When
observing a fly buzzing around the room, we should have the impression
that it is not the fly, but rather the space that lies behind it that is
moving. After all, the fly is always fixed in our central point of
view. But how does the brain convey the impression of a fly in motion in
a motionless field? With the help of functional magnetic resonance
imaging (fMRI) scientists from the Werner Reichardt Centre for
Integrative Neuroscience and the Max Planck Institute for Biological
Cybernetics in Tübingen have identified two areas of the brain that
compare the movements of the eye with the visual movements cast onto the
retina so as to correctly perceive objects in motion.

The
two areas of the brain that are particularly good at reacting to
external movements, even during eye movements, are known as V3A and V6.
They are located in the upper half in the posterior part of the brain.
Area V3A shows a high degree of integration: it reacts to movements
around us regardless of whether or not we follow the moving object with
our eyes. But the area does not react to visual movements on the retina
when eye movements produce them. Area V6 has similar characteristics. In
addition, it can perform these functions when we are moving forwards.
The calculations the brain has to perform are more complicated in this
case: the three-dimensional, expanding forward movement is superimposed
onto the two-dimensional lateral movements that are caused by eye
movements.

The
scientists Elvira Fischer and Andreas Bartels from the Werner Reichardt
Centre for Integrative Neuroscience and the Max Planck Institute for
Biological Cybernetics have investigated these areas with the help of
functional magnetic resonance imaging (fMRI). fMRI is a procedure that
can measure brain activity based on local changes in blood flow and
oxygen consumption. Participants in the study were shown various visual
scenarios whilst undergoing fMRI scanning. For example, they had to
follow a small dot with their eyes while it moved across a screen from
one side to the other. The patterned background was either stationary or
moved at varying speeds, sometimes slower, faster or at the same speed
as the dot. Sometimes the dot was stationary while only the background
moved. In a total of six experiments the scientists measured brain
activity in more than a dozen different scenarios. From this they have
been able to discover that V3A and V6, unlike other visual areas in the
brain, have a pronounced ability to compare eye movements with the
visual signals on the retina.

“I
am especially fascinated by V3A because it reacts so strongly and
selectively to movements in our surroundings. It sounds trivial, but it
is an astonishing capability of the brain,” explains Andreas Bartels,
project leader of the study.

Whether
it is ourselves who move or something else in our surroundings is a
problem about which we seldom think, since at the subconscious level our
brain constantly calculates and corrects our visual impression. Indeed,
patients who have lost this ability to integrate movements in their
surroundings with their eye movements can no longer recognize what it is
that ultimately is moving: the surroundings or themselves. Every time
they move their eyes these patients feel dizzy. Studies such as this
bring us one step closer to an understanding of the causes of such
illnesses.

The
study was a collaboration between the Werner Reichardt Centre for
Integrative Neuroscience and the department for Human Perception,
Cognition and Action of Heinrich Bülthoff as well as the department for
Physiology of Cognitive Processes of Nikos Logothetis at the Max Planck
Institute for Biological Cybernetics.

Source: Max Planck Institute for Biological Cybernetics