But to what extent is language an actual requirement for conditional statements mentally processed off-line, especially in novel situations? Perhaps one could find out by carrying out the same experiment on a patient who has Wernicke’s aphasia. Given the claim that the patient can produce sentences like “If Blaka is bigger than Guli, then Lika tuk,” the question is whether she understands the transitivity implied in the sentence. If so, would she pass the three-boxes test we designed for chimps? Conversely, what about a patient with Broca’s aphasia, who purportedly has a broken syntax box? He no longer uses “ifs,” “buts,” and “thens” in his sentences and doesn’t comprehend these words when he hears or reads them. Would such a patient nevertheless be able to pass the three-boxes test, implying he doesn’t need the syntax module to understand and deploy the rules of deductive if-then inferences in a versatile manner? One could ask the same question of a number of other rules of logic as well. Without such experiments the interface between language and thought will forever remain a nebulous topic reserved for philosophers.
I have used the three-boxes idea to illustrate that one can, in principle, experimentally disentangle language and thought. But if the experiment proves impractical to carry out, one could conceivably confront the patient with cleverly designed video games that embody the same logic but do not require explicit verbal instructions. How good would the patient be at such games? And indeed, can the games themselves be used to slowly coax language comprehension back into action?
Another point to consider is that the ability to deploy transitivity in abstract logic may have evolved initially in a social context. Ape A sees ape B bullying and subduing ape C, who has on previous occasions successfully subdued A. Would A then spontaneously retreat from B, implying the ability to employ transitivity? (As a control, one would have to show that A doesn’t retreat from B if B is only seen subduing some other random ape C.)
The three-boxes test given to Wernicke’s aphasics might help us to disentangle the internal logic of our thought processes and the extent to which they interact with language. But there is also a curious emotional aspect to this syndrome that has received scant attention, namely, aphasics’ complete indifference—indeed, ignorance—of the fact that they are producing gibberish and their failure to register the expression of incomprehension on the faces of people they are talking to. Conversely, I once wandered into a clinic and started saying “Sawadee Khrap. Chua alai? Kin Krao la yang?” to an American patient and he smiled and nodded acknowledgment. Without his language comprehension module he couldn’t tell nonsense speech and normal speech apart, whether the speech emerged from his own mouth or from mine. My postdoctoral colleague Eric Altschuler and I have often toyed with the idea of introducing two Wernicke’s aphasics to each other. Would they talk incessantly to each other all day, and without getting bored? We joked about the possibility that Wernicke’s aphasics are not talking gibberish; maybe they have a private language comprehensible only to each other.
WE HAVE BEEN speculating on the evolution of language and thought, but still haven’t resolved it. (The three-boxes experiment or its video-game analog hasn’t been tried yet.) Nor have we considered the modularity of language itself: the distinction between semantics and syntax (including what we defined earlier in the chapter as recursive embedding, for example, “The girl who killed the cat that ate the rat started to sing”). Presently, the strongest evidence for the modularity of syntax comes from neurology, from the observation that patients with a damaged Wernicke’s area produce elaborate, grammatically correct sentences that are devoid of meaning. Conversely, in patients who have a damaged Broca’s area but an intact Wernicke’s area, like Dr. Hamdi, meaning is preserved, but there is no syntactic deep structure. If semantics (“thought”) and syntax were mediated by the same brain region or by diffuse neural networks, such an “uncoupling” or dissociation of the two functions couldn’t occur. This is the standard view presented by psycholinguists, but is it really true? The fact that the deep structure of language is deranged in Broca’s aphasia is beyond question, but does it follow that this brain region is specialized exclusively for key aspects of language such as recursion and hierarchical embedding? If I lop off your hand you can’t write, but your writing center is in the angular gyrus, not in your hand. To counter this argument psycholinguists usually point out that the converse of this syndrome occurs when Wernicke’s area is damaged: Deep structure underlying grammar is preserved but meaning is abolished.
My postdoctoral colleagues Paul McGeoch and David Brang and I decided to take a closer look. In an influential and brilliant paper written in 2001 in the journal Science, the linguist Noam Chomsky and cognitive neuroscientist Marc Hauser surveyed the whole field of psycholinguistics and the conventional wisdom that language is unique to humans (and probably modular). They found that almost every aspect of language could be seen in other species, after adequate training, such as in chimps, but the one aspect that makes the deep grammatical structure in humans unique is recursive embedding. When people say that deep structure and syntactic organization are normal in Wernicke’s aphasia, they are usually referring to the more obvious aspects, such as the ability to generate a fully formed sentence employing nouns, prepositions, and conjunctions but carrying no meaningful content (“John and Mary went to the joyful bank and paid hat”). But clinicians have long known that, contrary to popular wisdom, the speech output of Wernicke’s aphasics isn’t entirely normal even in its syntactic structure. It’s usually somewhat impoverished. However, these clinical observations were largely ignored because they were made long before recursion was recognized as the sine qua non of human language. Their true importance was missed.
When we carefully examined the speech output of many Wernicke’s aphasics, we found that, in addition to the absence of meaning, the most striking and obvious loss was in recursive embedding. Patients spoke in loosely strung together phrases using conjunctions: “Susan came and hit John and took the bus and Charles fell down,” and so forth. But they could almost never construct recursive sentences such as “John who loved Julie used a spoon.” (Even without setting “who loved Julie” off with commas, we know instantly that John used the spoon, not Julie.) This observation demolishes the long-standing claim that Broca’s area is a syntax box that is autonomous from Wernicke’s area. Recursion may turn out to be a property of Wernicke’s area, and indeed may be a general property common to many brain functions. Furthermore, we mustn’t confuse the issue of functional autonomy and modularity in the modern human brain with the question of evolution: Did one module provide a substrate for the other or even evolve into another, or did they evolve completely independently in response to different selection pressures?