RATS HAVE A DOUBLE VIEW OF THE WORLD

Scientists from the Max
Planck Institute for Biological Cybernetics
in Tübingen, using miniaturised high-
speed cameras and high-speed
behavioural tracking, discovered that rats
move their eyes in opposite directions in
both the horizontal and the vertical plane
when running around. Each eye moves in
a different direction, depending on the
change in the animal's head position.

An
analysis of both eyes' field of view found
that the eye movements exclude the
possibility that rats fuse the visual
information into a single image like
humans do. Instead, the eyes move in
such a way that enables the space above
them to be permanently in view --
presumably an adaptation to help them
deal with the major threat from
predatory birds that rodents face in their
natural environment.

Like many mammals, rats have their eyes
on the sides of their heads. This gives
them a very wide visual field, useful for
detection of predators. However, three-
dimensional vision requires overlap of the
visual fields of the two eyes. Thus, the
visual system of these animals needs to
meet two conflicting demands at the
same time; on the one hand maximum
surveillance and on the other hand
detailed binocular vision.

The research team from the Max Planck
Institute for Biological Cybernetics have
now, for the first time, observed and
characterised the eye movements of
freely moving rats. They fitted minuscule
cameras weighing only about one gram to
the animals' heads, which could record
the lightning-fast eye movements with
great precision.

The scientists also used
another new method to measure the
position and direction of the head,
enabling them to reconstruct the rats'
exact line of view at any given time.

The Max Planck scientists' findings came
as a complete surprise. Although rats
process visual information from their eyes
through very similar brain pathways to
other mammals, their eyes evidently
move in a totally different way. "Humans
move their eyes in a very stereotypical
way for both counteracting head
movements and searching around. Both
our eyes move together and always
follow the same object. In rats, on the
other hand, the eyes generally move in
opposite directions," explains Jason Kerr
from the Max Planck Institute for
Biological Cybernetics.

In a series of behavioural experiments,
the neurobiologists also discovered that
the eye movements largely depend on
the position of the animal's head. "When
the head points downward, the eyes
move back, away from the tip of the
nose. When the rat lifts its head, the
eyes look forward: cross-eyed, so to
speak. If the animal puts its head on one
side, the eye on the lower side moves up
and the other eye moves down." says
Jason Kerr.

In humans, the direction in which the
eyes look must be precisely aligned,
otherwise an object cannot be fixated. A
deviation measuring less than a single
degree of the field of view is enough to
cause double vision. In rats, the opposing
eye movements between left and right
eye mean that the line of vision varies by
as much as 40 degrees in the horizontal
plane and up to 60 degrees in the vertical
plane. The consequence of these unusual
eye movements is that irrespective of
vigorous head movements in all planes,
the eyes movements always move in such
a way to ensure that the area above the
animal is always in view simultaneously
by both eyes -something that does not
occur in any other region of the rat's
visual field.

These unusual eye movements that rats
possess appear to be the visual system's
way of adapting to the animals' living
conditions, given that they are preyed
upon by numerous species of birds.
Although the observed eye movements
prevent the fusion of the two visual
fields, the scientists postulate that
permanent visibility in the direction of
potential airborne attackers dramatically
increases the animals' chances of survival.