What Gives Us Fingertip Dexterity?

"We think that the human nervous system employs a surprisingly time-critical and neurally demanding strategy for this common and seemingly trivial task of tapping and then pushing accurately, which is a necessary component of dexterous manipulation," said Valero-Cuevas, who holds a joint appointment in the USC School of Dentistry's division of Biokinesiology and Physical Therapy.

"Our data suggest that specialized neural circuitry may have evolved for the hand because of the time-critical neural control that is necessary for executing the abrupt transition from motion (tap) to static force (push)," he said. "In the tap-push exercise, we found that the brain must be switching from the tap command to the push command while the fingertip is still in motion. Neurophysiological limitations prevent an instantaneous or perfect switch, so we speculate that there must be specialized circuits and strategies that allow people to do so effectively.

"If the transition between motor commands is not well timed and executed, your initial forces will be misdirected and you simply won't be able to pick up an egg, a wine glass or a small bead quickly," he said.

The findings begin to explain why it takes young children years to develop fine finger muscle coordination and skills such as precision pinching or manipulation, and why fine finger manipulation is so vulnerable to neurological diseases and aging, Valero-Cuevas said.

But perhaps even more importantly, he said, the findings suggest a functional explanation for an important evolutionary feature of the human brain: its disproportionately large sensory and motor centers associated with hand function.

"If, indeed, the nervous system faced evolutionary pressures to be able to anticipate and precisely control routine tasks like rapid precision pinch, the cortical structures for sensorimotor integration for finger function would probably need to be pretty well developed in the brain," Valero-Cuevas said.

"That would give us the neural circuits needed for careful timing of motor actions and fine control of finger muscles," he said. "Thus, our work begins to propose some functional justifications for the evolution of specialized brain areas controlling dexterous manipulation of the fingertips in humans."

By understanding the neuromuscular principles behind dexterous manipulation, Valero-Cuevas hopes to help those who have lost the use of their hands by guiding rehabilitation and helping to develop the next generation of prosthetics. In addition, the work will allow industry to build machines that have versatility comparable to that of the human hand.

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Posted to the site on 8th February 2008