What gives us fingertip dexterity?

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What gives us fingertip dexterity?

http://www.eurekalert.org/pub_releases/2008-02/uosc-wgu020708.php

In a novel experiment, a USC biomedical engineer examines the

intricate circuitry between hand manipulation skills and specialized

neural circuits in the brain

Quickly moving your fingertips to tap or press a surface is essential

for everyday life to, say, pick up small objects, use a BlackBerry or

an iPhone. But researchers at the University of Southern California

say that this seemingly trivial action is the result of a complex

neuro-motor-mechanical process orchestrated with precision timing by

the brain, nervous system and muscles of the hand.

USC Viterbi School of Engineering biomedical engineer Francisco

Valero-Cuevas is working to understand the biological, neurological

and mechanical features of the human hand that enable dexterous

manipulation and makes it possible for a person to grasp and crack an

egg, fasten a button, or fumble with a cell phone to answer a call.

" When you look at the hand, you think, `five fingers, what could be

more straightforward " ' " Valero-Cuevas said, " but really we don't

understand well what a hand is bio-mechanically, how it is controlled

neurologically, how disease impairs it, and how treatment can best

restore its function. It is difficult to know how each of its 30-plus

muscles contributes to everyday functions like using a cell phone or

performing the many finger tasks it takes to dress yourself. "

In a study published online in the Feb. 6, 2008 issue of The Journal

of Neuroscience, titled " Neural Control Of Motion-to-Force

Transitions with the Fingertip, " Valero-Cuevas and co-author

Madhusudhan Venkadesan of Cornell University's Department of

Mathematics asked volunteers to tap and push against a surface with

their forefinger while the researchers recorded the fingertip force

and electrical activity in all of the muscles of the hand.

These researchers, in a first-of-a-kind experiment, recorded 3D

fingertip force plus the complete muscle coordination pattern

simultaneously using intramuscular electromyograms from all seven

muscles of the index finger. Subjects were asked to produce a

downward tapping motion, followed by a well-directed vertical

fingertip force against a rigid surface. The researchers found that

the muscle coordination pattern clearly switched from that for motion

to that for force (~65 ms) before contact. Venkadesan's mathematical

modeling and analysis revealed that the underlying neural control

also switched between mutually incompatible strategies in a time-

critical manner.

" 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.

" As an analogy, I ask people to imagine going through life wearing

winter gloves, " he said. " If you can grasp things in only the

grossest of ways without fine manipulation, life is pretty difficult.

Yet millions of people worldwide go through life without the full use

of their hands. Diseases and aging processes that affect the hand

function tend to disproportionately degrade the quality of life, and

we want to reverse that. "

The research was supported by the Whitaker Foundation, the National

Science Foundation and the National Institutes of Health.