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Courtesy of New Scientist Magazine
If just a handful of neurons is all it takes to make a robot act like an
insect, is there less to animal behavior than meets the eye, asks Duncan Graham-Rowe
IN A DARK LABORATORY, a solitary creature shuffles around, searching, lured by the love
song of a male she will never meet. She has spent her entire existence in these unnatural
surroundings and is doomed never to see the outside world, forced to comply with the whims
of researchers. Yet she is unlikely to excite the wrath of animal rights activists nor
provoke much sympathy from anyone. Indeed, hers is barely an existence at all since she
consists of only a few wheels and a bundle of electronic components. Still, at least it's
one less cage to clean out.
The cybercricket's unassuming appearance belies what is a landmark in robotics. It behaves
just like a real cricket-and not just outwardly. It simulates the cricket right down to
the neurons, and is one of the first attempts to reproduce the pattern of neural signaling
found in a living creature. It's also proving to be a challenge to biologists, leaving
them open to the charge that they read too much into animal behavior, and see life as
altogether more complex than it is.
Decades of research have allowed biologists to theorize that certain behaviors are
associated with certain patterns of neural activity. But while they can observe behavior
and record neural signaling, to prove that the two are causally linked is extremely
difficult. This is where robots are starting to make their mark.
The key to these "biobots" is a better understanding of how natural and
artificial neural systems work. Roboticists can now mimic in metal and silicon the natural
pathways that link optical and acoustic sensors-eyes and ears-to motors, which act as
surrogate muscles. This gives biologists the chance to simulate particular patterns of
neural activity and see whether or not they prompt the anticipated behaviors.
The first biobots were very simple, designed to test biological theories. For example,
ethologists believe that in order to navigate, rats construct cognitive maps of their
surroundings. One popular way to test this theory was to place a rat in an open
rectangular box and train it to return to a single location where food had been hidden.
Researchers would then place the rat in a different but identical box to see whether it
would go to the same spot in search of food. Sure enough, it did. The widely held
conclusion from this experiment was that the rat remembered the location of the hidden
food by relating it to the geometry of the box. In other words, it constructed a cognitive
map.
But Henrik Lund, a computer scientist at Aarhus University in Denmark, and Orazio Miglino,
a psychologist at the University of Naples, were not convinced that this conclusion could
be drawn from the rat's behavior. They thought ethologists were reading too much into the
findings. They wanted to make a robot that could mimic the rat's behavior, without a
memory. If the robot had no memory, they reasoned, it could have no map.
Lund and Miglino chose a standard lab robot called a Khepera, a small three-wheeled device
that looks like an electronic biscuit. They added two infrared sensors that act like
radars, four touch sensors and a pair of motors to power the two drive wheels. The robot's
behavior was dictated by an artificial neural network, a cluster of simple mathematical
processors, or nodes, that are designed to behave like nerve cells in the brain.
But while rats have millions of neurons, the robot had only 10 nodes. Its sensors
connected directly to eight of the nodes, which in turn fed two nodes that controlled the
motors. With these basic resources, the researchers then tried to fashion a machine
capable of finding a particular spot in a box using a computer program called a genetic
algorithm
In an artificial neural network, a node adds together the signals arriving at its inputs,
and when the sum reaches a threshold it fires off a signal to other nodes (or to one of
the motors). In addition, every input to a node is given a weighting which either
increases or reduces the incoming signal. A genetic algorithm treats the nodes' thresholds
and weights as though they are genes being passed from one generation to the next.
It creates dozens of different potential robot designs with randomly chosen genes-values
for the thresholds and weights-and tests each one in turn to see how well it finds the
selected spot. It then allows the best performers to "breed" by swapping genes
with other successful robots. The new generation of machines is then tested again. Over
many generations, this process allows the best robot for searching out the target to
evolve.
Lund and Miglino found that their best robot could locate the food's position as easily as
a rat, simply by seeing and feeling its way round the box. "We wanted to . . . look
for the minimum conditions to solve this task," says Lund. Their success showed that
the open box experiment was not enough on its own to support the theory that rats use
cognitive maps. Personally, Lund believes that rats do construct these maps, but he wanted
to show that biologists were reading too much into their results.
