The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

But now I modify the experiment: I put a colored luminous spot on each of the boxes—red (on the big box), blue (intermediate box), and green (small box)—and have the boxes lying separately on the floor. I bring you into the room for the first time and expose you to the boxes long enough for you to realize which box has which spot. Then I switch the room lights off so that only the luminous colored dots are visible. Finally, I bring a luminous reward into the dark room and dangle it from the ceiling.

If you have a normal brain you will, without hesitation, put the red-dotted box at the bottom, the blue-dotted box in the middle, and the green-dotted box on top, and then climb to the top of the pile to retrieve the dangling reward. (Let’s assume the boxes have handles sticking out that you use to pick them up with, and that the boxes have been made equal weight so that you can’t use tactile cues to distinguish them.) In other words, as a human being you can create arbitrary symbols (loosely analogous to words) and then juggle them entirely in your brain, doing a virtual-reality simulation to discover a solution. You could even do this if during the first phase you were shown only the red-and green-dotted boxes, and then separately shown the green-and blue-dotted boxes, followed finally in the test phase by seeing the red-and green-dotted boxes alone. (Assume that stacking even two boxes gives you better access to the reward.) Even though the relative sizes of the boxes were not currently visible during these three viewing stages, I bet you could now juggle the symbols entirely in your head to establish the transitivity using conditional (if-then) statements—“If red is bigger than blue and blue is bigger than green, then red must be bigger than green”—and then proceed to stack the green box on the red box in the dark to reach the reward. An ape would almost certainly fail at this task, which requires off-line (out of sight) manipulation of arbitrary signs, the basis of language.

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.)