Caltech scientists decipher the neurological basis of timely movement

[Guest guest]

Caltech scientists decipher the neurological basis of timely movement

http://www.eurekalert.org/pub_releases/2008-06/ciot-csd060608.php

Contrary to what one might imagine, the way in which each of us

interacts with the world is not a simple matter of seeing (or

touching, or smelling) and then reacting. Even the best baseball

hitter eyeing a fastball does not swing at what he sees. The neurons

and neural connections that make up our sensory systems are far too

slow for this to work. " Everything we sense is a little bit in the

past, " says A. Andersen of the California Institute of

Technology, who has now uncovered the trick the brain uses to get

around this puzzling problem.

Work by Andersen, the G. Boswell Professor of Neuroscience at

Caltech, and his colleagues Grant Mulliken of MIT and Sam Musallam of

McGill University, offers the first neural evidence that voluntary

limb movements are guided by our brain's prediction of what will

happen an instant into the future. " The brain is generating its own

version of the world, a 'forward model,' which allows you to know

where you actually are in real time. It takes the delays out of the

system, " Andersen says.

The research in Andersen's laboratory is focused on understanding the

neurobiological underpinnings of brain processes, including the

senses of sight, hearing, balance, and touch, and the neural

mechanisms of action. The lab is working toward the development of

implanted neural prosthetic devices that would serve as an interface

between severely paralyzed individuals' brain signals and their

artificial limbs--allowing thoughts to control movement.

Research along these lines conducted at the University of Pittsburgh

and Carnegie Mellon University recently allowed monkeys to feed

themselves using a robotic limb that they controlled only with their

thoughts. Their thoughts were picked up via an array of electrodes

sitting on top of the primary motor cortex, a lower level brain

region responsible for carrying out motor functions.

Andersen's group focuses on a more high-level area of cortex called

the posterior parietal cortex (PPC), which is where sensory stimuli

are actually transformed into movement plans.

In their experiments, Andersen and his colleagues trained two monkeys

to use a joystick to move a cursor on a computer screen from a small

red circle into a green circle, while keeping their gaze fixed on the

red circle. The monkeys typically generated curved trajectories, but

to increase the curvature one monkey was trained to move the cursor

around an obstacle. The obstacle (a large blue circle) was placed

between the initial location of the cursor and the target circle, and

the monkey had to guide the cursor around the obstacle, without

touching it, and over to the green circle. As the monkeys conducted

the tasks, electrodes measured the activity of neurons in the PPC.

This allowed Andersen and his colleagues to monitor signals--commands

for movement--in real time.

The studies showed that neurons in the PPC produce signals that

represent the brain's estimation of the current and upcoming movement

of the cursor. " An internal estimate of the current state of the

cursor can be used immediately by the brain to rapidly correct a

movement, avoiding having to rely entirely on late-arriving sensory

information, which can result in slow and unstable control, " Mulliken

says.

" The idea is that you feed back the command you make for movement

into those areas of the brain that plan the movement (i.e., the

PPC), " Andersen says. " The signal about the movement taking place is

adjusted to be perfectly aligned in time with the actual movement--

what you're moving in your head matches with what you're moving in

the real world. " The effect is akin to an athlete visualizing his

performance in his mind. Studies have previously shown that these

simulations of movement trajectories run through the posterior

parietal cortex, and run at actual speed, taking the same amount of

time as the activity would in real life.

In the Pittsburgh robotic arm study, the neural signal driving the

robotic limb was what is known as a " trajectory signal, " which

represents the path that must be taken to move from one point to

another, like using a computer mouse to drag an object across a

screen. Previously Andersen's lab had shown that a different signal

in the posterior parietal cortex, called the " goal signal, " can also

be used to directly jump an object from one point to another.

" This goal signal is much faster for reaching a goal than a

trajectory signal, " Andersen says. " Fast goal decoding is very

advantageous for rapid sequences such as typing. Our new study shows

that the posterior parietal cortex codes the trajectory as well as

the goal, which makes this brain area an attractive target for neural

prosthesis. Not only does this increase the versatility and the

number of prosthetic applications, but it also makes the decoding

easier since the trajectories can be better estimated if the goal is

known. "

The paper, " Forward Estimation of Movement State in Posterior

Parietal Cortex, " will be published in a future print issue the

Proceedings of the National Academy of Sciences but is now available

online. First author, Grant Mulliken, was a graduate student at

Caltech and is now a postdoctoral fellow at the Massachusetts

Institute of Technology; coauthor Sam Musallam was a postdoctoral

fellow at Caltech and is currently an assistant professor at McGill

University in Montreal, Canada.