Sight, hearing, touch, smell and taste: these five senses allow us to perceive the world around
us so that we can respond. We reach for something we see. We follow an intriguing sound,
flee from acrid smoke, spit out something inedible, and feel our way along in the dark. In each
case, the way the brain processes sensory information leads to a precise motor response.
This activity is managed at the level of the nerve cells, synapses and neuronal networks in the
cortex of the brain, and it leads to some interesting questions that are almost philosophical.
How does the brain distinguish between external sensory perceptions and stimulations that
arise internally? Did I just call out or did someone else?
These are the questions that Dr. James Poulet from the Max Delbrück Center for Molecular
Medicine Berlin-Buch and the cluster of excellence NeuroCure have been studying for more
than ten years. Poulet is the winner of this year's Paul Ehrlich and Ludwig Darmstaedter Prize
for Young Researchers. The young British scientist has received the award because, as stated
by the Scientific Council of the Paul Ehrlich Foundation, "His research furthers our
understanding of the neuronal basis of behavior."
The young neurobiologist focuses on voluntary behavior that requires some sort of decisionmaking
process rather than stereotypical reflexes. Taking your hand off a hot surface is a
reflex, while responding to a touch is a voluntary reaction. "We want to know how neuronal
activities alter behavior," Poulet explains. "We are interested in the causal connection
between sensory perception, the activity of neurons and the motor response that results from
them." After starting with crickets, Poulet has now turned to mice. He is primarily interested
in brain regions called the primary somatosensory and motor cortex. Here sensory perceptions
are translated into behavioral responses.
Poulet has now trained the mice to stretch out a forepaw to probe the nearby environment.
New optical and electrophysiological techniques are giving Dr. Poulet a look directly into the
brains of alert, active mice, allowing him to record and manipulate the activity of specific
neurons. "These processes underlie healthy behavior and are disrupted during certain
diseases," Poulet says. "If we can determine which neuronal signals underlie sensory
perception and how these signals are linked to an appropriate behavioral response, it may give
us a key to understanding both healthy and defective activity. Before we can know what is
pathological, we should know what is normal."
Why can't we tickle ourselves?
One of James Poulet's early interests was to study how the brain discriminates between selfgenerated
and external sensory perceptions. Am I touching my own hand or am I being
touched? If the distance increases between me and another person, is it because I am moving
away or because the other person is? The brain has to discriminate between these situations
because they evoke different behaviors. It manages this task using an internal feedback
mechanism that has additional effects: it ensures that we don't damage our ears when we
shout and makes us unable to tickle ourselves. Rather than causing us to laugh ourselves
nearly to death, self-tickling barely evokes a weak smile. This intelligent feedback process is
known as "corollary discharge," and Poulet's investigations of it began in the singing cricket.
On warm summer nights, male crickets attract females by rhythmically rubbing its forewings
together at a level that exceeds 100 decibels. That level of noise can be compared to the sound
of a chainsaw, the thundering of disco music or the din of a pneumatic drill. "We wondered
why male crickets don't go deaf from the noise," explains Poulet. "How do they shut it out?"
The young prizewinner demonstrated that as male crickets begin to chirp, they "turn down" or
inhibit very specific neurons responsible for hearing; when they stop they remove the
inhibition. This "on" and "off" switch protects crickets against deafness – but they can still
hear the approach of an enemy. That's crucial because their loud mating call not only attracts
interested females but broadcasts a cricket's location to predators and rivals. Poulet and his
co-workers identified a precise neuron that tunes down the hearing system – the so-called
“corollary discharge neuron." Other organisms have these neurons as well, but virtually
nothing is known about them.
Poulet and his co-workers also investigated how females react to the chirping. Here the
question was whether they approach males as a reflex or whether it is a reaction to complex
acoustic recognition in the females' brain? Poulet showed that the females display an
approach movement in response to individual sounds that resembles a reflex rather than a
complex behavioral response. More support for this comes from the fact that once the
approach process has been triggered by the right song, females react to any acoustic stimulus.
At that point only the sound is important; it doesn't even have to be the right one.
What's the difference between dozing and being wide awake?
James Poulet is also interested in what is generally known as brain states. The concept can
easily be illustrated by an example. Someone is sitting dozing in the sun with his eyes closed,
thinking of nothing in particular. Suddenly, he is startled awake by some internal or external
signal. It may be an unusual sound, an unexpected breeze, or the recollection of a missed
appointment. From one second to the next, the person who was dozing is wide awake. "We
were wondering what happens in the brain in such a situation," Poulet says. "How does one
brain state differ from the other?" In 1929 Hans Berger, the inventor of the EEG, first showed
that different brain states exist in the awake human brain. EEG measures brainwaves, and
when you are sitting still with your eyes closed, they are different than when your eyes are
open and you are looking around. Poulet found something similar in mice in a state of quiet
wakefulness, compared to mice that are moving their whiskers. In the quiet mice, neurons are
excited in a rhythmic and orderly pattern, whereas their excitation is much more
desynchronized in actively moving, "whisking" mice. This apparently gives them greater
flexibility to react to the tasks at hand. "We know that brain states are part of the brain's
normal functions," says Poulet. "Our aim is to discover how they come about and the role
they play in regulating sensory perception and motor behaviour."