Inside the animal
Such experiments can be carried out by computer models, but Lund argues that these are no
substitute for the real world. "One main advantage of using a real robot in a real
experiment is that one is sure to have the same environmental stimuli as in the real
animal experiment," he says. Even so, the minimal robot approach is of only limited
value because it reveals nothing about what goes on inside the animal. The cybercricket,
by contrast, is designed to do just this.
Female crickets home in on potential partners by listening to their song-the sound we hear
as chirping. The sound is made of short bursts, or syllables, of a single
"carrier" frequency which is specific to different species. This behavior,
called phonotaxis, has been studied for decades in the cricket Gryllus bimaculatus, both
by watching its behavior and by studying its nervous system. But showing how the two are
linked has proved very difficult.
Immobilized or sedated
"It's not impossible, but it's definitely challenging and not many people are in a
position to do it," says Barbara Webb, a psychologist at the University of Nottingham
and one of the architects of the cybercricket. A living animal must have electrodes placed
in its brain to record activity from selected neurons. It is then placed in a spherical
treadmill so its movements can be tracked without it going anywhere. "You have to
immobilize the animal or sedate it to keep the electrodes in place," explains Webb.
"But the question is, can it still behave normally?"
The ideal way to study the insect would be to isolate any neurons thought to be relevant
to phonotaxis from the rest of the brain, play some cricket love songs and see what
happens. But this just isn't practical. Webb reasoned, however, that with a robot it might
work. "I wanted to find an animal system that I could build a complete robot model
of. Where the neurons would control the behavior when implemented in the real world,"
she says.
Webb began building the robot in 1991 while at the University of Edinburgh with John
Hallam. Later, they were joined by Lund. As if mimicking the cricket's neural system was
not tough enough, the team also had to copy its bizarre auditory system. G. bimaculatus
has two ear drums on its forelegs that are connected to two holes in its body via internal
tubes (see Diagram). This structure sets up phase differences in front of and behind the
ear drums that allow the cricket to tell which direction a sound is coming from. But this
works only if the sound is at the carrier frequency of the species.
To simulate the auditory system's performance, Webb and her colleagues used a collection
of amplifiers and delay lines, with four microphones positioned on a Khepera robot. A
digital signal from these "ears" fed into a neural network. "There was a
particular pair of neurons that I copied," says Webb. Neuroscientists had shown that
these were crucial to phonotaxis. "I actually tried to tune them so that the model
neurons had the same firing patterns as the cricket ones do," she says.
Next, the researchers added extra nodes to see how many more the system needed before it
displayed phonotaxis. Biologists had speculated that it would need 20 or so. "But it
turns out you only need to add two," says Webb. In her network, the left input node
feeds an output node which controls the Khepera's left motor-the same arrangement is
repeated on the right side. To complete the network, each input node inhibits any signals
to the opposite output node (see Diagram)
This surprising simplicity has been a hallmark of the cybercricket project. The robot was
conceived as a way to test the popular theory that phonotaxis needs two neural control
systems-one to recognize a male's call and the other to locate it. But when Webb played a
call song to the four-node robot, using the same experimental set-up as that used for live
crickets, not only did it recognize the sound but also moved towards it. "The two
systems are just one," says Webb.
This unexpected bonus raised the question of whether a real cricket's neural system is
simpler than anyone had supposed. To investigate this, the team decided to see if the
automaton could replicate other life-like behaviors. They played the robot two call songs
that it could recognize. Live crickets prefer songs with a faster syllable rate. So did
the robot.
For a cricket to choose between two suitors, some biologists argue that crickets must have
their preferences somehow encoded in their brains. But the robot showed this was
unnecessary. The combination of the right auditory system and simple neural network
selects call songs with faster syllable rates automatically.
This suggested to Webb that her simpler neural system is the same as that of a living
cricket. Biologists, she says, seem to overcomplicate things, partly because they
anthropomorphise. "People look at behavior and assume that there are intentions
involved like recognition and goal seeking," she says. "When they look to the
neurons they try to find neurons that correspond to those intentions." The problem is
that the neurons might not exist, because other animals do not work like humans.
But not all biologists are convinced of the robot's relevance to a real cricket.
"There is no problem in studying the behavior that emerges from simple systems,"
says Gerald Pollack, a neuroethologist at McGill University in Montreal who has worked on
crickets. "The problem is [that] most biological systems are not simple," says
Pollack, who argues that even if the robot cricket mimicked the animal's behavior
perfectly, it still wouldn't constitute evidence that robots and animals are doing it the
same way.
