San Francisco
EVEN the most accident-prone motorist can glance at road signs without
veering off the highway. But experiments on monkeys have only now begun to
reveal how our brains avoid such catastrophes. Richard Andersen and his
colleagues at the California Institute of Technology in Pasadena have identified
a group of brain cells that enable us to keep heading in the same direction even
when our eyes are moving.
We can tell which way we are going because, as we travel forwards, the world
ahead of us appears to expand. To take a well-known image, as the Starship
Enterprise zooms through space, its crew sees a pattern of stars flying outwards
from a central point which defines the direction of movement.
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But if a crew member鈥檚 eyes follow the course of one particular star moving
to the left, the entire field of view will appear to move to the right. When
this apparent movement is superimposed on the expanding star pattern, the point
from which the pattern is expanding will appear to move to the left (see
Figure). Without a mechanism to compensate for eye movements, the crew of the
Enterprise would instinctively change the direction of their craft whenever they
glanced sideways.
Until now, this mechanism has eluded neuroscientists. But in the latest issue
of Science (vol 273, p 1544), Andersen鈥檚 team provides the first clues
to how it works. The researchers placed rhesus monkeys in front of an expanding
pattern of dots like the pattern of stars seen from the bridge of the
Enterprise. They then trained each monkey to move its eyes so that it tracked a
single, large dot which was part of the expanding pattern and moving to the
left.
Andersen and his team recorded the electrical activity of nerve cells in part
of the brain鈥檚 visual cortex called the dorsomedial superior temporal area. This
contains cells that respond to moving patterns, including some that fire most
strongly when presented with an expanding scene that is indicative of forward
movement.
The researchers first identified brain cells that fired strongly in response
to the pattern of dots expanding from a central point in each monkey鈥檚 field of
view. When each animal鈥檚 eyes moved left to follow the large dot, some of these
neurons dramatically decreased their rate of firing鈥攖hey evidently did not
register that the expanding pattern had been distorted as a result of eye
movement.
Other neurons, however, continued firing as before, having compensated for
the fact that eye movement had shifted the central point of the expanding
pattern to the left. Andersen argues that the brain uses these 鈥渟hifting鈥�
neurons to determine the true direction of movement.
Next, the researchers tried moving the screen鈥攕till with the same
expanding display鈥攕o that the large dot remained straight ahead of the
monkey, which was able to follow it without moving its eyes. In this case, both
sets of neurons changed their pattern of firing in the same way, responding as
if the direction of movement had shifted. This suggests that the key to the
compensation mechanism is a signal direct from our moving eyes which feeds back
to the shifting neurons, says Andersen.
Now that Andersen鈥檚 team has identified the neurons that keep us on a steady
course, the next problem is to work out how the brain receives the information
needed to compensate for eye movement. 鈥淭here is evidence that there is
compensation in the right direction, and in some cases of sufficient magnitude
to fully compensate for eye movement,鈥� says Bill Warren, a neuroscientist at
Brown University in Providence, Rhode Island. 鈥淏ut the detailed mechanism is
still unclear.鈥