EYE muscles have a primitive mind of their own. A new study suggests that the
nerve cells which control eye muscles form a network that coordinates eye
position, information that had been thought to originate from deeper within the
brain. The result overthrows a hundred-year-old theory of how binocular vision
develops.
Eye coordination is essential for depth perception and to avoid double
vision. The eyes are controlled by nerve cells called motor neurons, which are
directly connected to eye muscles. The motor neurons, in turn, are connected to
neurons deeper in the brain called premotor neurons.
The debate over how binocular movement is achieved started more than 100
years ago in Germany. The physicist Hermann von Helmholtz argued that separate
signals are sent to each eye, and the brain learns to combine these signals to
coordinate movement.
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But his contemporary, physiologist Ewald Hering, insisted that the eyes are
not sent individual commands but instead receive one of two commands: 鈥渕ove
together in this direction鈥� or 鈥渕ove towards each other鈥�. This second theory is
now so firmly entrenched that neuroscientists often don鈥檛 bother to record the
position of both eyes in vision experiments, says Mike King of the University of
Mississippi in Jackson.
King and his colleague Wu Zhou were forced to make such measurements because
of the particular problem they were studying: how rhesus monkeys focus on laser
spots on a horizontal screen just below eye level. If the spot is moved directly
towards one eye, that eye remains fixed, so to get a full set of data they
recorded the position of both eyes, as well as neuron activity.
If both eyes are sent the same signal, as Hering claimed, then the activity
of any premotor neuron shouldn鈥檛 have encoded information specific to the
movement of one eye. 鈥淭hat鈥檚 why when I plotted out the data, I was shocked,鈥�
says King. Of 96 individual premotor neurons measured, four-fifths showed bursts
of activity correlating with the movement of only one eye鈥攁 result
impossible to reconcile with Hering鈥檚 model.
The next surprise was that motor neurons seem to work together, the team says
in this week鈥檚 Nature (vol 393, p 692). Even when one eye moved
independently, the motor neurons connected to the muscles of the other eye
changed their firing rates.
鈥淚t鈥檚 very good work and just destroys prevailing wisdoms,鈥� comments Michael
Goldberg, a vision researcher at the National Eye Institute near Washington DC.
鈥淭he challenge is for theoreticians to explain all this.鈥�
King speculates that motor neurons may be far more sophisticated than
previously believed. Rather than acting only as on/off switches for eye muscles,
he says that they could form a neural network that learns to orchestrate eye
movements, as Helmholtz envisioned.
Similar motor neuron networks may control other groups of closely associated
muscles, King adds. 鈥淲hen you look at the coordination you need to play a piano,
it looks suspiciously similar.鈥