Supernumerary digits and functional extras

Newton saw in the human hand proof of the divine; Darwin saw a key to our species' success. Many others, too, have described the hand in hyperbolic terms, as a paragon of design, a cornerstone of human uniqueness, an engine of our achievements. But what makes the human hand so powerful? Is it the proportions of the fingers? Is it the opposability of the thumb? Or, could it be none of this? Could it be that the real power of our hands lies—not in the physical design—but elsewhere, out of sight?

My guest today is Dr. Matt Longo. Matt is Professor of Cognitive Neuroscience at Birkbeck, University of London. He's the author of the recent book, The Invisible Hand, a wide-ranging tour of the human hand and how it's geared into the brain.

Here, Matt and I talk about the difference between the "visible hand"—that is, its physical structure—and the "invisible hand"—its representation in the brain. We consider the evolution of the visible hand and whether there really is anything truly distinctive or impressive about it. We talk about the biology of touch. We tour the invisible hand, discussing how—through cortical magnification—the hand becomes over-represented in the brain's sensory maps. We catalogue a few ways that the hands can go awry. And we talk about whether we should feel any nostalgia for all the hand-based activities and crafts that we're losing. Along the way, we also touch on star-nosed moles and raccoons; tetrapods and the primitive archetype; hand dominance; the parallel between a horse's knee and a human's wrist; tool use, plasticity and abstraction; homunculi; the rubber-hand illusion; supernumerary fingers; the Third Thumb project; and the question of what it might unlock if dolphins had hands.

Alright, friends, this is a fun one. On to my interview with Dr. Matt Longo!

Notes

3:00 – For discussion of the many traits and behaviors that have been proposed as uniquely human, see our earlier audio essay.

5:00 – For an example of the "if only dolphins had hands" thought experiment, see here.

8:00 – See The Principles of Anatomy as Seen in the Hand by Frederic Wood Jones.

10:30 – Dr. Longo's book, The Invisible Hand, is available open access here.

16:00 – For discussion of how—in horses and other species—the five digits have been reduced or otherwise tweaked over evolution, see here. For an image showing examples of homology between the human forelimb and the forelimbs of other creatures, see here.

19:00 – For a brief discussion of "thumb opposability," see here. For an influential discussion of hand morphology and human hand grips, see work by Mary Marzke here.

30:00 – For our earlier episode on the brain's many maps, see here.

34:00 – For a discussion of Penfield's work and the idea of a "homunculus" in the brain, see here.

42:00 – For an illustration of a "homunculus" with big lips and hands, see here.

44:30 – For more on the star-nosed mole and its distinctive appendage, see here.

49:00 – For the report that first coined the term "numbsense," see here. For recent work on "anarchic hand," see here. For more on phantom limbs, see here. For a classic study of the "rubber hand illusion," see here.

59:30 – For a discussion of hand-dominance across primate species, see here.

1:03:00 – More on the "Third Thumb" project.

1:06:00 – A classic case of "motor equivalence" is seen in handwriting.

Recommendations

Marco Catani, 'A little man of some importance'

Tracy Kivell, 'Evidence in hand: Recent discoveries and the early evolution of human manual manipulation'

Hands, by John Napier

Many Minds is a project of the Diverse Intelligences Summer Institute, which is made possible by a generous grant from the John Templeton Foundation to Indiana University. The show is hosted and produced by Kensy Cooperrider, with help from Assistant Producer Urte Laukaityte and with creative support from DISI Directors Erica Cartmill and Jacob Foster. Our artwork is by Ben Oldroyd.

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