Still, as the cybercricket displays more real-life behaviors, Webb feels that her case
improves. "You might say one behavior is not very hard to reproduce," says Webb.
"But if you carry out a whole series of different experiments and you replicate all
of them, then it starts to become a stronger and stronger argument."
But she recognizes that studying behavior alone can take her only so far. The robot allows
her to make predictions about what neuroscientists should find, and ideally she wants
someone to find out if her predictions are correct. If the firing patterns and behaviors
are identical in creature and Khepera, can there still be doubt that they work in the same
way?
Whatever the outcome of this debate, many scientists agree that, at the very least, robots
are valuable research tools. This is the attitude taken at the University of Zürich in
Switzerland, by a team made up of roboticists and zoologists. Their target is ant
navigation.
Ant bearings
The desert ant Cataglyphis forages up to a couple of hundred meters from its nest and
returns home almost unerringly in a straight line. An essential part of this ability, say
biologists, stems from the insect's skill in using polarized light as a compass. As light
from the Sun is scattered by molecules in the atmosphere, it is partially polarized. This
effect creates a pattern in the sky that is symmetrical about the solar meridian, a line
that cuts through the Sun and its zenith point (see Diagram)
Cataglyphis has receptors in its compound eye that are sensitive to different orientations
of polarized light, and researchers believe that these feed three neurons-called POL
neurons. Each neuron gives a peak output for a different orientation, measured as the
angle between the direction in which the light is polarized and the axis of the ant's
body. Yet precisely how the ant makes use of these signals is still unknown, says
Dimitrios Lambrinos, one of the team's roboticists.
One theory to explain the ant's abilities, called the scanning model, is based on just one
POL neuron. It suggests that when an insect goes foraging, it first rotates through 360
degrees until it locates the meridian. The effect is the same as looking through two
polarized lenses and rotating one until the light coming through both is brightest. Once
the ant has this fix, it heads off in its chosen direction.
To test such theories, the team built Sahabot 2, a six-wheeled "ant" with three
sets of polarized light sensors, to simulate those in Cataglyphis. For the POL neurons,
the researchers took a "black box" approach. They didn't want to simulate the
actual firing of neurons, as Webb had done, they merely wanted to reproduce the same
inputs and outputs to and from the neural system. This job fell to three amplifiers, each
one tuned-just as in real ants-to give a maximum output at different angles between the
direction of polarization and the robot's axis.
This year, Lambrinos and his colleagues let the robot loose in Tunisia, where Cataglyphis
lives. In all the experiments, the cyberant behaved very similarly to real ants,
travelling more than 100 meters and returning to its start point with an error of less
than 60 centimeters.
But the robot performed worst when programmed to follow the scanning model. In the noisy
environment of the desert, each amplifier generated only a flattened peak, which made
finding the maximum point-the ant's reference direction-a relatively tough task. But what,
asked the researchers, if they combined the outputs of all three amplifiers? By
subtracting the output of two amplifiers from the third, the team created a sharply
defined peak that the robot sensed with greater accuracy. When the robot trundled off with
this new program onboard, its error rate fell.
It was a revelation, says Lambrinos. Previously, no one understood why there were three
POL neurons, since the scanning model needed only one. It took a test with a robot in the
same environment as living ants to provide a better model.
To go further, the team wants to follow Webb's lead down to the level of individual
neurons-to study the firing patterns in the tiny ant brain and to recreate them in
silicon. This may be easier for the Swiss team than for Webb because it is
multidisciplinary. "One of the reasons this is so successful is that we are working
together," says Lambrinos. "If we don't agree with each other then we know other
biologists won't agree with us either."
Robot modeling is fast becoming a valuable tool for biologists to test theories and create
new ones. And it will continue to grow in popularity, according to Webb, not least because
of the time savings. "It takes a lot more time to find out what one neuron does than
to build one," she says.
And copying nature may hold lessons not just for biologists, says Webb. The cricket
project suggests that computer scientists could get far more out of their neural networks
if they tried. "Someone making a standard neural network would never bother making a
network with only four neurons because they would assume that it simply wouldnt be
enough," she says. But as the lovelorn cybercricket found, sometimes four is all you
need.
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