it difficult for a vocabulary to grow as large as the process of emergence might otherwise prefer.

The bouba/kiki dynamic, along with other similarity-detecting processes in the brain, can be imagined as the basis of the creation of an endless series of metaphors, which could correspond to a boundless vocabulary. But the metaphor of the sun, if this explanation is right, might come about only in a situation where the vocabulary is at least somewhat limited. Imagine that you had an endless capacity for vocabulary at the same time that you were inventing language. In that case you could make up an arbitrary new word for each new thing you had to say. A compressed vocabulary might engender less arbitrary, more evocative words.

Maybe the modest brain capacity of early hominids was the source of the limitation of vocabulary size. Whatever the cause, an initially limited vocabulary might be necessary for the emergence of an expressive language. Of course the vocabulary can always grow later on, once the language has established itself. Modern English has a huge vocabulary. Try Googling "hystricine."