At Ernest Hemingway’s old home in Key West, Fla., you’ll find bromeliads and date palms, a hand-crafted wooden yacht and an in-ground pool—the only one within 100 miles at the time of its construction. You’ll also find roughly 50 cats, most of which have six toes on their front feet instead of five. As legend has it, after a booze-soaked evening at a local bar, Hemingway was given their six-toed ancestor, “Snow White,” by a waylaid ship captain. On the high seas, these so-called polydactyl cats were cherished for their superior balance and unmatched mousing abilities. Snow White and cats like her also highlight an important concept in neuroscience: the developing brain is flexible enough to wire up to whatever body it finds itself attached to—even if that body is more complex than a “typical” one.
This awe-inspiring flexibility helps the brain translate the extraordinary physical variability among animals—in terms of body size, shape and form—to variability in their behaviors and capabilities. Without it, an animal would not be able to realize the adaptive advantages that come with variability. For example, ancient giraffes with a longer neck had the advantage of being able to feed on higher leaves and so passed the trait on to their descendants. But that neck would have been useless if their brain could not adapt to control it. Likewise, if Michael Phelps hadn’t been able to learn to use his oversize wingspan and feet, he would never have won 23 gold medals.
Are there limits to the brain’s flexibility? The answer may depend on when you ask. For example, certain behaviors and abilities—such as some aspects of language and binocular vision—must be learned during critical periods in early childhood. As you age, it becomes harder for existing neurons to form new connections, and you become more “hardwired.”
But this fact doesn’t tell us how flexible a new brain might be—or how large and complex an animal a single brain could control. After all, even Brontosaurus (welcome back, belatedly), 72 feet in length and weighing in at tens of thousands of pounds had but one brain. Because understanding the brain’s limits might help us transcend them, it should come as no surprise that some of neuroscience’s moon-shot projects—from restoring movement after spinal cord injuries to operating robotic limbs with one’s mind—are focused on the limits of the brain’s flexibility. But how can we study these limits? It turns out that Key West’s expert mousers provide a hint.
In a thrilling paper published recently in Nature Communications, researchers set out to study the abilities of people with extra fingers. This condition, known as polydactyly, affects roughly two in every 1,000 newborns. But because extra fingers are not generally expected to be functional—and perhaps also because of the stigma attached to unusual physical features—they are usually removed. Yet this is not always the case: some people with polydactyly decide not to have their additional fingers removed. And by studying a mother and son pair who opted to keep their left and right hands’ sixth finger, the researchers made a series of discoveries about its function. These discoveries speak to a remarkable flexibility on the part of the brain and body and suggest that biological variability should be celebrated rather than scorned.
The first discovery was an anatomical one: rather than sharing materials with its neighbors, the sixth finger—in both mother and son—had its own muscles, nerves and tendons. It had comparable strength and independence of movement to the other fingers.
How could a sixth finger become functional? Take a lesson from the mechanisms of representative democracy: new districts appear as populations move and grow, but those districts only gain a voice in Congress if they have a representative in Washington, D.C. Likewise, as your brain grows and develops, it builds a “map” of your body. You cannot feel or move the parts of your body that are not represented on this map. The discovery that the mother and son could move their sixth finger therefore prompted researchers to dig into how that finger is represented in their brain.
The scientists found that each of the subjects’ six fingers were represented by distinct areas of the brain’s motor cortex. Consistent with these findings, the subjects had awareness of where in space all six digits were, even when they could not see their hand. Finally, using a cleverly designed video game, the researchers showed that their six-fingered subjects could perform tasks with one hand that most people would need both hands to achieve. Together, these findings suggest that the brain is not hardwired for five fingers but could foreseeably represent as many digits as might appear on a body. For the trivia buffs out there, the living record for digits goes to an Indian carpenter with 14 fingers and 14 toes and, presumably, a most interesting brain.
While the present study is limited to just two subjects, its implications are nonetheless far-reaching. Much effort has been spent to understand phantom limb syndrome, but this study is among the first to shed light on a converse phenomenon. In that regard, it is a foundational work for scientists trying to build interfaces between minds and machines. The clear demonstration that a sixth finger can shift and expand the function of the hand is a strong argument for the ability of the human brain to control machines more complex than the human body. Put another way, if there is a limit on the brain’s flexibility, this study did not find it.
The investigation of polydactyl hands and the brains that control them is a test case for the advantages of researching the unusual. Scientists take great pains to control and standardize: for example, the potency of a particular drug might be determined in laboratory mice that are virtually identical. But there are those few who chase the exotic and anomalous: heat-seeking vipers, cold-adapted octopuses or, as in the present case, additional digits. To understand the advantages of studying the unusual, it helps to reflect on the advantages of being unusual: if legend is to be believed, the high-seas game of cat-and-mouse broke in favor not of the mouse but of Hemingway’s supernumerary cat.